  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.
