Benign termination of mega-ampere (MA) level runaway current has been convincingly demonstrated in recent JET and DIII-D experiments, establishing it as a leading candidate for runaway mitigation on ITER. This comes in the form of a runaway flush by parallel streaming loss along stochastic magnetic field lines formed by global magnetohydrodynamic instabilities, which are found to correlate with a low-$Z$ injection that purges the high-$Z$ impurities from a post-thermal-quench plasma. Here, we show the competing physics that govern the postflush reconstitution of the runaway current in an ITER-like reactor where significantly higher current is expected. The trapped “runaways” are found to dominate the seeding for runaway reconstitution, and the incomplete purge of high-$Z$ impurities helps drain the seed but produces a more efficient avalanche, two of which compete to produce a 2–3 MA step in current drop before runaway reconstitution of the plasma current.

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• Fast particle effects in plasmas
• Magnetohydrodynamics
• Relativistic kinetic theory