We present an ultrafast neural network model, QLKNN, which predicts core tokamak transport heat and particle fluxes. QLKNN is a surrogate model based on a database of 3 × 108 flux calculations of the quasilinear gyrokinetic transport model, QuaLiKiz. The database covers a wide range of realistic tokamak core parameters. Physical features such as the existence of a critical gradient for the onset of turbulent transport were integrated into the neural network training methodology. We have coupled QLKNN to the tokamak modeling framework JINTRAC and rapid control-oriented tokamak transport solver RAPTOR. The coupled frameworks are demonstrated and validated through application to three JET shots covering a representative spread of H-mode operating space, predicting the turbulent transport of energy and particles in the plasma core. JINTRAC-QLKNN and RAPTOR-QLKNN are able to accurately reproduce JINTRAC-QuaLiKiz T i, e and ne profiles, but 3-5 orders of magnitude faster. Simulations which take hours are reduced down to only a few tens of seconds. The discrepancy in the final source-driven predicted profiles between QLKNN and QuaLiKiz is on the order of 1%-15%. Also the dynamic behavior was well captured by QLKNN, with differences of only 4%-10% compared to JINTRAC-QuaLiKiz observed at mid-radius, for a study of density buildup following the L-H transition. Deployment of neural network surrogate models in multi-physics integrated tokamak modeling is a promising route toward enabling accurate and fast tokamak scenario optimization, uncertainty quantification, and control applications. DOI dataset used for paper: 10.5281/zenodo.3497066

VL - 27 UR - https://arxiv.org/abs/1911.05617 IS - 2 U1 -FP

U2 -IMT

U5 - ec86515e5a6b89214cdee1922d2f857d ER - TY - JOUR T1 - Predictive multi-channel flux-driven modelling to optimise ICRH tungsten control and fusion performance in JET JF - Nuclear Fusion Y1 - 2020 A1 - Casson, F. J. A1 - Patten, H. A1 - Bourdelle, C. A1 - Breton, S. A1 - Citrin, J. A1 - Koechl, F. A1 - Sertoli, M. A1 - Angioni, C. A1 - Baranov, Y. A1 - Bilato, R. A1 - Belli, E. A1 - Challis, C. D. A1 - Corrigan, G. A1 - Czarnecka, A. A1 - Ficker, O. A1 - Frassinetti, L. A1 - Garzotti, L. A1 - Goniche, M. A1 - Graves, J. P. A1 - Johnson, T. A1 - Kirov, K. A1 - Knight, P. J. A1 - Lerche, E. A1 - Mantsinen, M. A1 - Mylnar, J. A1 - Valisa, M. A1 - JET Contributors AB - The evolution of the JET high performance hybrid scenario, including central accumulation of the tungsten (W) impurity, is reproduced with predictive multi-channel integrated modelling over multiple confinement times using first-principle based core transport models. Eight transport channels (Ti,Te,j,nD,nBe,nNi,nW, omega) are modelled predictively, with self-consistent sources, radiation and magnetic equilibrium, yielding a system with multiple non-linearities: This system can reproduce the observed radiative temperature collapse after several confinement times. W is transported inward by neoclassical convection driven by the main ion density gradients and enhanced by poloidal asymmetries due to centrifugal acceleration. The slow evolution of the bulk density profile sets the timescale for W accumulation. Modelling this phenomenon requires a turbulent transport model capable of accurately predicting particle and momentum transport (QuaLiKiz) and a neoclassical transport model including the effects of poloidal asymmetries (NEO) coupled to an integrated plasma simulator (JINTRAC). The modelling capability is applied to optimise the available actuators to prevent W accumulation, and to extrapolate in power and pulse length. Central NBI heating is preferred for high performance, but gives central deposition of particles and torque which increase the risk of W accumulation by increasing density peaking and poloidal asymmetry. The primary mechanism for ICRH to control W in JET is via its impact through turbulence in reducing main ion density peaking (which drives inward neoclassical convection), increased temperature screening and turbulent W diffusion. The anisotropy from ICRH also reduces poloidal asymmetry, but this effect is negligible in high rotation JET discharges. High power ICRH near the axis can sensitively mitigate against W accumulation, and dominant ion heating (e.g. He-3 minority) is predicted to provide more resilience to W accumulation than dominant electron heating (e.g. H minority) in the JET hybrid scenario. Extrapolation to DT plasmas finds 17.5 MW of fusion power and improved confinement compared to DD, due to reduced ion-electron energy exchange, and increased Ti/Te stabilisation of ITG instabilities. The turbulence reduction in DT increases density peaking and accelerates the arrival of W on axis; this may be mitigated by reducing the penetration of the beam particle source with an increased pedestal density. PB - IOP Publishing VL - 60 IS - 6 U1 - FP U2 - IMT U5 - 85e09691b3e9df9d0bb4bc6800d91b57 ER - TY - JOUR T1 - On hybrid scenarios in KSTAR JF - Nuclear Fusion Y1 - 2020 A1 - Na, Y. S. A1 - Lee, Y. H. A1 - Byun, C. S. A1 - Kim, S. K. A1 - Lee, C. Y. A1 - Park, M. S. A1 - Yang, S. M. A1 - Kim, B. A1 - Jeon, Y. M. A1 - Citrin, J. A1 - Choi, G. J. A1 - Juhn, J. W. A1 - Kang, J. S. A1 - Kim, H. S. A1 - Kim, J. H. A1 - Ko, W. H. A1 - Kwon, J. M. A1 - Lee, W. C. A1 - Woo, M. H. A1 - Yi, S. A1 - Yoon, S. W. A1 - Yun, G. S. A1 - KSTAR Team AB - We report the status of hybrid scenario experiments in Korea Superconducting Tokamak Advanced Research (KSTAR). The hybrid scenario is defined as stationary discharges with B N >= 2.4 and H_89 >= 2.0 at q_95 < 6.5 without or with very mild sawtooth activities in KSTAR. It is being developed towards reactor-relevant conditions. High performance of B N <~ 3.0, H_89 <~ 2.4 and G-factor (= B N H_89/q2_95) <~ 0.46 has been achieved and sustained for >~ 40 T_E at ne/nGW ~ 0.7 with heating power of <~ 5 MW. Some KSTAR hybrid discharges exhibit a unique feature of a slow transition from conventional H-mode to hybrid mode after the third neutral beam injection. The reason for the confinement enhancement is extensively studied in this transition period of a representative discharge exhibiting a common feature of KSTAR hybrid scenarios. 0D performance analysis with magnetohydrodynamic activities, 1D kinetic profile dynamics, power balance analysis, linear gyro-kinetic analysis and edge pedestal stability analysis were conducted. The enhancement is thought to be from both the core and the pedestal. The improvement in the core region of the ion energy channel is observed from the linear gyro-kinetic analysis considering the electromagnetic, the fast ion, the Shafranov shift, W ExB, and the magnetic shear effect. The electromagnetic finite B stabilisation plays a role in the inner core region at p tor ~ 0.35 together with the fast ion effect. The alpha stabilisation effect is also found at p tor ~ 0.5. W ExB, which could reduce the linear growth of the ion temperature gradient mode in the outer core region at p tor ~ 0.5 - 0.7 with the highest contribution from the toroidal rotation. Regarding the improvement in the pedestal, Shafranov shift broadens the stability boundary of the pedestal in support of the diamagnetic effect. The pedestal height and width could be reproduced by the EPED model, while a realistic current profile is used to calculate the internal inductance for Shafranov shift. Based on these findings, a comprehensive confinement enhancement mechanism has been proposed by considering the core-edge interplay. PB - IOP Publishing VL - 60 IS - 8 U1 - FP U2 - IMT U5 - 08abb98c65fe64d143886c32ece5e2c0 ER - TY - JOUR T1 - Core tungsten transport in WEST long pulse L-mode plasmas JF - Nuclear Fusion Y1 - 2020 A1 - Yang, X. A1 - Manas, P. A1 - Bourdelle, C. A1 - Artaud, J. F. A1 - Sabot, R. A1 - Camenen, Y. A1 - Citrin, J. A1 - Clairet, F. A1 - Desgrange, C. A1 - Devynck, P. A1 - Dittmar, T. A1 - Ekedahl, A. A1 - Fedorczak, N. A1 - Gil, L. A1 - Loarer, T. A1 - Lotte, P. A1 - Meyer, O. A1 - Morales, J. A1 - Peret, M. A1 - Peysson, Y. A1 - Stephens, C. D. A1 - Urbanczyk, G. A1 - Vezinet, D.. A1 - Zhang, L. A1 - Gong, X. AB - Tungsten transport is investigated in WEST long pulse L-mode plasmas operated with the strike point on the actively cooled upper tungsten divertor. The pulses are mostly heated by lower hybrid waves. It is experimentally found that tungsten does not centrally accumulate throughout these similar to 30 s reproducible discharges despite large normalised electron density gradients R/L-ne. To explain these observations, turbulent and neoclassical transport of electrons and tungsten ions are computed with GKW Peeters A.G. et al (2009 Computer Phys. Commnun. 180 2650) and NEO Belli E. and Candy J. (2008 Plasma Phys. Control. Fusion 50 095010), Belli E. and Candy J. (2012 Plasma Phys. Control. Fusion 54 015015) respectively. Additionally, interpretative integrated modelling simulations are also performed to keep data coherency despite the lack of measurements of some quantities such as the Ti profiles. Modelled R/Lne are found consistent with interferometry inversions and the tungsten peaking factor R/L-nW remains comparable to R/L-ne due to dominant turbulent diffusivities inside r/a = 0.3-0.8. In the central region r/a < 0.3 neoclassical W transport dominates but the convective velocities are several order of magnitudes lower compared to plasmas with toroidal rotation velocities induced by a neutral beam injection (NBI) torque. Finally, nitrogen is seeded in these pulses leading to an enhanced energy content which is consistent with stabilised ion temperature gradient modes from dilution. PB - IOP Publishing VL - 60 IS - 8 U1 - FP U2 - IMT U5 - 01c2c12463f6cb1eb28bf607a3bd6131 ER - TY - JOUR T1 - Understanding LOC/SOC phenomenology in tokamaks JF - Nuclear Fusion Y1 - 2020 A1 - Rice, J. E. A1 - Citrin, J. A1 - Cao, N. M. A1 - Diamond, P. H. A1 - Greenwald, M. J. A1 - Grierson, B. A. AB - Phenomenology of Ohmic energy confinement saturation in tokamaks is reviewed. Characteristics of the linear Ohmic confinement (LOC) and saturated Ohmic confinement (SOC) regimes are documented and transformations in all transport channels across the LOC/SOC transition are described, including rotation reversals, 'non-local' cut-off and density peaking, in addition to dramatic changes in fluctuation intensity. Unification of results from nearly 20 devices indicates that the LOC/SOC transition occurs at a critical value of the product of the density, edge safety factor and device major radius, and that this product increases with toroidal magnetic field. Comparison with gyro-kinetic simulations suggests that the effects of sub-dominant TEMs are important in the LOC regime while ITG mode turbulence dominates with SOC. PB - IOP Publishing VL - 60 IS - 10 U1 - FP U2 - IMT U5 - 2dac9f3023a2c3da7543a1acda20f190 ER - TY - JOUR T1 - Flux-driven integrated modelling of main ion pressure and trace tungsten transport in ASDEX Upgrade JF - Nuclear Fusion Y1 - 2019 A1 - Linder, O. A1 - Citrin, J. A1 - Hogeweij, G. M. D. A1 - Angioni, C. A1 - Bourdelle, C. A1 - Casson, F. J. A1 - Fable, E. A1 - Ho, A. A1 - Koechl, F. A1 - Sertoli, M. A1 - EUROfusion MST1 Team A1 - ASDEX Upgrade Team AB -Neoclassical and turbulent heavy impurity transport in tokamak core plasmas are determined by main ion temperature, density and toroidal rotation profiles. Thus, in order to understand and prevent experimental behaviour of W accumulation, flux-driven integrated modelling of main ion heat and particle transport over multiple confinement times is a vital prerequisite. For the first time, the quasilinear gyrokinetic code QuaLiKiz is applied for successful predictions of core kinetic profiles in an ASDEX Upgrade H-mode discharge in the turbulence dominated region within the integrated modelling suite JETTO. Neoclassical contributions are calculated by NCLASS; auxiliary heat and particle deposition profiles due to NBI and ECRH are prescribed from previous analysis with TRANSP. Turbulent and neoclassical contributions are insufficient in explaining main ion heat and particle transport inside the q = 1 surface, necessitating the prescription of further transport coefficients to mimic the impact of MHD activity on central transport. The ion to electron temperature ratio at the simulation boundary at p tor=0.85 stabilizes ion scale modes while destabilizing ETG modes when significantly exceeding unity. Careful analysis of experimental measurements using Gaussian process regression techniques is carried out to explore reasonable uncertainties. In following trace W impurity transport simulations performed with additionally NEO, neoclassical transport under consideration of poloidal asymmetries alone is found to be insufficient to establish hollow central W density profiles. Reproduction of these conditions measured experimentally is found possible only when assuming the direct impact of a saturated (m, n) = (1, 1) MHD mode on heavy impurity transport.

VL - 59 IS - 1 U1 -FP

U2 -IMT

U5 - 6d42c99c474c747f17f4b44236cecb27 ER - TY - JOUR T1 - Growth rates of ITG modes in the presence of flow shear JF - Physics of Plasmas Y1 - 2019 A1 - Dagnelie, V. I. A1 - Citrin, J. A1 - Jenko, F. A1 - M. J. Pueschel A1 - Gorler, T. A1 - Told, D. A1 - Doerk, H. AB -Plasma microinstabilities in toroidal magnetic confinement devices can be driven unstable by a radial ion temperature gradient and stabilized by rotational flow shear. In this study, we argue that these nonlinear dynamics can be captured by the linear stabilization of Floquet modes. To that end, we propose a novel method (the τAC method) to calculate growth rates by averaging over linear Floquet modes. The τAC method is compared to nonlinear and other linear approaches and is shown to work well at low parallel velocity gradient drive. As such, the method provides a promising approach to explore the parameter dependencies of flow shear stabilization.

VL - 26 IS - 1 U1 -FP

U2 -IMT

U5 - e61faaa5de6eb7214a9347a2996ee794 ER - TY - JOUR T1 - On microinstabilities and turbulence in steep-gradient regions of fusion devices JF - Plasma Physics and Controlled Fusion Y1 - 2019 A1 - M. J. Pueschel A1 - Hatch, D. R. A1 - Ernst, D. R. A1 - Guttenfelder, W. A1 - Terry, P. W. A1 - Citrin, J. A1 - Connor, J. W. AB - Higher excitation states along the background field of plasma instabilities dominate over standard, ground-state eigenmodes once the gradient drive becomes sufficiently strong. At this point, mode parity can no longer be used as the sole identifier of microtearing activity. Not only ion- and electron-temperature-gradient-driven modes occur in higher states, but even trapped electron modes, where decorrelation mechanisms enable odd-parity mode structures without a vanishing bounce average. Nonlinearly, higher-order Hermite states imprint their structures on the turbulence. Even at considerably strong drive, however, quasilinear models can recover nonlinear fluxes, as long as all subdominantly unstable eigenmodes are included. Due to flux contributions from such modes, gradient scalings of fluxes can be stronger than linear expectations. VL - 61 IS - 3 U1 - FP U2 - IMT U5 - 223f039c6443fe920ab2b51949e5babd ER - TY - JOUR T1 - Application of Gaussian process regression to plasma turbulent transport model validation via integrated modelling JF - Nuclear Fusion Y1 - 2019 A1 - Ho, A. A1 - Citrin, J. A1 - Auriemma, F. A1 - Bourdelle, C. A1 - Casson, F. J. A1 - Kim, H. T. A1 - Manas, P. A1 - Szepesi, G. A1 - Weisen, H. A1 - JET Contributors AB -This paper outlines an approach towards improved rigour in tokamak turbulence transport model validation within integrated modelling. Gaussian process regression (GPR) techniques were applied for profile fitting during the preparation of integrated modelling simulations allowing for rigourous sensitivity tests of prescribed initial and boundary conditions as both fit and derivative uncertainties are provided. This was demonstrated by a JETTO integrated modelling simulation of the JET ITER-like-wall H-mode baseline discharge #92436 with the QuaLiKiz quasilinear turbulent transport model, which is the subject of extrapolation towards a deuterium–tritium plasma. The simulation simultaneously evaluates the time evolution of heat, particle, and momentum fluxes over ~10 confinement times, with a simulation boundary condition at rho tor=0.85. Routine inclusion of momentum transport prediction in multi-channel flux-driven transport modelling is not standard and is facilitated here by recent developments within the QuaLiKiz model. Excellent agreement was achieved between the fitted and simulated profiles for n e , T e , T i , and omega tor within 2x, but the simulation underpredicts the mid-radius T i and overpredicts the core n e and T e profiles for this discharge. Despite this, it was shown that this approach is capable of deriving reasonable inputs, including derivative quantities, to tokamak models from experimental data. Furthermore, multiple figures-of-merit were defined to quantitatively assess the agreement of integrated modelling predictions to experimental data within the GPR profile fitting framework.

VL - 59 IS - 5 U1 -FP

U2 -IMT

U5 - 9b651392bcad55886e0c0848df55a9f0 ER - TY - JOUR T1 - First principles and integrated modelling achievements: towards trustful fusion power predictions for JET and ITER JF - Nuclear Fusion Y1 - 2019 A1 - J. Garcia A1 - Dumont, R. A1 - Joly, J. A1 - Morales, J. A1 - Garzotti, L. A1 - Bache, T. A1 - Baranov, Y. A1 - Casson, F. A1 - Citrin, J. A1 - Ho, A. A1 - Challis, C. A1 - Kirov, K. A1 - Mailloux, J. A1 - Saarelma, S. A1 - Nocente, M. A1 - Bañón Navarro, A. A1 - Gorler, T. A1 - Gallart, D. A1 - Mantsinen, M. AB -Predictability of burning plasmas is a key issue for designing and building credible future fusion devices. In this context, an important effort of physics understanding and guidance is being carried out in parallel to the on-going JET experimental campaigns in H, D and T by performing analyses and modelling towards an improvement of the understanding of DT physics for the optimization of the JET-DT neutron yield and fusion born alpha particle physics. Extrapolations to JET-DT from recent experiments using the maximum power available have been performed including some of the most sophisticated codes and a broad selection of models. There is a general agreement that 11-15MW of fusion power can be expected in DT for the hybrid and baseline scenarios. On the other hand, in high beta, torque and fast ion fraction conditions, isotope effects could be favourable leading to higher fusion yield. It is shown that alpha particles related physics, such as TAE destabilization or fusion power electron heating, could be studied in ITER relevant JET-DT plasmas.

PB - Tegenlicht Meet Up 040 CY - Eindhoven, Netherlands VL - 59 IS - 8 U1 -FP

U2 -IMT

U5 - 979e9d36956a6fb000f8d9e5060d8989 ER - TY - JOUR T1 - Saturation and nonlinear electromagnetic stabilization of ITG turbulence JF - Physics of Plasmas Y1 - 2019 A1 - Whelan, G. G. A1 - M. J. Pueschel A1 - Terry, P. W. A1 - Citrin, J. A1 - McKinney, I. J. A1 - Guttenfelder, W. A1 - Doerk, H. AB - Energy transfer in ion-temperature-gradient-driven (ITG) turbulence and its role in modeling transport are examined for finite normalized plasma pressure β for a number of test cases and experimental discharges. The analysis shows that like the zero-β case, finite-β ITG turbulence saturates by nonlinear energy transfer to stable modes mediated by a zonal flow. Electromagnetic effects reliably increase stable mode amplitudes but affect heat fluxes only at the ≈5% level. The most important change with increased β is an increase in the correlation time of the triplet interaction of the unstable mode, stable mode, and zonal flow, thus providing a heightened nonlinear energy transfer efficiency, which allows the instability to saturate at lower amplitude. The heat flux is examined in connection with nonlinear electromagnetic stabilization, the phenomenon where the flux falloff with β is more pronounced than the falloff predicted by quasilinear transport models. The inclusion of the triplet correlation time in the quasilinear model captures most of the nonlinearly enhanced stabilization for the configurations studied here VL - 26 IS - 8 U1 - FP U2 - IMT U5 - 24be14b025f9701f0f5d92a5b38ecb07 ER - TY - JOUR T1 - Overview of the JET preparation for deuterium–tritium operation with the ITER like-wall JF - Nuclear Fusion Y1 - 2019 A1 - Joffrin, E. A1 - Abduallev, S. A1 - Abhangi, M A1 - Abreu, P. A1 - Afanasev, V. A1 - Citrin, J. A1 - Ho, A. A1 - Hogeweij, G. M. D. A1 - Marin, M. A1 - G. van Rooij A1 - Shumack, A. E. A1 - Jaulmes, F. A1 - Felici, F. A1 - den Harder, N. A1 - Tsalas, M. A1 - Afzal, M. A1 - Aggarwal, K. M. A1 - Ahlgren, T. A1 - Aho-Mantila, L. A1 - Aiba, N. A1 - et al. AB - For the past several years, the JET scientific programme (Pamela et al 2007 Fusion Eng. Des. 82 590) has been engaged in a multi-campaign effort, including experiments in D, H and T, leading up to 2020 and the first experiments with 50%/50% D–T mixtures since 1997 and the first ever D–T plasmas with the ITER mix of plasma-facing component materials. For this purpose, a concerted physics and technology programme was launched with a view to prepare the D–T campaign (DTE2). This paper addresses the key elements developed by the JET programme directly contributing to the D–T preparation. This intense preparation includes the review of the physics basis for the D–T operational scenarios, including the fusion power predictions through first principle and integrated modelling, and the impact of isotopes in the operation and physics of D–T plasmas (thermal and particle transport, high confinement mode (H-mode) access, Be and W erosion, fuel recovery, etc). This effort also requires improving several aspects of plasma operation for DTE2, such as real time control schemes, heat load control, disruption avoidance and a mitigation system (including the installation of a new shattered pellet injector), novel ion cyclotron resonance heating schemes (such as the three-ions scheme), new diagnostics (neutron camera and spectrometer, active Alfvèn eigenmode antennas, neutral gauges, radiation hard imaging systems...) and the calibration of the JET neutron diagnostics at 14 MeV for accurate fusion power measurement. The active preparation of JET for the 2020 D–T campaign provides an incomparable source of information and a basis for the future D–T operation of ITER, and it is also foreseen that a large number of key physics issues will be addressed in support of burning plasmas. VL - 59 IS - 11 U1 - FP U2 - IMT U5 - 307a9a4ea8f67c2769e405c252ba0db4 ER - TY - JOUR T1 - Control of the hydrogen:deuterium isotope mixture using pellets in JET JF - Nuclear Fusion Y1 - 2019 A1 - Valovic, M. A1 - Baranov, Y. A1 - Boboc, A. A1 - Buchanan, J. A1 - Citrin, J. A1 - Delabie, E. A1 - Frassinetti, L. A1 - Fontdecaba, J. A1 - Garzotti, L. A1 - Giroud, C. A1 - McKean, R. A1 - Lerche, E. A1 - Kiptily, V. A1 - Kochl, F. A1 - Marin, M. A1 - Maslov, M. A1 - Menmuir, S. A1 - Tvalashvili, G. A1 - Weisen, H. A1 - JET Contributors AB - Deuterium pellets are injected into an initially pure hydrogen H-mode plasma in order to control the hydrogen:deuterium (H:D) isotope mixture. The pellets are deposited in the outer 20% of the minor radius, similar to that expected in ITER, creating transiently hollow electron density profiles. A H:D isotope mixture of approximately 45%:55% is obtained in the core with a pellet fuelling throughput of @pel = 0.045 P aux / T e.ped similar to previous pellet fuelling experiments in pure deuterium. Evolution of the H:D mix in the core is reproduced using a simple model, although deuterium transport could be higher at the beginning of the pellet train compared with the flat-top phase. VL - 59 IS - 10 U1 - FP U2 - IMT U5 - 4e740b87f1a943a47d0fb40534bcfcce ER - TY - JOUR T1 - Density peaking in JET - determined by fuelling or transport? JF - Nuclear Fusion Y1 - 2019 A1 - Tala, T. A1 - Nordman, H. A1 - Salmi, A. A1 - Bourdelle, C. A1 - Citrin, J. A1 - Czarnecka, A. A1 - Eriksson, F. A1 - Fransson, E. A1 - Giroud, C. A1 - Hillesheim, J. A1 - Maggi, C. A1 - Mantica, P. A1 - Mariani, A. A1 - Maslov, M. A1 - Meneses, L. A1 - Menmuir, S. A1 - Mordijck, S. A1 - Naulin, V. A1 - Oberparleiter, M. A1 - Sips, G. A1 - Tegnered, D. A1 - Tsalas, M. A1 - Weisen, H. A1 - JET Contributors AB - Core density profile peaking and electron particle transport have been extensively studied by performing several dimensionless collisionality (υ *) scans with other matched dimensionless profiles in various plasma operation scenarios on the Joint European Torus (JET). This is the first time when electron particle transport coefficients in the H-mode have been measured on JET with high resolution diagnostics, and therefore we are in a position to distinguish between the neutral beam injection (NBI) source and inward electron particle pinch in contributing to core density peaking. The NBI particle source is found to contribute typically 50%–60% to the electron density peaking in JET H-mode plasmas where T e/T i ~ 1 or smaller and at υ * = 0.1–0.5 (averaged between r/a = 0.3–0.8), and being independent of υ * within that range. In these H-mode plasmas, the electron particle transport coefficients, D e and v e, are small, thus giving rise to the large influence of NBI fueling with respect to transport effect on peaking. In L-mode plasma conditions, the role of the NBI source is small, typically 10%–20%, and the electron particle transport coefficients are large. These dimensionless υ * scans give the best possible data for model validation. TGLF simulations are in good agreement with the experimental results with respect to the role of NBI particle source versus inward pinch in affecting density peaking, both for the H-mode and L-mode υ * scans. It predicts, similarly to experimental results, that typically about half of the peaking originates from the NBI fuelling in the H-mode and 10%–20% in the L-mode. GENE simulation results also support the key role of NBI fuelling in causing a peaked density profile in JET H-mode plasma (T e/T i ~ 1 and υ * = 0.1–0.5) and, in fact, give an even higher weight on NBI fuelling than that experimentally observed or predicted by TGLF. For the non-fuelled H-mode plasma at higher T e/T i = 1.5 and lower β N and υ *, both TGLF and GENE predict peaked density profiles, therefore agreeing well with experimental steady-state density peaking. Overall, the various modelling results give a fairly good confidence in using TGLF and GENE in predicting density peaking in quite a wide range of plasma conditions in JET. VL - 59 IS - 12 U1 - FP U2 - IMT U5 - 082a12a8da2101f77aa3f00d71ab98a6 ER - TY - JOUR T1 - High Z neoclassical transport: Application and limitation of analytical formulae for modelling JET experimental parameters JF - Physics of Plasmas Y1 - 2018 A1 - Breton, S. A1 - Casson, F. J. A1 - Bourdelle, C. A1 - Angioni, C. A1 - Belli, E. A1 - Camenen, Y. A1 - Citrin, J. A1 - Garbet, X. A1 - Sarazin, Y. A1 - Sertoli, M. A1 - JET Contributors AB -Heavy impurities, such as tungsten (W), can exhibit strongly poloidally asymmetric density profiles in rotating or radio frequency heated plasmas. In the metallic environment of JET, the poloidal asymmetry of tungsten enhances its neoclassical transport up to an order of magnitude, so that neoclassical convection dominates over turbulent transport in the core. Accounting for asymmetries in neoclassical transport is hence necessary in the integrated modeling framework. The neoclassical drift kinetic code, NEO [E. Belli and J. Candy, Plasma Phys. Controlled Fusion P50, 095010 (2008)], includes the impact of poloidal asymmetries on W transport. However, the computational cost required to run NEO slows down significantly integrated modeling. A previous analytical formulation to describe heavy impurity neoclassical transport in the presence of poloidal asymmetries in specific collisional regimes [C. Angioni and P. Helander, Plasma Phys. Controlled Fusion 56, 124001 (2014)] is compared in this work to numerical results from NEO. Within the domain of validity of the formula, the factor for reducing the temperature screening due to poloidal asymmetries had to be empirically adjusted. After adjustment, the modified formula can reproduce NEO results outside of its definition domain, with some limitations: When main ions are in the banana regime, the formula reproduces NEO results whatever the collisionality regime of impurities, provided that the poloidal asymmetry is not too large. However, for very strong poloidal asymmetries, agreement requires impurities in the Pfirsch-Schlüter regime. Within the JETTO integrated transport code, the analytical formula combined with the poloidally symmetric neoclassical code NCLASS [W. A. Houlberg et al., Phys. Plasmas 4, 3230 (1997)] predicts the same tungsten profile as NEO in certain cases, while saving a factor of one thousand in computer time, which can be useful in scoping studies. The parametric dependencies of the temperature screening reduction due to poloidal asymmetries would need to be better characterised for this faster model to be extended to a more general applicability.

VL - 25 IS - 1 U1 -FP

U2 -IMT

U5 - 4124380d82634de2c37d95ae7f073ef4 ER - TY - JOUR T1 - Effects of nitrogen seeding on core ion thermal transport in JET ILW L-mode plasmas JF - Nuclear Fusion Y1 - 2018 A1 - Bonanomi, N. A1 - Mantica, P. A1 - Citrin, J. A1 - Giroud, C. A1 - Lerche, E. A1 - Sozzi, C. A1 - Taylor, D. A1 - Tsalas, M. A1 - Van Eester, D. A1 - JET Contributors AB -A set of experiments was carried out in JET ILW (Joint European Torus with ITER-Like Wall) L-mode plasmas in order to study the effects of light impurities on core ion thermal transport. N was puffed into some discharges and its profile was measured by active Charge Exchange diagnostics, while ICRH power was deposited on- and off-axis in (3 He) - D minority scheme in order to have a scan of local heat flux at constant total power with and without N injection. Experimentally, the ion temperature profiles are more peaked for similar heat fluxes when N is injected in the plasma. Gyro-kinetic simulations using the GENE code indicate that a stabilization of Ion Temperature Gradient driven turbulent transport due to main ion dilution and to changes in T_e /T_i and s / q is responsible of the enhanced peaking. The quasi-linear models TGLF and QuaLiKiz are tested against the experimental and the gyro-kinetic results.

VL - 58 IS - 2 U1 -FP

U2 -IMT

U5 - a5afa1cd02ed07bb15c57c8516a6c1c8 ER - TY - JOUR T1 - Non-Maxwellian fast particle effects in gyrokinetic GENE simulations JF - Physics of Plasmas Y1 - 2018 A1 - Di Siena, A. A1 - Gorler, T. A1 - Doerk, H. A1 - Bilato, R. A1 - Citrin, J. A1 - Johnson, T. A1 - Schneider, M. A1 - Poli, E. A1 - JET Contributors AB -Fast ions have recently been found to significantly impact and partially suppress plasma turbulence both in experimental and numerical studies in a number of scenarios. Understanding the underlying physics and identifying the range of their beneficial effect is an essential task for future fusion reactors, where highly energetic ions are generated through fusion reactions and external heating schemes. However, in many of the gyrokinetic codes fast ions are, for simplicity, treated as equivalent-Maxwellian-distributed particle species, although it is well known that to rigorously model highly non-thermalised particles, a non-Maxwellian background distribution function is needed. To study the impact of this assumption, the gyrokinetic code GENE has recently been extended to support arbitrary background distribution functions which might be either analytical, e.g., slowing down and bi-Maxwellian, or obtained from numerical fast ion models. A particular JET plasma with strong fast-ion related turbulence suppression is revised with these new code capabilities both with linear and nonlinear gyrokinetic simulations. It appears that the fast ion stabilization tends to be less strong but still substantial with more realistic distributions, and this improves the quantitative power balance agreement with experiments.

VL - 25 UR - https://arxiv.org/abs/1802.04561 IS - 4 U1 -FP

U2 -IMT

U5 - 11b445a4af8c06819215a9cc1c28cdad ER - TY - JOUR T1 - Observation of enhanced ion particle transport in mixed H/D isotope plasmas on JET JF - Nuclear Fusion Y1 - 2018 A1 - Maslov, M. A1 - King, D. A1 - Viezzer, E. A1 - Keeling, D. L. A1 - Giroud, C. A1 - Tala, T. A1 - Salmi, A. A1 - Marin, M. A1 - Citrin, J. A1 - Bourdelle, C. A1 - Solano, E. R. A1 - JET Contributors AB -Particle transport in tokamak plasmas has been intensively studied in the past, particularly in relation to density peaking and the presence of anomalous inward particle convection in L- and H-modes. While in the L-mode case the presence of the anomalous inward pinch has previously been unambiguously demonstrated, particle transport in the H-mode was unclear. The main difficulty of such studies is that particle diffusion and convection could not be measured independently in steady-state conditions in the presence of a core particle flux. Therefore, it is usually not possible to separate the transport effect(inward convection), from the source effect (slow diffusion of particles introduced to the plasma core by neutral beam injection heating). In this work we describe experiments done on JET with mixtures of two hydrogenic isotopes: H and D. It is demonstrated that in the case of several ion species, convection and diffusion can be separated in a steady plasma without implementation of perturbative techniques such as gas puff modulation. Previous H-mode density peaking studies suggested that for this relatively high electron collisionality plasma scenario, the observed density gradient is mostly driven by particle source and low particle diffusivity D < 0.5 * χ eff. Transport coefficients derived from observation of the isotope profiles in the new experiments far exceed that value—ion particle diffusion is found to be as high as D ≥ 2 * χ eff, combined with a strong inward convection. Apparent disagreement with previous findings was explained by significantly faster transport of ion components with respect to the electrons, which could not be observed in a single main ion species plasma. This conclusion is confirmed by quasilinear gyrokinetic simulations.

VL - 58 IS - 7 U1 -FP

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U5 - 811540f379fb24aad4246914a85abf67 ER - TY - JOUR T1 - First principle integrated modeling of multi-channel transport including Tungsten in JET JF - Nuclear Fusion Y1 - 2018 A1 - Breton, S. A1 - Casson, F. A1 - Bourdelle, C. A1 - Citrin, J. A1 - Baranov, Y. A1 - Camenen, Y. A1 - Challis, C. A1 - Corrigan, G. A1 - J. Garcia A1 - Garzotti, L. A1 - Henderson, S. A1 - Koechl, F. A1 - Militello-Asp, E. A1 - O'Mulane, M. A1 - Putterich, T. A1 - Sertoli, M. A1 - Valisa, M. AB -For the first time, over five confinement times, the self-consistent flux driven time evolution of heat, momentum transport and particle fluxes of electrons and multiple ions including Tungsten (W) is modeled within the integrated modeling platform JETTO [Romanelli M et al PFR 2014], using first principle-based codes: namely, QuaLiKiz [Bourdelle C. et al. PPCF 2016] for turbulent transport and NEO [Belli E A and Candy J PPCF 2008] for neoclassical transport. For a JET-ILW pulse, the evolution of measured temperatures, rotation and density profiles are successfully predicted and the observed W central core accumulation is obtained. The poloidal asymmetries of the W density modfying its neoclassical and turbulent transport are accounted for. Actuators of the W core accumulation are studied: removing the central particle source annihilates the central W accumulation whereas the suppression of the torque reduces significantly the W central accumulation. Finally, the presence of W slightly reduces main ion heat turbulent transport through complex nonlinear interplays involving radiation, effective charge impact on ITG and collisionality.

VL - 58 IS - 9 U1 -FP

U2 -IMT

U5 - 8b217fe93b83353631a28ae3b1981d0a ER - TY - JOUR T1 - Fast H isotope and impurity mixing in ion-temperature-gradient turbulence JF - Nuclear Fusion Y1 - 2018 A1 - Bourdelle, C. A1 - Camenen, Y. A1 - Citrin, J. A1 - Marin, M. A1 - Casson, F. J. A1 - Kochl, F. A1 - Maslov, M. A1 - JET Contributors AB - In ion-temperature-gradient (ITG) driven turbulence, the resonance condition leads to ion particle turbulent transport coefficients significantly larger than electron particle turbulent transport coefficients. This is shown in nonlinear gyrokinetic simulations and explained by an analytical quasilinear model. It is then illustrated by JETTO-QuaLiKiz integrated modelling. Large ion particle transport coefficients implies that the ion density profiles are uncorrelated to the corresponding ion source, allowing peaked isotope density profiles even in the absence of core source. This also implies no strong core accumulation of He ash. Furthermore, the relaxation time of the individual ion profiles in a multi-species plasma can be significantly faster than the total density profile relaxation time which is constrained by the electrons. This leads to fast isotope mixing and fast impurity transport in FM regimes. In trapped-electron- mode (TEM) turbulence, in presence of electron heating about twice the ion heating, the situation is the inverse: ion particle turbulent transport coefficients are smaller than their electron counterpart. VL - 58 IS - 7 U1 - FP U2 - IMT U5 - 412f415e9060f52a4a0e26b173073035 ER - TY - JOUR T1 - Real-time-capable prediction of temperature and density profiles in a tokamak using RAPTOR and a first-principle-based transport model JF - Nuclear Fusion Y1 - 2018 A1 - Felici, F. A1 - Citrin, J. A1 - Teplukhina, A. A. A1 - Redondo, J. A1 - Bourdelle, C. A1 - Imbeaux, F. A1 - Sauter, O. A1 - JET Contributors A1 - EUROfusion MST1 Team AB -The RAPTOR code is a control-oriented core plasma profile simulator with various applications in control design and verification, discharge optimization and real-time plasma simulation. To date, RAPTOR was capable of simulating the evolution of poloidal flux and electron temperature using empirical transport models, and required the user to input assumptions on the other profiles and plasma parameters. We present an extension of the code to simulate the temperature evolution of both ions and electrons, as well as the particle density transport. A proof-of-principle neural-network emulation of the quasilinear gyrokinetic QuaLiKiz transport model is coupled to RAPTOR for the calculation of first-principle-based heat and particle turbulent transport. These extended capabilities are demonstrated in a simulation of a JET discharge. The multi-channel simulation requires ~0.2 s to simulate 1 second of a JET plasma, corresponding to ~20 energy confinement times, while predicting experimental profiles within the limits of the transport model. The transport model requires no external inputs except for the boundary condition at the top of the H-mode pedestal. This marks the first time that simultaneous, accurate predictions of T e, T i and n e have been obtained using a first-principle-based transport code that can run in faster-than-real-time for present-day tokamaks.

VL - 58 IS - 9 U1 -FP

U2 -IMT

U5 - cd2e1ae7f2c45a47c0d8427ae170617d ER - TY - JOUR T1 - Separation of transport in slow and fast time-scales using modulated heat pulse experiments (hysteresis in flux explained) JF - Nuclear Fusion Y1 - 2018 A1 - van Berkel, M. A1 - Vandersteen, G. A1 - Zwart, H. J. A1 - Hogeweij, G. M. D. A1 - Citrin, J. A1 - Westerhof, E. A1 - Peumans, D. A1 - M. R. de Baar AB -Old and recent experiments show that there is a direct response to the heating power of transport observed in modulated ECH experiments both in tokamaks and stellarators. This is most apparent for modulated experiments in the Large Helical Device (LHD) and in Wendelstein 7 advanced stellarator (W7-AS). In this paper we show that: 1) This power dependence can be reproduced by linear models and as such hysteresis (in flux) has no relationship to hysteresis as defined in the literature; 2) Observations of "hysteresis" (in flux) and a direct response to power can be perfectly reproduced by introducing an error in the estimated deposition profile as long as the errors redistribute the heat over a large radius; 3) Non-local models depending directly on the heating power can also explain the experimentally observed Lissajous curves (hysteresis); 4) How non-locality and deposition errors can be recognized in experiments and how they affect estimates of transport coefficients; 5) That non-linear-non-local transport models offer a path in discerning deposition errors from non-local fast transport components otherwise experimentally indistinguishable. To show all this, transport needs to be analyzed by separating the transport in a slow (diffusive) time-scale and a fast (heating/non-local) time-scale, which can only be done in the presence of perturbations. (DOI dataset, OA: 10.4121/uuid:5fcf4247-da0e-4119-adcd-fc90b85b7f03)

VL - 58 IS - 10 U1 -FP

U2 -IMT

U5 - 29d19fbcbf73c33399d56c66632d0a33 ER - TY - JOUR T1 - Impact of electron-scale turbulence and multi-scale interactions in the JET tokamak JF - Nuclear Fusion Y1 - 2018 A1 - Bonanomi, N. A1 - Mantica, P. A1 - Citrin, J. A1 - Gorler, T. A1 - Teaca, B. A1 - JET Contributors AB -Experimental observations in JET tokamak plasmas and gyrokinetic simulations point to an important role, for electron heat transport, of electron-scale instabilities and of their interaction with ion-scale instabilities. Since these effects are maximized for strong electron heating and ion-scale modes close to marginal stability, these findings are of high relevance for ITER plasmas, featuring both conditions. Gyrokinetic and quasi-linear transport models accounting for multi-scale effects are assessed against JET experimental results.

VL - 58 IS - 12 U1 -FP

U2 -IMT

U5 - f26a3ff2caa8654cb095ff8fe469feb8 ER - TY - JOUR T1 - Comparison between measured and predicted turbulence frequency spectra in ITG and TEM regimes JF - Plasma Physics and Controlled Fusion Y1 - 2017 A1 - Citrin, J. A1 - Arnichand, H. A1 - Bernardo, J. A1 - Bourdelle, C. A1 - Garbet, X. A1 - Jenko, F. A1 - Hacquin, S. A1 - M. J. Pueschel A1 - Sabot, R. AB -The observation of distinct peaks in tokamak core reflectometry measurements—named quasi-coherent-modes (QCMs)—are identified as a signature of trapped-electron-mode (TEM) turbulence (Arnichand et al 2016 Plasma Phys. Control. Fusion 58 014037). This phenomenon is investigated with detailed linear and nonlinear gyrokinetic simulations using the Gene code. A Tore-Supra density scan is studied, which traverses through a linear (LOC) to saturated (SOC) ohmic confinement transition. The LOC and SOC phases are both simulated separately. In the LOC phase, where QCMs are observed, TEMs are robustly predicted unstable in linear studies. In the later SOC phase, where QCMs are no longer observed, ion-temperature-gradient (ITG) modes are identified. In nonlinear simulations, in the ITG (SOC) phase, a broadband spectrum is seen. In the TEM (LOC) phase, a clear emergence of a peak at the TEM frequencies is seen. This is due to reduced nonlinear frequency broadening of the underlying linear modes in the TEM regime compared with the ITG regime. A synthetic diagnostic of the nonlinearly simulated frequency spectra reproduces the features observed in the reflectometry measurements. These results support the identification of core QCMs as an experimental marker for TEM turbulence.

VL - 59 IS - 6 U1 -FP

U2 -IMT

U5 - a74f9500780401307ab96cdb2d2ab014 ER - TY - JOUR T1 - Overview of the JET results in support to ITER JF - Nuclear Fusion Y1 - 2017 A1 - X. Litaudon A1 - Abduallev, S. A1 - Abhangi, M. A1 - Citrin, J. A1 - den Harder, N. A1 - Hogeweij, G. M. D. A1 - Jaulmes, F. A1 - Shumack, A. A1 - Tsalas, M. A1 - G. J. van Rooij A1 - et al. AB - The 2014–2016 JET results are reviewed in the light of their significance for optimising the ITER research plan for the active and non-active operation. More than 60 h of plasma operation with ITER first wall materials successfully took place since its installation in 2011. New multi-machine scaling of the type I-ELM divertor energy flux density to ITER is supported by first principle modelling. ITER relevant disruption experiments and first principle modelling are reported with a set of three disruption mitigation valves mimicking the ITER setup. Insights of the L–H power threshold in Deuterium and Hydrogen are given, stressing the importance of the magnetic configurations and the recent measurements of fine-scale structures in the edge radial electric. Dimensionless scans of the core and pedestal confinement provide new information to elucidate the importance of the first wall material on the fusion performance. H-mode plasmas at ITER triangularity (H = 1 at β N ~ 1.8 and n/n GW ~ 0.6) have been sustained at 2 MA during 5 s. The ITER neutronics codes have been validated on high performance experiments. Prospects for the coming D–T campaign and 14 MeV neutron calibration strategy are reviewed. VL - 57 IS - 10 U1 - FP U2 - IMT U5 - 38b74a22f1f4822d531a5a047a2ecc1c ER - TY - JOUR T1 - Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution JF - Nuclear Fusion Y1 - 2017 A1 - Meyer, H. A1 - Eich, T. A1 - Citrin, J. A1 - Classen, I. A1 - Hogeweij, D. A1 - Jaulmes, F. A1 - Kappatou, A. A1 - van den Brand, H. A1 - Vanovac, B. A1 - Vijvers, W. A. J. A1 - Westerhof, E. A1 - et al. AB - Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement H H(98,y2) =approx 0.95. Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes. VL - 57 IS - 10 U1 - FP U2 - PEPD U5 - f781e58d912e0c330cdf9b05c806267b ER - TY - JOUR T1 - Overview of the TCV tokamak program: scientific progress and facility upgrades JF - Nuclear Fusion Y1 - 2017 A1 - Coda, S. A1 - Ahn, J. A1 - Albanese, R. A1 - Alberti, S. A1 - Alessi, E. A1 - Citrin, J. A1 - Hogeweij, D. A1 - Vijvers, W. A. J. A1 - EUROfusion MST1 Team A1 - et al. AB - The TCV tokamak is augmenting its unique historical capabilities (strong shaping, strong electron heating) with ion heating, additional electron heating compatible with high densities, and variable divertor geometry, in a multifaceted upgrade program designed to broaden its operational range without sacrificing its fundamental flexibility. The TCV program is rooted in a three-pronged approach aimed at ITER support, explorations towards DEMO, and fundamental research. A 1 MW, tangential neutral beam injector (NBI) was recently installed and promptly extended the TCV parameter range, with record ion temperatures and toroidal rotation velocities and measurable neutral-beam current drive. ITER-relevant scenario development has received particular attention, with strategies aimed at maximizing performance through optimized discharge trajectories to avoid MHD instabilities, such as peeling-ballooning and neoclassical tearing modes. Experiments on exhaust physics have focused particularly on detachment, a necessary step to a DEMO reactor, in a comprehensive set of conventional and advanced divertor concepts. The specific theoretical prediction of an enhanced radiation region between the two X-points in the low-field-side snowflake-minus configuration was experimentally confirmed. Fundamental investigations of the power decay length in the scrape-off layer (SOL) are progressing rapidly, again in widely varying configurations and in both D and He plasmas; in particular, the double decay length in L-mode limited plasmas was found to be replaced by a single length at high SOL resistivity. Experiments on disruption mitigation by massive gas injection and electron-cyclotron resonance heating (ECRH) have begun in earnest, in parallel with studies of runaway electron generation and control, in both stable and disruptive conditions; a quiescent runaway beam carrying the entire electrical current appears to develop in some cases. Developments in plasma control have benefited from progress in individual controller design and have evolved steadily towards controller integration, mostly within an environment supervised by a tokamak profile control simulator. TCV has demonstrated effective wall conditioning with ECRH in He in support of the preparations for JT-60SA operation. VL - 57 IS - 10 U1 - FP U2 - PEPD U5 - 2a93b89d78ec281b0b5b9970ec417422 ER - TY - JOUR T1 - Tractable flux-driven temperature, density, and rotation profile evolution with the quasilinear gyrokinetic transport model QuaLiKiz JF - Plasma Physics and Controlled Fusion Y1 - 2017 A1 - Citrin, J. A1 - Bourdelle, C. A1 - Casson, F. J. A1 - Angioni, C. A1 - Bonanomi, N. A1 - Camenen, Y. A1 - Garbet, X. A1 - Garzotti, L. A1 - Gorler, T. A1 - Gurcan, O. D. A1 - Koechl, F. A1 - Imbeaux, F. A1 - Linder, O. A1 - van de Plassche, K. A1 - Strand, P. A1 - Szepesi, G. A1 - JET Contributors AB - Quasilinear turbulent transport models are a successful tool for prediction of core tokamak plasma profiles in many regimes. Their success hinges on the reproduction of local nonlinear gyrokinetic fluxes. We focus on significant progress in the quasilinear gyrokinetic transport model QuaLiKiz (Bourdelle et al 2016 Plasma Phys. Control. Fusion 58 014036), which employs an approximated solution of the mode structures to significantly speed up computation time compared to full linear gyrokinetic solvers. Optimisation of the dispersion relation solution algorithm within integrated modelling applications leads to flux calculations x10 6-7 faster than local nonlinear simulations. This allows tractable simulation of flux-driven dynamic profile evolution including all transport channels: ion and electron heat, main particles, impurities, and momentum. Furthermore, QuaLiKiz now includes the impact of rotation and temperature anisotropy induced poloidal asymmetry on heavy impurity transport, important for W-transport applications. Application within the JETTO integrated modelling code results in 1 s of JET plasma simulation within 10 h using 10 CPUs. Simultaneous predictions of core density, temperature, and toroidal rotation profiles for both JET hybrid and baseline experiments are presented, covering both ion and electron turbulence scales. The simulations are successfully compared to measured profiles, with agreement mostly in the 5%–25% range according to standard figures of merit. QuaLiKiz is now open source and available at www.qualikiz.com. VL - 59 UR - https://arxiv.org/abs/1708.01224 IS - 12 U1 - FP U2 - IMT U5 - 5234d3fadaa3c4d0ad98a555a43c362c ER - TY - JOUR T1 - Stellarator Turbulence: Subdominant Eigenmodes and Quasilinear Modeling JF - Physical Review Letters Y1 - 2016 A1 - M. J. Pueschel A1 - Faber, B. J. A1 - Citrin, J. A1 - Hegna, C. C. A1 - Terry, P. W. A1 - Hatch, D. R. PB - American Physical Society VL - 116 IS - 8 U1 - FP U2 - CPP U5 - d13ff85b6bf2a1fef927ff144d95eb66 ER - TY - JOUR T1 - Core turbulent transport in tokamak plasmas: bridging theory and experiment with QuaLiKiz JF - Plasma Physics and Controlled Fusion Y1 - 2016 A1 - Bourdelle, C. A1 - Citrin, J. A1 - Baiocchi, B. A1 - Casati, A. A1 - Cottier, P. A1 - Garbet, X. A1 - Imbeaux, F. A1 - JET Contributors AB -Nonlinear gyrokinetic codes allow for detailed understanding of tokamak core turbulent transport. However, their computational demand precludes their use for predictive profile modeling. An alternative approach is required to bridge the gap between theoretical understanding and prediction of experiments. A quasilinear gyrokinetic model, QuaLiKiz (Bourdelle et al 2007 Phys. Plasmas 14 112501), is demonstrated to be rapid enough to ease systematic interface with experiments. The derivation and approximation of this approach are reviewed. The quasilinear approximation is proven valid over a wide range of core plasma parameters. Examples of profile prediction using QuaLiKiz coupled to the CRONOS integrated modeling code (Artaud et al 2010 Nucl. Fusion 50 043001) are presented. QuaLiKiz is being coupled to other integrated modeling platforms such as ETS and JETTO. QuaLiKiz quasilinear gyrokinetic turbulent heat, particle and angular momentum fluxes are available to all users. It allows for extensive stand-alone interpretative analysis and for first principle based integrated predictive modeling.

VL - 58 IS - 1 U1 -FP

U2 -PDG

U5 - af81b465b951460b1fab6740bf4a96ce ER - TY - JOUR T1 - Identification of trapped electron modes in frequency fluctuation spectra JF - Plasma Physics and Controlled Fusion Y1 - 2016 A1 - Arnichand, H. A1 - Citrin, J. A1 - Hacquin, S. A1 - Sabot, R. A1 - Kramer-Flecken, A. A1 - Garbet, X. A1 - Bourdelle, C. A1 - Bottereau, C. A1 - Clairet, F. A1 - Giacalone, J. C. A1 - Guimaraes-Filho, Z. O. A1 - Guirlet, R. A1 - Hornung, G. A1 - Lebschy, A. A1 - Lotte, P. A1 - Maget, P. A1 - Medvedeva, A. A1 - Molina, D. A1 - Nikolaeva, V. A1 - Prisiazhniuk, D. A1 - Tore Supra team A1 - ASDEX Upgrade Team VL - 58 U1 -FP

U2 -CPP

U5 - b89f99c7c6f1824655dca8faaaae5de9 ER - TY - JOUR T1 - Simulation of core turbulence measurement in Tore Supra ohmic regimes JF - Physics of Plasmas Y1 - 2016 A1 - Hacquin, S. A1 - Citrin, J. A1 - Arnichand, H. A1 - Sabot, R. A1 - Bourdelle, C. A1 - Garbet, X. A1 - Kramer-Flecken, A. A1 - Tore Supra team AB - This paper reports on a simulation of reflectometry measurement in Tore Supra ohmic discharges, for which the experimental observations as well as gyrokinetic non-linear computations predict a modification of turbulence spectrum between the linear (LOC) and the saturated ohmic confinement (SOC) regimes. Synthetic reflectometry simulations coupling full-wave computations with gyrokinetic data are carried out. This allows a direct comparison between the gyrokinetic non-linear predictions and experimental observations. The synthetic diagnostic results are found in a good agreement with the experimental findings; in particular, they reproduce well the quasi-coherent peak in the fluctuation spectrum of LOC regimes dominated by a trapped electron mode turbulence. It is also shown that such synthetic tools are valuable for (i) an enhanced interpretation of the reflectometry measurement (for instance, through the investigation of the 2D effects) and (ii) a better understanding of the turbulence properties (for instance, via the analysis of its poloidal asymmetry). VL - 23 IS - 9 U1 - FP U2 - IMT U3 - FP120 U5 - 4b1cc2d83ad4908defa7bd6025412a28 ER - TY - JOUR T1 - Gyrokinetic study of turbulence suppression in a JET-ILW power scan JF - Plasma Physics and Controlled Fusion Y1 - 2016 A1 - Doerk, H. A1 - Challis, C. A1 - Citrin, J. A1 - J. Garcia A1 - Gorler, T. A1 - Jenko, F. A1 - JET Contributors AB -For exploring tokamak operation regimes that deliver both high beta and good energy confinement, power scans at JET with ITER-like wall have been performed. Relatively weak degradation of the confinement time coincides with increased core temperature of the ions at high power. The changes in core turbulence characteristics during a power scan with an optimized (broad) q profile are analyzed by means of nonlinear gyrokinetic simulations. The increase in beta is crucial for stabilizing ion temperature gradient driven turbulence, accompanied by increased ion to electron temperature ratio, the presence of a dynamic fast ion species, as well as the geometric stabilization by increased thermal and suprathermal pressure. A sensitivity study with respect to the q profile reveals that electromagnetic effects are more pronounced at larger values of q. Further, it is confirmed that turbulence suppression due to rotation becomes less effective in such strongly electromagnetic systems. Electrostatic simplified models may thus perform well in present-day devices, in which high beta is often correlated with high rotation, but provide poor extrapolation towards low rotation devices. Implications for ITER and reactor plasmas are discussed.

VL - 58 SN - 0741-3335 UR - http://www.euro-fusionscipub.org/wp-content/uploads/WPJET1PR16_15828_submitted.pdf IS - 11 U1 -FP

U2 -IMT

U3 - FP120 U5 - 4c63548b9ee1a5f0fa742d00cde333d3 ER - TY - CONF T1 - Non-Maxwellian background effects in gyrokinetic simulations with GENE T2 - Journal of Physics: Conference Series Y1 - 2016 A1 - Di Siena, A. A1 - Gorler, T. A1 - Doerk, H. A1 - Citrin, J. A1 - Johnson, T. A1 - Schneider, M. A1 - Poli, E. A1 - JET Contributors AB - The interaction between fast particles and core turbulence has been established as a central issue for a tokamak reactor. Recent results predict significant enhancement of electromagnetic stabilisation of ITG turbulence in the presence of fast ions. However, most of these simulations were performed with the assumption of equivalent Maxwellian distributed particles, whereas to rigorously model fast ions, a non-Maxwellian background distribution function is needed. To this aim, the underlying equations in the gyrokinetic code GENE have been re-derived and implemented for a completely general background distribution function. After verification studies, a previous investigation on a particular JET plasma has been revised with linear simulations. The plasma is composed by Deuterium, electron, Carbon impurities, NBI fast Deuterium and ICRH 3 He. Fast particle distributions have been modelled with a number of different analytic choices in order to study the impact of non-Maxwellian distributions on the plasma turbulence: slowing down and anisotropic Maxwellian. Linear growth rates are studied as a function of the wave number and compared with those obtained using an equivalent Maxwellian. Generally, the choice of the 3 He distribution seems to have a stronger impact on the microinstabilities than that of the fast Deuterium. JF - Journal of Physics: Conference Series VL - 775 IS - 1 U1 - FP U2 - IMT U3 - FP120 U5 - 615b48402fe85900d67891c750171d82 ER - TY - JOUR T1 - Electron density evolution during a fast, non-diffusive propagation of a magnetic field in a multi-ion-species plasma JF - Physics of Plasmas Y1 - 2016 A1 - Doron, R. A1 - Rubinstein, B. A1 - Citrin, J. A1 - Arad, R. A1 - Maron, Y. A1 - Fruchtman, A. A1 - Strauss, H. R. A1 - Mehlhorn, T. A. VL - 23 IS - 12 U1 - FP U2 - IMT U5 - e56a71bf32325ae612dd62bf76b5ed5a ER - TY - JOUR T1 - Benchmarking the GENE and GYRO codes through the relative roles of electromagnetic and E × B stabilization in JET high-performance discharges JF - Plasma Physics and Controlled Fusion Y1 - 2016 A1 - Bravenec, R. A1 - Citrin, J. A1 - Candy, J. A1 - Mantica, P. A1 - Gorler, T. A1 - JET Contributors VL - 58 SN - 0741-3335 IS - 12 U1 - FP U2 - IMT U3 - FP120 U5 - dfe13900d6054d43680fe07fea522554 ER - TY - JOUR T1 - Discriminating the trapped electron modes contribution in density fluctuation spectra JF - Nuclear Fusion Y1 - 2015 A1 - Arnichand, H. A1 - Sabot, R. A1 - Hacquin, S. A1 - Kramer-Flecken, A. A1 - Bourdelle, C. A1 - Citrin, J. A1 - Garbet, X. A1 - Giacalone, J. C. A1 - Guirlet, R. A1 - Hillesheim, J. C. A1 - Meneses, L. VL - 55 IS - 9 U1 -FP

U2 -CPP-HT

U5 - f9d934bd0dfe31427ac6596b2e59bc30 ER - TY - JOUR T1 - Electromagnetic stabilization of tokamak microturbulence in a high- β regime JF - Plasma Physics and Controlled Fusion Y1 - 2015 A1 - Citrin, J. A1 - J. Garcia A1 - T. Görler A1 - Jenko, F. A1 - Mantica, P. A1 - Told, D. A1 - Bourdelle, C. A1 - Hatch, D. R. A1 - Hogeweij, G. M. D. A1 - Johnson, T. A1 - M. J. Pueschel A1 - Schneider, M. AB -The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low- β JET L-mode discharge illustrates the relation between ITG stabilization and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high- β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high- β generalization of comparable observations found at low- β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

VL - 57 IS - 1 U1 -FP

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U5 - e0af60be4a46d18811b729f6beb4c203 ER - TY - JOUR T1 - Key impact of finite-beta and fast ions in core and edge tokamak regions for the transition to advanced scenarios JF - Nuclear Fusion Y1 - 2015 A1 - J. Garcia A1 - Challis, C. A1 - Citrin, J. A1 - Doerk, H. A1 - Giruzzi, G. A1 - T. Görler A1 - Jenko, F. A1 - Maget, P. A1 - JET Contributors AB -Extensive linear and non-linear gyrokinetic simulations and linear magnetohydrodynamic (MHD) analyses performed for JET hybrid discharges with improved confinement have shown that the large population of fast ions found in the plasma core under particular heating conditions has a strong impact on core microturbulence and edge MHD by reducing core ion heat fluxes and increasing pedestal pressure in a feedback mechanism. In the case of the ITER like wall, it is shown how this mechanism plays a decisive role for the transition to plasma regimes with improved confinement and it can explain the weak power degradation obtained in dedicated power scans. The mechanism is found to be highly dependent on plasma triangularity as it changes the balance between the improvement in the plasma core and the edge. The feedback mechanism can play a similar role in the ITER hybrid scenario as in the JET discharges analysed due to its high triangularity plasmas and the large amount of fast ions generated in the core by the heating systems and the alpha power.

VL - 55 UR - http://www.iop.org/Jet/article?EFDP13061&EFDP13061 IS - 5 U1 -FP

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U5 - cb08bcfe1152e2a818b87b6c975ced7e ER - TY - JOUR T1 - L to H mode transition: parametric dependencies of the temperature threshold JF - Nuclear Fusion Y1 - 2015 A1 - Bourdelle, C. A1 - Chone, L. A1 - Fedorczak, N. A1 - Garbet, X. A1 - Beyer, P. A1 - Citrin, J. A1 - Delabie, E. A1 - Dif-Pradalier, G. A1 - Fuhr, G. A1 - Loarte, A. A1 - Maggi, C. F. A1 - Militello, F. A1 - Sarazin, Y. A1 - Vermare, L. A1 - JET Contributors VL - 55 IS - 7 U1 - FP U2 - PDG U5 - d57b66cd0db4e8ad6047c945111fa5a7 ER - TY - JOUR T1 - Real-time capable first principle based modelling of tokamak turbulent transport JF - Nuclear Fusion Y1 - 2015 A1 - Citrin, J. A1 - Breton, S. A1 - Felici, F. A1 - Imbeaux, F. A1 - Aniel, T. A1 - Artaud, J. F. A1 - Baiocchi, B. A1 - Bourdelle, C. A1 - Camenen, Y. A1 - J. Garcia VL - 55 IS - 9 U1 - FP U2 - CPP-HT U5 - 90cae35de21da646d707098e7f9862de ER - TY - JOUR T1 - Development of advanced inductive scenarios for ITER JF - Nuclear Fusion Y1 - 2014 A1 - Luce, T. C. A1 - Challis, C. D. A1 - Ide, S. A1 - Joffrin, E. A1 - Kamada, Y. A1 - Polizer, P. A. A1 - Schweinzer, J. A1 - Sips, A.C.C. A1 - Stober, J. A1 - Giruzzi, G. A1 - Kessel, C. E. A1 - Murakami, M. A1 - Na, Y.-S. A1 - Park, J. M. A1 - Polevoi, A. R. A1 - Budny, R. V. A1 - Citrin, J. A1 - J. Garcia A1 - Hayashi, N. A1 - Hobirk, J. A1 - Hudson, B. F. A1 - Imbeaux, F. A1 - Isayama, A. A1 - McDonald, D. C. A1 - Nakano, T. A1 - Oyama, N. A1 - Parail, V.V. A1 - Petrie, T. W. A1 - Petty, C. C. A1 - Suzuki, T. A1 - Wade, M. R. AB -Since its inception in 2002, the International Tokamak Physics Activity topical group on Integrated Operational Scenarios (IOS) has coordinated experimental and modelling activity on the development of advanced inductive scenarios for applications in the ITER tokamak. The physics basis and the prospects for applications in ITER have been advanced significantly during that time, especially with respect to experimental results. The principal findings of this research activity are as follows. Inductive scenarios capable of higher normalized pressure (beta(N)>= 2.4) than the ITER baseline scenario (beta(N) = 1.8) with normalized confinement at or above the standard H-mode scaling are well established under stationary conditions on the four largest diverted tokamaks (AUG, DIII-D, JET, JT-60U), demonstrated in a database of more than 500 plasmas from these tokamaks analysed here. The parameter range where high performance is achieved is broad in q(95) and density normalized to the empirical density limit. MHD modes can play a key role in reaching stationary high performance, but also define the limits to achieved stability and confinement. Projection of performance in ITER from existing experiments uses empirical scalings and theory-based modelling. The status of the experimental validation of both approaches is summarized here. The database shows significant variation in the energy confinement normalized to standard H-mode confinement scalings, indicating the possible influence of additional physics variables absent from the scalings. Tests using the available information on rotation and the ratio of the electron and ion temperatures indicate neither of these variables in isolation can explain the variation in normalized confinement observed. Trends in the normalized confinement with the two dimensionless parameters that vary most from present-day experiments to ITER, gyroradius and collision frequency, are significant. Regression analysis on the multi-tokamak database has been performed, but it appears that the database is not conditioned sufficiently well to yield a new scaling for this type of plasma. Coordinated experiments on size scaling using the dimensionless parameter scaling approach find a weaker scaling with normalized gyroradius than the standard H-mode scaling. Preliminary studies on scaling with collision frequency show a favourable scaling stronger than the standard H-mode scaling. Coordinated modelling activity has resulted in successful benchmarking of modelling codes in the ITER regime. Validation of transport models using these codes on present-day experiments is in progress, but no single model has been shown to capture the variations seen in the experiments. However, projection to ITER using these models is in general agreement with the favourable projections found with the empirical scalings.

VL - 54 SN - 0029-5515 IS - 1 U1 -FP

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U5 - bd8e6e10ad1da6f54b5c768c2cad759e ER - TY - JOUR T1 - Ion temperature profile stiffness: non-linear gyrokinetic simulations and comparison with experiment JF - Nuclear Fusion Y1 - 2014 A1 - Citrin, J. A1 - Jenko, F. A1 - Mantica, P. A1 - Told, D. A1 - Bourdelle, C. A1 - Dumont, R. A1 - J. Garcia A1 - Haverkort, J. W. A1 - Hogeweij, G. M. D. A1 - Johnson, T. A1 - M. J. Pueschel AB -Recent experimental observations at JET show evidence of reduced ion temperature profile stiffness. An extensive set of nonlinear gyrokinetic simulations are performed based on the experimental discharges, investigating the physical mechanism behind the observations. The impact on the ion heat flux of various parameters that differ within the data-set are explored. These parameters include the safety factor, magnetic shear, toroidal flow shear, effect of rotation on the magnetohydrodynamic equilibrium, R/L-n, beta(e), Z(eff), T-e/T-i, and the fast-particle content. While previously hypothesized to be an important factor in the stiffness reduction, the combined effect of toroidal flow shear and low magnetic shear is not predicted by the simulations to lead to a significant reduction in ion heat flux, due both to an insufficient magnitude of flow shear and significant parallel velocity gradient destabilization. It is however found that nonlinear electromagnetic effects due to both thermal and fast-particle pressure gradients, even at low beta(e), can significantly reduce the ion heat flux, and is a key factor in explaining the experimental observations. A total of four discharges are examined, at both inner and outer radii. For all cases studied, the simulated and experimental ion heat flux values agree within reasonable variations of input parameters around the experimental uncertainties.

VL - 54 SN - 0029-5515; 1741-4326 IS - 2 U1 -FP

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U5 - ba87938e30199199a6a17bf4846326c5 ER - TY - JOUR T1 - L to H mode transition: on the role of Z(eff) JF - Nuclear Fusion Y1 - 2014 A1 - Bourdelle, C. A1 - Maggi, C. F. A1 - Chone, L. A1 - Beyer, P. A1 - Citrin, J. A1 - Fedorczak, N. A1 - Garbet, X. A1 - Loarte, A. A1 - Militello, F. A1 - Romanelli, M. A1 - Sarazin, Y. AB - In this paper, the nature of the primary instability present in the pedestal forming region prior to the transition into H mode is analysed using a gyrokinetic code on JET-ILWprofiles. The linear analysis shows that the primary instability is of resistive nature, and can therefore be stabilized by increased temperature, hence power. The unstable modes are identified as being resistive ballooning modes. Their growth rates decrease for temperatures increasing towards the experimentally measured temperature at the L-H transition. The growth rates are larger for lower effective charge Z(eff). This dependence is shown to be in qualitative agreement with recent and past experimental observations of reduced Z(eff) associated with lower L-H power thresholds. VL - 54 SN - 0029-5515; 1741-4326 IS - 2 U1 - FP U2 - CPP-HT U5 - 243547f2c07e1c57e76e9b4147717183 ER - TY - JOUR T1 - Modelling of JET hybrid scenarios with GLF23 transport model: E × B shear stabilization of anomalous transport JF - Nuclear Fusion Y1 - 2014 A1 - Voitsekhovitch, I. A1 - da Silva Aresta Belo, P. A1 - Citrin, J. A1 - Fable, E. A1 - Ferreira, J. A1 - J. Garcia A1 - Garzotti, L. A1 - Hobirk, J. A1 - Hogeweij, G. M. D. A1 - Joffrin, E. A1 - Kochl, F. A1 - X. Litaudon A1 - Moradi, S. A1 - Nabais, F. A1 - JET-EFDA Contributors A1 - EU-ITM ITER Scenario Modelling group KW - E x B shear stabilization KW - hybrid scenario KW - transport modelling AB -The E × B shear stabilization of anomalous transport in JET hybrid discharges is studied via self-consistent predictive modelling of electron and ion temperature, ion density and toroidal rotation velocity performed with the GLF23 model. The E × B shear stabilization factor (parameter α E in the GLF23 model) is adjusted to predict accurately the four simulated quantities under different experimental conditions, and the uncertainty in α E determined by 15% deviation between simulated and measured quantities is estimated. A correlation of α E with toroidal rotation velocity and E × B shearing rate is found in the low density plasmas, suggesting that the turbulence quench rule may be more complicated than assumed in the GLF23 model with constant α E . For the selected discharges the best predictive accuracy is obtained by using weak/no E × B shear stabilization (i.e. α E ≈ 0) at low toroidal angular frequency (Ω < 60 krad s −1 ), even in the scenarios with the current overshoot, and α E = 0.9 at high frequency (Ω > 100 krad s −1 ). Interestingly, a weak E × B shear stabilization of anomalous transport is found in the medium density strongly rotating discharge. An importance of linear β e stabilization in this discharge is estimated and compared to the low density discharge with equally high β e . The toroidal rotation velocity is well predicted here by assuming that the momentum diffusion coefficient is a fraction of thermal ion diffusivity. Taking into account the α E and Prandtl number with their uncertainties determined in the modelling of JET hybrid discharges, the performance of ITER hybrid scenario with optimized heat mix (33 MW of NBI and 20 MW of ECCD) is estimated showing the importance of toroidal rotation for achieving Q > 5.

VL - 54 UR - http://www.iop.org/Jet/article?EFDP13041&EFDP13048 IS - 9 U1 -FP

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U5 - da2cf17455ede8d649edf343a88bdd87 ER - TY - JOUR T1 - Quasi-coherent modes and electron-driven turbulence JF - Nuclear Fusion Y1 - 2014 A1 - Arnichand, H. A1 - Sabot, R. A1 - Hacquin, S. A1 - Kramer-Flecken, A. A1 - Garbet, X. A1 - Citrin, J. A1 - Bourdelle, C. A1 - Hornung, G. A1 - Bernardo, J. A1 - Bottereau, C. A1 - Clairet, F. A1 - G.L. Falchetto A1 - Giacalone, J. AB -This letter reports on quasi-coherent (QC) modes observed in fluctuation spectra from Tore Supra and TEXTOR reflectometers. QC modes have characteristics in between coherent and broad-band fluctuations as they oscillate around a given frequency but have a wide spectrum. They are ballooned at the LFS midplane and appear usually on a frequency ranging from 30 to 120 kHz. In ohmic plasmas from both tokamaks, QC modes are detected only in linear ohmic confinement (LOC) regime and disappear in saturated ohmic confinement (SOC) regime. Linear simulations from Tore Supra predict that the LOC and SOC regimes are dominated by electron and ion modes respectively. Measurements of the perpendicular velocity of density fluctuations have been made from the top of TEXTOR by poloidal correlation reflectometry. They suggest that QC modes have a phase velocity ∼400 m s −1 higher in the electron diamagnetic direction than lower frequency fluctuations. Additionally, the onset of QC modes during electron cyclotron resonance heating has been observed in a Tore Supra region where turbulence is suspected to be driven by electron modes. These experimental results and instability calculations show a correlation between onsets of QC modes and predictions of trapped electron modes.

VL - 54 IS - 12 U1 -FP

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U5 - e609e9cd25503eb84da96d315a3025c4 ER - TY - JOUR T1 - Nonlinear Stabilization of Tokamak Microturbulence by Fast Ions JF - Physical Review Letters Y1 - 2013 A1 - Citrin, J. A1 - Jenko, F. A1 - Mantica, P. A1 - Told, D. A1 - Bourdelle, C. A1 - J. Garcia A1 - Haverkort, J. W. A1 - Hogeweij, G. M. D. A1 - Johnson, T. A1 - M. J. Pueschel AB -Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al. [ Phys. Rev. Lett. 107 135004 (2011)], this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.

PB - American Physical Society VL - 111 U1 -FP

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U5 - b79552ae74a4f49813ab40d1fa18d6b3 ER - TY - JOUR T1 - A fast, magnetics-free flux surface estimation and q-profile reconstruction algorithm for feedback control of plasma profiles JF - Plasma Physics and Controlled Fusion Y1 - 2013 A1 - Hommen, G. A1 - de M. Baar A1 - Citrin, J. A1 - de Blank, H. J. A1 - Voorhoeve, R. J. A1 - de Bock, M. F. M. A1 - Steinbuch, M. AB -The flux surfaces' layout and the magnetic winding number q are important quantities for the performance and stability of tokamak plasmas. Normally, these quantities are iteratively derived by solving the plasma equilibrium for the poloidal and toroidal flux. In this work, a fast, non-iterative and magnetics-free numerical method is proposed to estimate the shape of the flux surfaces by an inward propagation of the plasma boundary shape, as can be determined for example by optical boundary reconstruction described in Hommen (2010 Rev. Sci. Instrum. 81 113504), toward the magnetic axis, as can be determined independently with the motional Stark effect (MSE) diagnostic. Flux surfaces are estimated for various plasma regimes in the ITER, JET and MAST tokamaks and are compared with results of CRONOS reconstructions and simulations, showing agreement to within 1% of the minor radius for almost all treated plasmas. The availability of the flux surface shapes combined with the pitch angles measured using MSE allow the reconstruction of the plasma q-profile, by evaluating the contour-integral over the flux surfaces of the magnetic field pitch angle. This method provides a direct and exact measure of the q-profile for arbitrary flux surface shapes, which does not rely on magnetic measurements. Results based on estimated flux surface shapes show agreement with CRONOS q-profiles of better than 10%. The impact of the shape of the flux surfaces on the q-profile, particularly the profiles of elongation and Shafranov shift, and offsets in plasma boundary and the magnetic axis are assessed. OFIT+ was conceived for real-time plasma profile control experiments and advanced tokamak operation, and provides quickly and reliably the mapping of actuators and sensors to the minor radius as well as the plasma q-profile, independent of magnetic measurements.

VL - 55 SN - 0741-3335 UR - http://www.euro-fusionscipub.org/wp-content/uploads/2014/11/EFDP12014.pdf IS - 2 U1 -FP

U2 -TP

U5 - 0b2acdc2ac4dbfe79e66d33160ef64d5 ER - TY - JOUR T1 - The dependence of ion heat transport on the ion to electron temperature ratio in JET non-rotating plasmas JF - Plasma Physics and Controlled Fusion Y1 - 2013 A1 - Migliano, P. A1 - Mantica, P. A1 - Baiocchi, B. A1 - Casati, A. A1 - Citrin, J. A1 - Giroud, C. A1 - Hawkes, N. A1 - Lerche, E. A1 - Tsalas, M. A1 - Van Eester, D. A1 - JET-EFDA Contributors AB -Detailed experimental studies of ion heat transport are carried out in JET to explore the T e / T i dependence of ion heat transport in ITER relevant range of parameters ( T e / T i ⩾ 1) using low rotation plasmas with dominant ion cyclotron resonance heating to avoid the coupling of the effects of T e / T i and rotation which affected previous experiments. This experimental setup has led to an accurate determination of the ion temperature gradient (ITG) threshold at varying T e / T i , offering unique opportunities for validation of the well-established theory of ITG driven modes. A rather mild decrease in threshold with increasing T e / T i in the interval of ITER interest was found. The new experimental result has found good agreement with theoretical predictions based on quasi-linear fluid and linear gyrokinetic models.

VL - 55 UR - http://www.euro-fusionscipub.org/wp-content/uploads/2014/11/EFDP12022.pdf U1 -FP

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U5 - d7125604ed111f3a6236f5a56210eb51 ER - TY - JOUR T1 - Global and local gyrokinetic simulations of high-performance discharges in view of ITER JF - Nuclear Fusion Y1 - 2013 A1 - Jenko, F. A1 - Told, D. A1 - T. Görler A1 - Citrin, J. A1 - Bañón Navarro, A. A1 - Bourdelle, C. A1 - Brunner, S. A1 - Conway, G. A1 - T. Dannert A1 - Doerk, H. A1 - Hatch, D. R. A1 - Haverkort, J. W. A1 - Hobirk, J. A1 - Hogeweij, G. M. D. A1 - Mantica, P. A1 - M. J. Pueschel A1 - Sauter, O. A1 - Villard, L. A1 - Wolfrum, E. A1 - ASDEX Upgrade Team AB -One of the key challenges for plasma theory and simulation in view of ITER is to enhance the understanding and predictive capability concerning high-performance discharges. This involves, in particular, questions about high- β operation, ion temperature profile stiffness, and the physics of transport barriers. The goal of this contribution is to shed light on these issues by means of physically comprehensive ab initio simulations with the global gyrokinetic code GENE, applied to discharges in TCV, ASDEX Upgrade, and JET—with direct relevance to ITER.

VL - 53 U1 -FP

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U5 - b0a2b4afe7068d0bbd3b37e5f2c39afe ER - TY - JOUR T1 - Modelling of hybrid scenario: from present-day experiments towards ITER JF - Nuclear Fusion Y1 - 2013 A1 - X. Litaudon A1 - Voitsekhovitch, I. A1 - Artaud, J. F. A1 - Belo, P. A1 - Bizarro, J. P. S. A1 - Casper, T. A1 - Citrin, J. A1 - Fable, E. A1 - Ferreira, J. A1 - J. Garcia A1 - Garzotti, L. A1 - Giruzzi, G. A1 - Hobirk, J. A1 - Hogeweij, G. M. D. A1 - Imbeaux, F. A1 - Joffrin, E. A1 - Koechl, F. A1 - Liu, F. A1 - Lonnroth, J. A1 - Moreau, D. A1 - Parail, V. A1 - Schneider, M. A1 - Snyder, P. B. A1 - ASDEX Upgrade Team A1 - JET-EFDA Contributors A1 - EU-ITM ITER Scenario Modelling group AB -The ‘hybrid’ scenario is an attractive operating scenario for ITER since it combines long plasma duration with the reliability of the reference H-mode regime. We review the recent European modelling effort carried out within the Integrated Scenario Modelling group which aims at (i) understanding the underlying physics of the hybrid regime in ASDEX-Upgrade and JET and (ii) extrapolating them towards ITER. JET and ASDEX-Upgrade hybrid scenarios performed under different experimental conditions have been simulated in an interpretative and predictive way in order to address the current profile dynamics and its link with core confinement, the relative importance of magnetic shear, s , and E × B flow shear on the core turbulence, pedestal stability and H–L transition. The correlation of the improved confinement with an increased s / q at outer radii observed in JET and ASDEX-Upgrade discharges is consistent with the predictions based on the GLF23 model applied in the simulations of the ion and electron kinetic profiles. Projections to ITER hybrid scenarios have been carried out focusing on optimization of the heating/current drive schemes to reach and ultimately control the desired plasma equilibrium using ITER actuators. Firstly, access condition to the hybrid-like q -profiles during the current ramp–up phase has been investigated. Secondly, from the interpreted role of the s / q ratio, ITER hybrid scenario flat-top performance has been optimized through tailoring the q -profile shape and pedestal conditions. EPED predictions of pedestal pressure and width have been used as constraints in the interpretative modelling while the core heat transport is predicted by GLF23. Finally, model-based approach for real-time control of advanced tokamak scenarios has been applied to ITER hybrid regime for simultaneous magnetic and kinetic profile control.

VL - 53 U1 -FP

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U5 - 1e865efa4611fccf9bfb441bffcf216a ER - TY - JOUR T1 - Optimizing the current ramp-up phase for the hybrid ITER scenario JF - Nuclear Fusion Y1 - 2013 A1 - Hogeweij, G. M. D. A1 - Artaud, J. F. A1 - Casper, T. A. A1 - Citrin, J. A1 - Imbeaux, F. A1 - Kochl, F. A1 - X. Litaudon A1 - Voitsekhovitch, I. A1 - ITM-TF ITER Scenario Modelling Group AB -The current ramp-up phase for the ITER hybrid scenario is analysed with the CRONOS integrated modelling suite. The simulations presented in this paper show that the heating systems available at ITER allow, within the operational limits, the attainment of a hybrid q profile at the end of the current ramp-up. A reference ramp-up scenario is reached by a combination of NBI, ECCD (UPL) and LHCD. A heating scheme with only NBI and ECCD can also reach the target q profile; however, LHCD can play a crucial role in reducing the flux consumption during the ramp-up phase. The optimum heating scheme depends on the chosen transport model, and on assumptions of parameters like n e peaking, edge T e,i and Z eff . The sensitivity of the current diffusion on parameters that are not easily controlled, shows that development of real-time control is important to reach the target q profile. A first step in that direction has been indicated in this paper. Minimizing resistive flux consumption and optimizing the q profile turn out to be conflicting requirements. A trade-off between these two requirements has to be made. In this paper it is shown that fast current ramp with L-mode current overshoot is at the one extreme, i.e. the optimum q profile at the cost of increased resistive flux consumption, whereas early H-mode transition is at the other extreme.

VL - 53 U1 -FP

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U5 - 88a9a6c3198d406bdfbbe7a0eea1a68d ER - TY - JOUR T1 - Predictive analysis of q-profile influence on transport in JET and ASDEX Upgrade hybrid scenarios JF - Plasma Physics and Controlled Fusion Y1 - 2012 A1 - Citrin, J. A1 - Hobirk, J. A1 - Schneider, M. A1 - Artaud, J. F. A1 - Bourdelle, C. A1 - Crombe, K. A1 - Hogeweij, G. M. D. A1 - Imbeaux, F. A1 - Joffrin, E. A1 - Koechl, F. A1 - Stober, J. KW - BARRIERS KW - CONFINEMENT KW - DIII-D TOKAMAK KW - HIGH-PERFORMANCE DISCHARGES KW - IMPROVED H-MODE KW - ITER KW - OPERATION KW - SIMULATIONS KW - STABILITY KW - TORE-SUPRA AB -Hybrid scenarios in present machines are often characterized by improved confinement compared with the IPB98(y, 2) empirical scaling law expectations. This work concentrates on isolating the impact of increased s/q at outer radii (where s is the magnetic shear) on core confinement in low-triangularity JET and ASDEX Upgrade (AUG) experiments. This is carried out by predictive heat and particle transport modelling using the integrated modelling code CRONOS coupled to the GLF23 turbulent transport model. For both machines, discharge pairs were analysed displaying similar pedestal confinement yet significant differences in core confinement. From these comparisons, it is found that s/q shaping at outer radii may be responsible for up to similar to 50% of the relative core confinement improvement observed in these specific discharges. This relative improvement is independent of the degree of rotational shear turbulence suppression assumed in the GLF23 model. However, employing the full GLF23 rotational shear model leads to an overprediction of the ion temperatures in all discharges analysed. Additional mechanisms for core confinement improvement are discussed and estimated. Further linear threshold analysis with QuaLiKiz is carried out on both pairs of discharges. This work aims to validate recent predictions of the ITER hybrid scenario also employing CRONOS/GLF23, where a high level of confinement and resultant fusion power sensitivity to the s/q profile was found.

VL - 54 SN - 0741-3335 N1 - ISI Document Delivery No.: 947PATimes Cited: 0Cited Reference Count: 56 U1 -FP

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U5 - bda1f42e30092cecb77d13ab81a2b6af ER - TY - JOUR T1 - Quasilinear transport modelling at low magnetic shear JF - Physics of Plasmas Y1 - 2012 A1 - Citrin, J. A1 - Bourdelle, C. A1 - Cottier, P. A1 - Escande, D. F. A1 - Gurcan, O. D. A1 - Hatch, D. R. A1 - Hogeweij, G. M. D. A1 - Jenko, F. A1 - M. J. Pueschel KW - GRADIENT-DRIVEN MODE KW - PARTICLE KW - STABILITY KW - TOKAMAKS KW - TORE-SUPRA KW - TURBULENCE AB -Accurate and computationally inexpensive transport models are vital for routine and robust predictions of tokamak turbulent transport. To this end, the QuaLiKiz [Bourdelle et al., Phys. Plasmas 14, 112501 (2007)] quasilinear gyrokinetic transport model has been recently developed. QuaLiKiz flux predictions have been validated by non-linear simulations over a wide range in parameter space. However, a discrepancy is found at low magnetic shear, where the quasilinear fluxes are significantly larger than the non-linear predictions. This discrepancy is found to stem from two distinct sources: the turbulence correlation length in the mixing length rule and an increase in the ratio between the quasilinear and non-linear transport weights, correlated with increased non-linear frequency broadening. Significantly closer agreement between the quasilinear and non-linear predictions is achieved through the development of an improved mixing length rule, whose assumptions are validated by non-linear simulations. (C) 2012 American Institute of Physics.

VL - 19 SN - 1070-664X N1 - ISI Document Delivery No.: 966JYTimes Cited: 0Cited Reference Count: 38 U1 -FP

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U5 - 79b0621193d64d70ed8b09d2bf00f7ca ER - TY - JOUR T1 - Measurements of the edge current evolution and comparison with neoclassical calculations during MAST H-modes using motional Stark effect JF - Plasma Physics and Controlled Fusion Y1 - 2012 A1 - de Bock, M. F. M. A1 - Citrin, J. A1 - Saarelma, S. A1 - Temple, D. A1 - Conway, N. J. A1 - Kirk, A. A1 - Meyer, H. A1 - Michael, C. A. KW - ARBITRARY COLLISIONALITY KW - BOOTSTRAP CURRENT KW - DIII-D KW - PEDESTAL KW - TOKAMAKS AB -Edge localized modes (ELMs), that are present in most tokamak H-(high confinement) modes, can cause significant damage to plasma facing components in fusion reactors. Controlling ELMs is considered necessary and hence it is vital to understand the underlying physics. The stability of ELMs is typically expressed in terms of the pressure gradient del p in the edge and the edge current density j(phi) Both del p and j(phi) are usually derived from profiles fitted to the measured edge density and temperature profiles, where for the calculation of j(phi) neoclassical theory is used. This paper presents direct measurements of the magnetic pitch angle gamma(m) evolution in the edge and the derived j(phi). These provide a method to validate the j(phi) as derived with neoclassical theory and they open up the possibility to find a complete, self-consistent set of edge profiles, that fit density, temperature and gamma(m) measurements, hence allowing for a more accurate stability analysis.

VL - 54 SN - 0741-3335 U1 -FP

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U5 - 87dc9949c54cfd4cccb26830a22a998a ER - TY - THES T1 - Turbulent transport in tokamak advanced scenarios Y1 - 2012 A1 - Citrin, J. PB - Technische Universiteit Eindhoven CY - Eindhoven, the Netherlands VL - PhD SN - 978-94-6191-533-7 UR - http://repository.tue.nl/747615 U1 - FP U2 - CPP-HT U5 - afc8f6f4939cb0df83b63815a926304c ER - TY - JOUR T1 - Optimizing the Current Ramp-Up Phase for Hybrid ITER Scenario JF - Plasma and Fusion Research Y1 - 2012 A1 - Hogeweij, G. M. D. A1 - Artaud, J. A1 - Casper, T. A1 - Citrin, J. A1 - Imbeaux, F. A1 - Kochl, F. A1 - X. Litaudon A1 - Voitsekhovitch, I. AB - This paper reports on a systematic effort to optimize the current ramp-up phase for the ITER hybrid scenario, and to assess the sensitivity of the results to the assumptions made. VL - 7 U1 - FP U2 - CPP-HT U4 - non, only conference fee (oral given) U5 - 430d712fff8a33fa43a15b1b9eef2d27 ER - TY - JOUR T1 - Ion heat transport studies in JET JF - Plasma Physics and Controlled Fusion Y1 - 2011 A1 - Mantica, P. A1 - Angioni, C. A1 - Baiocchi, B. A1 - Baruzzo, M. A1 - Beurskens, M. N. A. A1 - Bizarro, J. P. S. A1 - Budny, R. V. A1 - Buratti, P. A1 - Casati, A. A1 - Challis, C. A1 - Citrin, J. A1 - Colyer, G. A1 - Crisanti, F. A1 - Figueiredo, A. C. A. A1 - Frassinetti, L. A1 - Giroud, C. A1 - Hawkes, N. A1 - Hobirk, J. A1 - Joffrin, E. A1 - Johnson, T. A1 - Lerche, E. A1 - Migliano, P. A1 - Naulin, V. A1 - Peeters, A.G. A1 - Rewoldt, G. A1 - Ryter, F. A1 - Salmi, A. A1 - Sartori, R. A1 - Sozzi, C. A1 - Staebler, G. A1 - Strintzi, D. A1 - Tala, T. A1 - Tsalas, M. A1 - Van Eester, D. A1 - Versloot, T. A1 - de Vries, P. C. A1 - Weiland, J. KW - ASDEX UPGRADE KW - CONFINEMENT KW - H-MODE PLASMAS KW - INSTABILITIES KW - RATIO KW - SHEAR KW - T-E/T-I KW - THERMAL TRANSPORT KW - TOKAMAKS KW - TURBULENCE SIMULATIONS AB -Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of active charge exchange spectroscopy and the availability of multi-frequency ion cyclotron resonance heating with (3)He minority. The determination of ion temperature gradient (ITG) threshold and ion stiffness offers unique opportunities for validation of the well-established theory of ITG driven modes. Ion stiffness is observed to decrease strongly in the presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known omega(ExB) threshold up-shift and plays a major role in enhancing core confinement in hybrid regimes and ion internal transport barriers. The effects of T(e)/T(i) and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simulations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour.

VL - 53 SN - 0741-3335 IS - 12 N1 - ISI Document Delivery No.: 870BLTimes Cited: 0Cited Reference Count: 58Part 1-2 U1 -FP

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U5 - 3e267c444cd4d79c851bd379feaeb938 ER - TY - JOUR T1 - Current ramps in tokamaks: from present experiments to ITER scenarios JF - Nuclear Fusion Y1 - 2011 A1 - Imbeaux, F. A1 - Citrin, J. A1 - Hobirk, J. A1 - Hogeweij, G. M. D. A1 - Kochl, F. A1 - Leonov, V. M. A1 - Miyamoto, S. A1 - Nakamura, Y. A1 - Parail, V. A1 - Pereverzev, G. A1 - Polevoi, A. A1 - Voitsekhovitch, I. A1 - Basiuk, V. A1 - Budny, R. A1 - Casper, T. A1 - Fereira, J. A1 - Fukuyama, A. A1 - J. Garcia A1 - Gribov, Y. V. A1 - Hayashi, N. A1 - Honda, M. A1 - Hutchinson, I. H. A1 - Jackson, G. A1 - Kavin, A. A. A1 - Kessel, C. E. A1 - Khayrutdinov, R. R. A1 - Labate, C. A1 - X. Litaudon A1 - Lomas, P. J. A1 - Lonnroth, J. A1 - Luce, T. A1 - Lukash, V. E. A1 - Mattei, M. A1 - Mikkelsen, D. A1 - Nunes, I. A1 - Peysson, Y. A1 - Politzer, P. A1 - Schneider, M. A1 - Sips, G. A1 - Tardini, G. A1 - Wolfe, S. M. A1 - Zhogolev, V. E. KW - DATABASE KW - DISCHARGES KW - HYBRID KW - MODEL KW - SIMULATION KW - TRANSPORT AB -In order to prepare adequate current ramp-up and ramp-down scenarios for ITER, present experiments from various tokamaks have been analysed by means of integrated modelling in view of determining relevant heat transport models for these operation phases. A set of empirical heat transport models for L-mode (namely, the Bohm-gyroBohm model and scaling based models with a specific fixed radial shape and energy confinement time factors of H(96-L) = 0.6 or H(IPB98) = 0.4) has been validated on a multi-machine experimental dataset for predicting the l(i) dynamics within +/- 0.15 accuracy during current ramp-up and ramp-down phases. Simulations using the Coppi-Tang or GLF23 models (applied up to the LCFS) overestimate or underestimate the internal inductance beyond this accuracy (more than +/- 0.2 discrepancy in some cases). The most accurate heat transport models are then applied to projections to ITER current ramp-up, focusing on the baseline inductive scenario (main heating plateau current of I(p) = 15 MA). These projections include a sensitivity study to various assumptions of the simulation. While the heat transport model is at the heart of such simulations (because of the intrinsic dependence of the plasma resistivity on electron temperature, among other parameters), more comprehensive simulations are required to test all operational aspects of the current ramp-up and ramp-down phases of ITER scenarios. Recent examples of such simulations, involving coupled core transport codes, free-boundary equilibrium solvers and a poloidal field (PF) systems controller are also described, focusing on ITER current ramp-down.

VL - 51 SN - 0029-5515 IS - 8 N1 - ISI Document Delivery No.: 818DDTimes Cited: 1Cited Reference Count: 18 U1 -FP

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U5 - 0d6030560b4587c9666dfb14d9e62d73 ER - TY - JOUR T1 - Impact of heating and current drive mix on the ITER hybrid scenario JF - Nuclear Fusion Y1 - 2010 A1 - Citrin, J. A1 - Artaud, J. F. A1 - J. Garcia A1 - Hogeweij, G. M. D. A1 - Imbeaux, F. KW - ASDEX UPGRADE KW - CONFINEMENT KW - CURRENT-DENSITY PROFILE KW - DIII-D KW - DISCHARGES KW - IMPROVED H-MODE KW - PHYSICS BASIS KW - TEMPERATURE KW - TOKAMAKS KW - TRANSPORT AB - Hybrid scenario performance in ITER is studied with the CRONOS integrated modelling suite, using the GLF23 anomalous transport model for heat transport prediction. GLF23 predicted core confinement is optimized through tailoring the q-profile shape by a careful choice of current drive actuators, affecting the transport due to the predicted dependence of the turbulence level on the absolute q-profile values and magnetic shear. A range of various heating and current drive choices are examined, as are different assumptions on the pedestal height. The optimum q-profile shape is predicted to be one that maximizes the ratio of s/q throughout the bulk of the plasma volume. Optimizing the confinement allows a minimization of the plasma density required in order to achieve a defined target fusion power of 350 MW. A lower density then allows a lower total current (I-p) at the same Greenwald fraction (f(G)), thus aiding in maintaining q > 1 as desired in a hybrid scenario, and in minimizing the flux consumption. The best performance is achieved with a combination of NBI and ECCD (e.g. 33/37 MW NBI/ECCD for a scenario with a pedestal height of 4 keV). The q-profile shape and plasma confinement properties are shown to be highly sensitive to the positioning of the ECCD deposition. Comparisons with the lower performing cases where some or all of the ECCD power is replaced with LHCD or ICRH are shown (e. g. 33/20/17 MW NBI/ECCD/LHCD or NBI/ECCD/ICRH). The inclusion of LHCD reduces confinement due to deleterious shaping of the q-profile, and the inclusion of ICRH, particularly in a stiff model, does not lead to significantly increased fusion power and furthermore does not contribute to the non-inductive current fraction. For the optimum NBI/ECCD current drive mix, the predictions show that a satisfactory ITER hybrid scenario (P-fus similar to 350 MW, Q >= 5, q(min) close to 1) may be achieved with T-ped >= 4 keV. In addition, predicted performance sensitivity analysis was carried out for several assumed parameters, such as Z(eff) and density peaking. VL - 50 SN - 0029-5515 UR -