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

Stand-off runaway electron beam termination by tungsten particulates for tokamak disruption mitigation