A detailed cross-device investigation on the role of filamentary dynamics in high-density regimes has been performed within the EUROfusion framework, comparing the ASDEX Upgrade (AUG) and TCV tokamaks. Both devices run density ramp experiments at different levels of plasma current, keeping the toroidal field or q95 constant in order to disentangle the role of the parallel connection length and the current. During the scan at a constant toroidal field, in both devices the scrape-off layer (SOL) profiles tend to develop a clear SOL density shoulder at a lower edge density whenever the current is reduced. Different current behaviour is substantially reconciled in terms of the edge density normalized to the Greenwald fraction. During the scan at constant q95 AUG exhibits similar behaviour, whereas in TCV no upstream profile modification signature has been observed at lower current levels. The latter behaviour has been ascribed to the lack of target density rollover. The relation between the upstream density profile modification and detachment condition has been investigated. For both devices the relation between blob size and the SOL density e-folding length is found independent of the plasma current, with the observation of a clear increase in blob size and the edge density normalized to a Greenwald fraction. ASDEX Upgrade has also explored filamentary behaviour in the H-mode. The experiments in AUG have focused on the role of neutrals, performing discharges with and without cryogenic pumps, highlighting how high neutral pressure, not only in the divertor but also at the midplane, is needed in order to develop an H-mode SOL profile shoulder in AUG.

PB - IOP Publishing VL - 60 UR - http://eprints.whiterose.ac.uk/152047/1/Vianello_2019_NF_manuscript.pdf IS - 1 U1 -FP

U2 -PEPD

U5 - 2391efbf0432454f26047770a55623d8 ER - TY - JOUR T1 - Development of a real-time algorithm for detection of the divertor detachment radiation front using multi-spectral imaging JF - Nuclear Fusion Y1 - 2020 A1 - Ravensbergen, T. A1 - van Berkel, M. A1 - Silburn, S. A. A1 - Harrison, J. R. A1 - Perek, A. A1 - Verhaegh, K. A1 - Vijvers, W. A. J. A1 - Theiler, C. A1 - Kirk, A. A1 - de Baar, M. R. AB -In this paper we present a novel algorithm to extract the optical plasma boundary and radiation front for detached divertor plasmas. We show that reliable detection of the divertor leg and radiation front is possible using lightweight image processing tools. Using a non-tomographic approach, the detected divertor leg and radiation front can be mapped to the poloidal plane. This approach is fast and accurate enough for real-time control purposes, allowing in particular real-time plasma shape and detachment control, and post-shot detachment physics and dynamics analysis.

PB - IOP Publishing VL - 60 IS - 6 U1 -FP

U2 -ESC

U5 - 9989f938d3d22d9099bf28ba4a7799eb ER - TY - JOUR T1 - Correcting for non-periodic behaviour in perturbative experiments: application to heat pulse propagation and modulated gas-puff experiments JF - Plasma Physics and Controlled Fusion Y1 - 2020 A1 - van Berkel, M. A1 - van Kampen, R. J. R. A1 - Vandersteen, G. A1 - Kobayashi, T. A1 - Ravensbergen, T. A1 - Igami, H. A1 - Lammers, J. T. A1 - Oosterwegel, G. A1 - Galperti, C. A1 - Felici, F. A1 - de Baar, M. R. A1 - LHD Experiment Group A1 - TCV team AB -This paper introduces a recent innovation in dealing with non-periodic behavior often referred to as transients. These transients can be the result from unforced response due to the initial condition and other drifts which are a source of error when performing and interpreting Fourier analysis on measurement data. Fourier analysis is particularly relevant in system identification used to build feedback controllers and the analysis of various pulsed experiments such as heat pulse propagation studies. The basic idea behind the methodology is that transients are continuous complex-valued smooth functions in the Fourier domain which can be estimated from the Fourier data. Then, these smooth functions can be approximately subtracted from the data such that only periodic components are retained. The merit of the approach is shown in two experimental examples, i.e., heat pulse propagation (core transport analysis) and radiation front movement due to gas puffing. The examples show that the quality of the data is significantly improved such that it allows new interpretation of the results even for non-ideal measurements.

VL - 62 IS - 9 U1 -FP

U2 -ESC

U5 - e00c223abaa800570057a8357cb31b33 ER - TY - JOUR T1 - Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond JF - Nuclear Fusion Y1 - 2019 A1 - Coda, S. A1 - Agostini, M. A1 - Albanese, R. A1 - Alberti, S. A1 - Alessi, E. A1 - Allan, S. A1 - Hogeweij, G. M. D. A1 - Perek, A. A1 - Ravensbergen, T. A1 - Vijvers, W. A. J. A1 - Allcock, J. A1 - Ambrosino, R. A1 - Anand, H. A1 - Andrebe, Y. A1 - EUROfusion MST1 Team A1 - et al. AB - The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device's unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power 'starvation' reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added. VL - 59 IS - 11 U1 - FP U2 - PEPD U5 - d50a5905a84255c86ad6d49b1cfa1569 ER - TY - JOUR T1 - MANTIS: A real-time quantitative multispectral imaging system for fusion plasmas JF - Review of Scientific Instruments Y1 - 2019 A1 - Perek, A. A1 - Vijvers, W. A. J. A1 - Andrebe, Y. A1 - Classen, I. G. J. A1 - Duval, B. P. A1 - Galperti, C. A1 - Harrison, J. R. A1 - Linehan, B. A1 - Ravensbergen, T. A1 - Verhaegh, K. A1 - M. R. de Baar A1 - TCV team A1 - EUROfusion MST1 Team AB -This work presents a novel, real-time capable, 10-channel Multispectral Advanced Narrowband Tokamak Imaging System installed on the TCV tokamak, MANTIS. Software and hardware requirements are presented together with the complete system architecture. The image quality of the system is assessed with emphasis on effects resulting from the narrowband interference filters. Some filters are found to create internal reflection images that are correlated with the filters’ reflection coefficient. This was measured for selected filters where significant absorption (up to 65% within ∼70 nm of the filter center) was measured. The majority of this was attributed to the filter’s design, and several filters’ performance is compared. Tailored real-time algorithms exploiting the system’s capabilities are presented together with benchmarks comparing polling and event based synchronization. The real-time performance is demonstrated with a density ramp discharge performed on TCV. The behavior of spectral lines’ emission from different plasma species and their interpretation are qualitatively described.

VL - 90 IS - 12 U1 -FP

U2 -PEPD

U5 - 5355a89839c5513fe91f52dfac500f3d ER - TY - JOUR T1 - Density control in ITER: an iterative learning control and robust control approach JF - Nuclear Fusion Y1 - 2018 A1 - Ravensbergen, T. A1 - de Vries, P. C. A1 - Felici, F. A1 - Blanken, T. C. A1 - Nouailletas, R. A1 - Zabeo, L. AB - Plasma density control for next generation tokamaks, such as ITER, is challenging because of multiple reasons. The response of the usual gas valve actuators in future, larger fusion devices, might be too slow for feedback control. Both pellet fuelling and the use of feedforward-based control may help to solve this problem. Also, tight density limits arise during ramp-up, due to operational limits related to divertor detachment and radiative collapses. As the number of shots available for controller tuning will be limited in ITER, in this paper, iterative learning control (ILC) is proposed to determine optimal feedforward actuator inputs based on tracking errors, obtained in previous shots. This control method can take the actuator and density limits into account and can deal with large actuator delays. However, a purely feedforward-based density control may not be sufficient due to the presence of disturbances and shot-to-shot differences. Therefore, robust control synthesis is used to construct a robustly stabilizing feedback controller. In simulations, it is shown that this combined controller strategy is able to achieve good tracking performance in the presence of shot-to-shot differences, tight constraints, and model mismatches. VL - 58 IS - 1 U1 - FP U2 - PEPD U5 - 778c76d4e8292ffa9366cddbefea0f1c ER - TY - JOUR T1 - A systematic approach to optimize excitations for perturbative transport experiments JF - Physics of Plasmas Y1 - 2018 A1 - van Berkel, M. A1 - De Cock, A. A1 - Ravensbergen, T. A1 - Hogeweij, G. M. D. A1 - Zwart, H. J. A1 - Vandersteen, G. AB -In this paper, techniques for optimal input design are used to optimize the waveforms of perturbative experiments in modern fusion devices. The main focus of this paper is to find the modulation frequency for which the accuracy of the estimated diffusion coefficient is maximal. Mathematically, this problem can be formulated as an optimization problem in which the Fisher information matrix is maximized. First, this optimization problem is solved for a simplified diffusion model, while assuming a slab geometry and a semi-infinite domain. Later, the optimization is repeated under more general conditions such as a cylindrical geometry, finite domain, and simultaneous estimation of multiple transport coefficients. Based on the results of these optimizations, guidelines are offered to select the modulation frequency and to determine the optimality of the corresponding experiment. © 2018 EURATOM

VL - 25 IS - 8 U1 -FP

U2 -IMT

U5 - 83270781877438b0f4a781f307204968 ER -