Modelling of vertical displacement events in tokamaks: status and challenges ahead

Abstract

In this paper, we revisit the physics of mitigated and unmitigated Vertical Displacement Events (VDEs) and present a summary of present modelling efforts and code capabilities. While 3D MHD simulations of VDEs for AUG and JET are now approaching realistic plasma parameters (e.g. resistivity) and getting closer to experimental observations (e.g. CQ times and vessel forces), the time scales of ITER VDEs (∽500 ms) are still out of computational reach. Therefore, re-scaling of plasma parameters, model simplifications or demanding matrix pre-conditioning techniques are required. We describe such techniques and compare AUG, JET and ITER unmitigated VDE simulations in terms of the dynamics of the CQ and total vacuum vessel forces. In JET simulations, the safety factor ($q_{95}$) falls to unity during the current quench, which destabilizes a 1/1 kink mode. On the other hand, in AUG and ITER simulations, MHD activity is more effective preventing the drop of $q_{95}$, leading to comparatively smaller forces in these devices. 
We will also discuss the applicability of axisymmetric plasma simulations and their success on simulating mitigated disruptions. 
Finally we review the remaining modelling challenges and future plans for unmitigated disruptions. For example, MHD codes need to improve their Scrape-off-Layer (SOL) modelling, such as including the limitation of the halo current to the ion saturation current. Although crucial, this remains numerically challenging even for axisymmetric simulations.