  Stand-off runaway electron termination by injected tungsten particulates
offers a plausible option in the toolbox of disruption mitigation. Tungsten is
an attractive material choice for this application due to large electron
stopping power and high melting point. To assess the feasibility of this
scheme, we simulate runaway collisions with tungsten particulates using the
MCNP program for incident runaway energies ranging from 1 to 10 MeV. We assess
runaway termination from energetics and collisional kinematics perspectives.
Energetically, the simulations show that 99% of runaway beam energy is removed
by tungsten particulates on a timescale of 4-9 $\mu$s. Kinematically, the
simulations show that 99% of runaways are terminated by absorption or
backscattering on a timescale of 3-4 $\mu$s. By either metric, the runaway beam
is effectively terminated before the onset of particulate melting. Furthermore,
the simulations show that secondary radiation emission by tungsten particulates
does not significantly impact the runaway termination efficacy of this scheme.
Secondary radiation is emitted at lower particle energies than the incident
runaways and with a broad angular distribution such that the majority of
secondary electrons emitted will not experience efficient runaway
re-acceleration. Overall, the stand-off runaway termination scheme is a
promising concept for last-ditch runaway mitigation in ITER, SPARC, and other
future burning-plasma tokamaks.
