A benchmark exercise for the modeling of vertical displacement events(VDEs) is presented and applied to the 3D nonlinear magneto-hydrodynamic codesM3D-C1, JOREK and NIMROD. The simulations are based on a vertically unstableNSTX equilibrium enclosed by an axisymmetric resistive wall with rectangular crosssection. A linear dependence of the linear VDE growth rates on the resistivity ofthe wall is recovered for sufficiently large wall conductivity and small temperatures inthe open field line region. The benchmark results show good agreement between theVDE growth rates obtained from linear NIMROD and M3D-C1simulations as wellas from the linear phase of axisymmetric nonlinear JOREK, NIMROD and M3D-C1simulations. Axisymmetric nonlinear simulations of a full VDE performed with thethree codes are compared and excellent agreement is found regarding plasma locationand plasma currents as well as eddy and halo currents in the wall.

VL - 27 UR - https://arxiv.org/abs/1908.02387v2 IS - 2 U1 -FP

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U5 - c6dd5947070b72b6a34e337edb53b2f1 ER - TY - JOUR T1 - ELM-induced cold pulse propagation in ASDEX Upgrade JF - Plasma Physics and Controlled Fusion Y1 - 2019 A1 - Trier, E. A1 - Wolfrum, E. A1 - Willensdorfer, M. A1 - Yu, Q. A1 - Hoelzl, M. A1 - Orain, F. A1 - Ryter, F. A1 - Angioni, C. A1 - Bernert, M. A1 - Vanovac, B. A1 - Dunne, M. G. A1 - Denk, S. S. A1 - Fuchs, J. C. A1 - Fischer, R. A1 - Hennequin, P. A1 - Kurzan, B. A1 - Mink, F. A1 - Mlynek, A. A1 - Odstrcil, T. A1 - Schneider, P. A. A1 - Stroth, U. A1 - Tardini, G. A1 - ASDEX Upgrade Team A1 - EUROfusion MST1 Team AB - In ASDEX Upgrade, the propagation of cold pulses induced by type-I edge localized modes (ELMs) is studied using electron cyclotron emission measurements, in a dataset of plasmas with moderate triangularity. It is found that the edge safety factor or the plasma current are the main determining parameters for the inward penetration of the T e perturbations. With increasing plasma current the ELM penetration is more shallow in spite of the stronger ELMs. Estimates of the heat pulse diffusivity show that the corresponding transport is too large to be representative of the inter-ELM phase. Ergodization of the plasma edge during ELMs is a possible explanation for the observed properties of the cold pulse propagation, which is qualitatively consistent with non-linear magneto-hydro-dynamic simulations. VL - 61 IS - 4 U1 - FP U2 - TP U5 - 0279758ced9029d70a2148b4f3b6cd99 ER - TY - JOUR T1 - Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade JF - Plasma Physics and Controlled Fusion Y1 - 2018 A1 - Vanovac, B. A1 - Wolfrum, E. A1 - Denk, S. S. A1 - Mink, F. A1 - Laggner, F. M. A1 - Birkenmeier, G. A1 - Willensdorfer, M. A1 - Viezzer, E. A1 - Hoelzl, M. A1 - Freethy, S. J. A1 - Dunne, M. G. A1 - Lessig, A. A1 - Luhmann, N. C. A1 - ASDEX Upgrade Team A1 - EUROfusion MST1 Team AB -Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (T-e) and its fluctuations (delta T-e). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10-15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (Trad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s(-1). The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s(-1). Velocities are compared with the E x B background flow velocity and possible explanations for the origin of the low frequency mode are discussed.

VL - 60 IS - 4 U1 -FP

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U5 - b48d07d2ecab661001865b07ddb29bc2 ER - TY - JOUR T1 - Insights into type‐I edge localized modes and edge localized mode control from JOREK non‐linear magneto‐hydrodynamic simulations JF - Contributions to Plasma Physics Y1 - 2018 A1 - Hoelzl, M. A1 - Huijsmans, G. T. A. A1 - Orain, F. A1 - Artola, F. J. A1 - Pamela, S. A1 - Becoulet, M. A1 - van Vugt, D. A1 - Liu, F. A1 - Futatani, S. A1 - Vanovac, B. A1 - Lessig, A. A1 - Wolfrum, E. A1 - Mink, F. A1 - Trier, E. A1 - Dunne, M. A1 - Viezzer, E. A1 - Eich, T. A1 - Frassinetti, L. A1 - Gunter, S. A1 - Lackner, K. A1 - Krebs, I. A1 - ASDEX Upgrade Team A1 - EUROfusion MST1 Team KW - ballooning mode KW - ELM control KW - ELMs KW - JOREK KW - MHD KW - mode coupling KW - stochastic field KW - TOKAMAK AB - Edge localized modes (ELMs) are repetitive instabilities driven by the large pressure gradients and current densities in the edge of H‐mode plasmas. Type‐I ELMs lead to a fast collapse of the H‐mode pedestal within several hundred microseconds to a few milliseconds. Localized transient heat fluxes to divertor targets are expected to exceed tolerable limits for ITER, requiring advanced insights into ELM physics and applicable mitigation methods. This paper describes how non‐linear magneto‐hydrodynamic (MHD) simulations can contribute to this effort. The JOREK code is introduced, which allows the study of large‐scale plasma instabilities in tokamak X‐point plasmas covering the main plasma, the scrape‐off layer, and the divertor region with its finite element grid. We review key physics relevant for type‐I ELMs and show to what extent JOREK simulations agree with experiments and help reveal the underlying mechanisms. Simulations and experimental findings are compared in many respects for type‐I ELMs in ASDEX Upgrade. The role of plasma flows and non‐linear mode coupling for the spatial and temporal structure of ELMs is emphasized, and the loss mechanisms are discussed. An overview of recent ELM‐related research using JOREK is given, including ELM crashes, ELM‐free regimes, ELM pacing by pellets and magnetic kicks, and mitigation or suppression by resonant magnetic perturbation coils (RMPs). Simulations of ELMs and ELM control methods agree in many respects with experimental observations from various tokamak experiments. On this basis, predictive simulations become more and more feasible. A brief outlook is given, showing the main priorities for further research in the field of ELM physics and further developments necessary. VL - 58 IS - 6-8 U1 - FP U2 - IMT U5 - 71b9a6aa52e8f79ba2ebd4ec998f6c83 ER - TY - JOUR T1 - Characterization of low-frequency inter-ELM modes of H-mode discharges at ASDEX Upgrade JF - Nuclear Fusion Y1 - 2018 A1 - Vanovac, B. A1 - Wolfrum, E. A1 - Hoelzl, M. A1 - Willensdorfer, M. A1 - Cavedon, M. A1 - Harrer, G. A1 - Mink, F. A1 - Denk, S. S. A1 - Freethy, S. A1 - Dunne, M. G. A1 - Manz, P. A1 - Luhmann Jr., N. C. A1 - ASDEX Upgrade Team AB -The steep edge gradient region of tokamak plasmas in the high confinement regime is known to drive instabilities, which cause transport. Several diagnostics are used to allow for a high degree of characterization of low-frequency modes appearing in between type-I edge localizes modes (ELMs). These modes are dominantly observed in electron cyclotron emission (ECE) and ECE imaging measurements as a modulation of radiation temperature (delta T rad). In the radial magnetic field (B t) measurements, the frequency range of 4 kHz to 12 kHz is observed. The position of the mode is determined to be at the upper part of the steep gradient region, the poloidal mode velocity is changing from 1.5 ± 0.5 km s−1 to 2.5 ± 0.5 km s−1 and the toroidal mode number is 13 to 14. A comparison with the measured E x B velocity leads to the conclusion that the phase velocity of the mode is smaller than 3 km s−1 or zero. The poloidal structure of the modes is found to agree with the poloidal structure size associated with n = 13 as estimated from the equilibrium calculations. The modes are compared between two different heating phases during one discharge, and are found to differ in duration, velocity, frequency and toroidal mode number. The possibility of non-linear interaction between these modes and other, high frequency modes existing in the narrow pedestal, is assessed via bicoherence analysis. The presented analysis gives an unprecedented picture of the mode, its position, its structure and its velocity, calling for comparison with non-linear modelling.

VL - 58 IS - 11 U1 -FP

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U5 - 25d7dadbe7a8589f9e2dbdf0929416d0 ER - TY - JOUR T1 - Fast-ion redistribution and loss due to edge perturbations in the ASDEX Upgrade, DIII-D and KSTAR tokamaks JF - Nuclear Fusion Y1 - 2013 A1 - M. García-Muñoz A1 - Akaslompolo, S. A1 - Asunta, O. A1 - Boom, J. A1 - Chen, X. A1 - Classen, I.G.J. A1 - Dux, R. A1 - Evans, T. E. A1 - Fietz, S. A1 - Fisher, R. K. A1 - Fuchs, C. A1 - Geiger, B. A1 - Hoelzl, M. A1 - Igochine, V. A1 - Jeon, Y. M. A1 - Kim, J. A1 - Kim, J. Y. A1 - Kurzan, B. A1 - Lazanyi, N. A1 - Lunt, T. A1 - McDermott, R. M. A1 - Nocente, M. A1 - Pace, D. C. A1 - Rhodes, T. L. A1 - Rodriguez-Ramos, M. A1 - Shinohara, K. A1 - Suttrop, W. A1 - VanZeeland, M. A. A1 - Viezzer, E. A1 - Willensdorfer, M. A1 - Wolfrum, E. A1 - the ASDEX Upgrade A1 - DIII-D Team A1 - KSTAR Teams AB - The impact of edge localized modes (ELMs) and externally applied resonant and non-resonant magnetic perturbations (MPs) on fast-ion confinement/transport have been investigated in the ASDEX Upgrade (AUG), DIII-D and KSTAR tokamaks. Two phases with respect to the ELM cycle can be clearly distinguished in ELM-induced fast-ion losses. Inter-ELM losses are characterized by a coherent modulation of the plasma density around the separatrix while intra-ELM losses appear as well-defined bursts. In high collisionality plasmas with mitigated ELMs, externally applied MPs have little effect on kinetic profiles, including fast-ions, while a strong impact on kinetic profiles is observed in low-collisionality, low q 95 plasmas with resonant and non-resonant MPs. In low-collisionality H-mode plasmas, the large fast-ion filaments observed during ELMs are replaced by a loss of fast-ions with a broad-band frequency and an amplitude of up to an order of magnitude higher than the neutral beam injection prompt loss signal without MPs. A clear synergy in the overall fast-ion transport is observed between MPs and neoclassical tearing modes. Measured fast-ion losses are typically on banana orbits that explore the entire pedestal/scrape-off layer. The fast-ion response to externally applied MPs presented here may be of general interest for the community to better understand the MP field penetration and overall plasma response. VL - 53 UR - http://stacks.iop.org/0029-5515/53/i=12/a=123008 U1 - FP U2 - PDG U5 - 297ecfd209412ec1f2b5deab45ac5511 ER -