# Influence of massive material injection on avalanche runaway generation during tokamak disruptions

## TL;DR

This paper demonstrates that impurity injection during tokamak disruptions can significantly amplify runaway electron avalanches, affecting disruption mitigation strategies in devices like ITER.

## Contribution

It derives a new expression for avalanche growth rate in partially ionized plasmas and shows impurity injection can increase runaway amplification by tens of orders of magnitude.

## Key findings

- Impurity injection can greatly increase runaway avalanche growth.
- Maximum amplification can reach tens of orders of magnitude.
- Impurity density and species critically influence runaway dynamics.

## Abstract

In high-current tokamak devices such as ITER, a runaway avalanche can cause a large amplification of a seed electron population. We show that disruption mitigation by impurity injection may significantly increase the runaway avalanche growth rate in such devices. This effect originates from the increased number of target electrons available for the avalanche process in weakly ionized plasmas, which is only partially compensated by the increased friction force on fast electrons. We derive an expression for the avalanche growth rate in partially ionized plasmas and investigate the effects of impurity injection on the avalanche multiplication factor and on the final runaway current for ITER-like parameters. For impurity densities relevant for disruption mitigation, the maximum amplification of a runaway seed can be increased by tens of orders of magnitude compared to previous predictions. This motivates careful studies to determine the required densities and impurity species to obtain tolerable current quench parameters, as well as more detailed modeling of the runaway dynamics including transport effects.