Understanding generation and mitigation of runaway electrons in disruptions
is important for the safe operation of future tokamaks. In this paper we
investigate runaway dynamics in reactor-scale spherical tokamaks. We study both
the severity of runaway generation during unmitigated disruptions, as well as
the effect that typical mitigation schemes based on massive material injection
have on runaway production. The study is conducted using the numerical
framework DREAM (Disruption Runaway Electron Analysis Model). We find that, in
many cases, mitigation strategies are necessary to prevent the runaway current
from reaching multi-megaampere levels. Our results indicate that with a
suitably chosen deuterium-neon mixture for mitigation, it is possible to
achieve a tolerable runaway current and ohmic current evolution. With such
parameters, however, the majority of the thermal energy loss happens through
radial transport rather than radiation, which poses a risk of unacceptable
localised heat loads.Comment: 19 pages, 7 figure