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

U2 -IMT

U5 - c6dd5947070b72b6a34e337edb53b2f1 ER - TY - JOUR T1 - A new explanation of the sawtooth phenomena in tokamaks JF - Physics of Plasmas Y1 - 2020 A1 - Jardin, S. C. A1 - Krebs, I. A1 - Ferraro, N. M. AB - The ubiquitous sawtooth phenomena in tokamaks are so named because the central temperature rises slowly and falls rapidly, similar to the blades of a saw. First discovered in 1974, it has so far eluded a theoretical explanation that is widely accepted and consistent with experimental observations. We propose here a new theory for the sawtooth phenomena in auxiliary heated tokamaks, which is motivated by our recent understanding of "magnetic flux pumping." In this theory, the role of the (m, n) = (1, 1) mode is to generate a dynamo voltage, which keeps the central safety factor, q(0), just above 1.0 with low central magnetic shear. When central heating is present, the temperature on axis will increase until at some point, and the configuration abruptly becomes unstable to ideal MHD interchange modes with equal poloidal and toroidal mode numbers, m = n > 1. It is these higher order modes and the localized magnetic stochasticity they produce that cause the sudden crash of the temperature profile, not magnetic reconnection. Long time 3D MHD simulations demonstrate these phenomena, which appear to be consistent with many experimental observations. VL - 27 IS - 3 U1 - FP U2 - IMT U5 - fbd3c1487d138d21c1fdfe6423719556 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 -