Speakers
Description
The runaway electron (RE) beam benign termination observed in JET pulse 95135 has been modelled in [1] using the nonlinear MHD code JOREK [2]. The study demonstrated the role of magnetic stochasticity in causing RE loss. This demonstration was based on particular assumptions regarding the properties of the background plasma, such as resistivity, that might not be accurate due to the recombination following massive deuterium injection. The transport of REs was modelled through an ad-hoc diffusion due to the high computational demands of fully advective simulation. Moreover, an ideal wall boundary condition was used which tends to have a stabilizing effect on the MHD activity.
In this work, the same RE beam termination event was modelled using RE advection at the speed of light. Different resistivity values were also considered to assess the influence of this uncertain parameter. In particular, the influence of the plasma resistivity on the RE footprint on the wall was studied. It was found that higher resistivity results in MHD modes growing faster and up to larger amplitudes. The intense magnetic stochasticity observed with high resistivities results in a larger wetted area on the wall, contributing to a more benign termination of RE. Similar results were found for ITER in [3].
In addition, the comparison between the RE transport modelled with diffusion and advection showed a slower RE termination event with the advection model with respect to the diffusion due to a slower MHD modes growth rate, consistent with what is shown in [4]. At the same time, a resistive wall boundary condition [5, 6] leads to a faster RE termination due to the absence of the stabilizing effects caused by an ideal wall. As a result, with a background plasma resistivity corresponding to an electron temperature of 10 eV, the RE current loss occurs in 50 µs, which is comparable to the experimentally observed value of 20 µs [1]. With a higher resistivity, the loss time gets even closer to the experimental value.
Finally, in order to better understand the physics of RE beam termination, we also studied the evolution toward a highly MHD-unstable regime. In contrast to the above results, where the simulations were started from an already unstable state, this approach involves ramping up the plasma current from a stable equilibrium. This method provides a more self-consistent view of how benign terminations can arise.
[1] V Bandaru et al 2021 Plasma Phys. Control. Fusion 63 035024
[2] M Hölzl et al 2021 Nucl. Fusion 61 065001
[3] V. Bandaru et al 2024 Nucl. Fusion 64 076053
[4] H. Bergström et al 2025 Plasma Phys. Control. Fusion 67 035004
[5] P. Merkel et al. arXiv:1508.04911 (2015)
[6] M Hölzl et al 2012 J. Phys.: Conf. Ser. 401 012010
