Runaway electron dynamics following massive gas injection on the European medium sized tokamaks

The uncontrolled loss of a disruption-generated relativistic runaway electron (RE) beam is intolerable in large tokamaks, and therefore the issue of how to avoid or mitigate the RE beam is of prime importance for ITER. The tokamaks participating in the EUROfusion Medium Sized Tokamak (MST1) campaign are executing a coordinated experimental program to better understand RE generation, control and mitigation. On ASDEX Upgrade (AUG) runaway electrons are routinely generated in a well reproducible manner using in-vessel argon massive gas injection (MGI) into low-density, high temperature, circular discharges [1, 2]. The scaling of the initial runaway current has a nonlinear dependence on predisruption plasma-and MGI parameters, and is analyzed using 1D disruption-runaway simulations [3]. The dissipation of the RE current however scales very well with the amount and atomic number of the gas injected either to trigger the disruption, or afterwards to suppress the beam. The effect of argon and neon MGI on RE current dissipation shows good agreement with state-of-the-art kinetic models [4]. The main goal of RE experiments carried out on TCV [2, 5] - of which both quiescent and post-disruptive are possible - is to utilize TCV's flexible plasma shape and position control to determine the direct and indirect effects of geometry on RE generation and dissipation. While MGI on TCV cannot deliver material into the beam as efficiently as on AUG, we found that the plasma position with respect to the MGI valve plays a major role in gas penetration and subsequent RE dissipation. Further studies on these issues are planned for this summer campaign and results of these scans - along with theoretical comparisons - are planned to be presented in this contribution.