Effects of magnetic perturbations and radiation on the runaway avalanche

Abstract

The electron runaway phenomenon in plasmas depends sensitively on the momentum- space dynamics. However, efficient simulation of the global evolution of systems involving runaway electrons typically requires a reduced fluid description. This is needed, for example, in the design of essential runaway mitigation methods for tokamaks. In this paper, we present a method to include the effect of momentum-dependent spatial transport in the runaway avalanche growth rate. We quantify the reduction of the growth rate in the presence of electron diffusion in stochastic magnetic fields and show that the spatial transport can raise the effective critical electric field. Using a perturbative approach, we derive a set of equations that allows treatment of the effect of spatial transport on runaway dynamics in the presence of radial variation in plasma parameters. This is then used to demonstrate the effect of spatial transport in current quench simulations for ITER-like plasmas with massive material injection. We find that in scenarios with sufficiently slow current quench, owing to moderate impurity and deuterium injection, the presence of magnetic perturbations reduces the final runaway current considerably. Perturbations localised at the edge are not effective in suppressing the runaways, unless the runaway generation is off-axis, in which case they may lead to formation of strong current sheets at the interface of the confined and perturbed regions.

Effect of Two-Stage Shattered Pellet Injection on Tokamak Disruptions

“…The evolution of the magnetic field through the current quench might also be of interest, potentially including deliberately induced perturbations to increase the runaway losses. Even the comparatively smaller magnetic perturbations that may be present during the current quench can have a relatively large impact on the runaway generation and dissipation [33].…”

Modelling of runaway electron dynamics during argon-induced disruptions in ASDEX Upgrade and JET

“…The importance of transport losses is expected to be higher in a smaller machine, which might explain why the prediction of an experimentally excluded re-heating was less common for JET than for AUG. Time dependent radial profiles of the magnetic field fluctuations are difficult to obtain from experiment, although their amplitude during the current quench has often been found to be fairly small [43]. Numerical modelling of the transport due to magnetic fluctuations [44] would therefore require exploration of a fairly large additional parameter space and the effect of this on the results presented remains an open question.…”

Alpha particle driven Alfvénic instabilities in ITER post-disruption plasmas

“…The time evolution of the modes and their saturation amplitude is a critical question to determine their potency for runaway transport. Earlier studies showed that a magnetic perturbation with an amplitude of about δB/B ≈ 0.1% is sufficient to suppress runaway avalanche [71,72], while more recent research [73] decreases this threshold by about a factor of 2.…”