24 février 2022

Seminar by G. Brochard (University of California, Irvine, USA)

Gyrokinetic simulations of global MHD modes: verification and validation using DIII-D experiments, and application for ITER plasmas

Abstract: The transport of energetic particles (EP) in tokamaks constitutes a fundamental problem for burning plasmas, as it can adversely impact the plasma energy balance and threaten the integrity of the fusion device. First-principles simulations of EP transport are therefore critical to elaborate burning plasmas scenarii that can mitigate these effects. However to be realistic, first-principles simulations need to incorporate a wide range of physical scales, as EP transport can arise from microturbulence, Alfvén eigenmodes, global MHD instabilities, and from the nonlinear interactions between each of these transport channels.

The global gyrokinetic code (GTC) has been previously used for the study of EP transport induced by microturbulence and Alfvén eigenmodes. GTC is now applied for the simulations of macroscopic MHD modes in DIII-D and ITER plasmas. The code capability at simulating current-driven MHD modes is first demonstrated by performing averification and linear validation of the internal kink instability in DIII-D plasmas with gyrokinetic (GTC) and kinetic-MHD codes (GAM-solver, M3D-C1/K, NOVA, XTOR-K). Using realistic magnetic geometry and plasma profiles from a DIII-D discharge, these codes exhibit excellent agreements for the growth rate and mode structure of the n=1 internal kink mode in the ideal MHD limit by suppressing all kinetic effects. The simulated radial mode structures, obtained from linear simulations, are in reasonable agreement with the normalised electron cyclotron emission measurement after adjusting, within the experimental uncertainty, the safety factor q=1 flux-surface location in the equilibrium reconstruction. Furthermore, kinetic effects of thermal ions are found to decrease the kink growth rate in kinetic-MHD simulations, but increase the kink growth rate in gyrokinetic simulations, due to the additional drive of the ion temperature gradient and parallel electric field.

The validated MHD capability of GTC is then applied to study the fishbone instability in ITER plasmas. A prefusion ITER baseline scenario is considered in this analysis as part of the ITPA-EP activities. An associated DIII-D discharge is selected for experimental comparison of the simulations results, the selection being made in terms of similar q profile, normalized beta and plasmas profiles. The selected discharge exhibits clear n=1 fishbone bursts driven by fast ions from neutral beam injection (NBI). Fishbone modes are found to be triggered by energetic particles for both the DIII-D and ITER configurations, with similar mode structures. Preliminary nonlinear GTC results recover mode frequency down-chirping associated with resonant EP transport, that are key signatures of the fishbone instability.

Short bio: Dr. Guillaume Brochard is a post-doctoral fellow at the University of California, Irvine, in the team of Prof. Zhihong Lin. His research interests include energetic particle transport and instabilities in tokamak plasmas. He completed his PhD at CEA Cadarache / CPHT Ecole Polytechnique under the supervision of Dr. Hinrich Lütjens and Dr. Rémi Dumont.

Health conditions permitting, the talk will be preceded by a welcome breakfast at 10:00.

24 février 2022, 10h3012h00
Board room ou by Zoom