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The electron runaway phenomenon in plasmas depends sensitively on themomentum-space dynamics. However, efficient simulation of the global evolutionof systems involving runaway electrons typically requires a reduced fluiddescription. This is needed for example in the design of essential runawaymitigation methods for tokamaks. In this paper, we present a method to includethe effect of momentum-dependent spatial transport in the runaway avalanchegrowth rate. We quantify the reduction of the growth rate in the presence ofelectron diffusion in stochastic magnetic fields and show that the spatialtransport can raise the effective critical electric field. Using a perturbativeapproach we derive a set of equations that allows treatment of the effect ofspatial transport on runaway dynamics in the presence of radial variation inplasma parameters. This is then used to demonstrate the effect of spatialtransport in current quench simulations for ITER-like plasmas with massivematerial injection. We find that in scenarios with sufficiently slow currentquench, due to moderate impurity and deuterium injection, the presence ofmagnetic perturbations reduces the final runaway current considerably.Perturbations localized at the edge are not effective in suppressing therunaways, unless the runaway generation is off-axis, in which case they maylead to formation of strong current sheets at the interface of the confined andperturbed regions.

Effects of magnetic perturbations and radiation on the runaway avalanche