  In [V.A. Izzo et al 2022 Nucl. Fusion 62 096029], state-of-the-art modeling
of thermal and current quench (CQ) MHD coupled with a self-consistent evolution
of runaway electron (RE) generation and transport showed that a
non-axisymmetric (n = 1) in-vessel coil could passively prevent RE beam
formation during disruptions in SPARC, a compact high-field tokamak projected
to achieve a fusion gain Q > 2 in DT plasmas. However, such suppression
requires finite transport of REs within magnetic islands and re-healed flux
surfaces; conservatively assuming zero transport in these regions leads to an
upper bound of RE current ~1 MA compared to ~8.7 MA of pre-disruption plasma
current. Further investigation finds that core-localized electrons, within r/a
< 0.3 and with kinetic energies 0.2-15 MeV, contribute most to the RE plateau
formation. Yet only a relatively small amount of transport, i.e. a diffusion
coefficient ~18 $\mathrm{m^2/s}$, is needed in the core to fully mitigate these
REs. Properly accounting for (i) the CQ electric field's effect on RE transport
in islands and (ii) the contribution of significant RE currents to disruption
MHD may help achieve this.
