Collisionless cooling of perpendicular electron temperature in the thermal quench of a magnetized plasma

Abstract

Abstract Thermal quench of a nearly collisionless plasma against an isolated cooling boundary or region is an undesirable off-normal event in magnetic fusion experiments, but an ubiquitous process of cosmological importance in astrophysical plasmas. Parallel transport theory of ambipolar-constrained tail electron loss is known to predict rapid cooling of the parallel electron temperature $$T_{e\parallel },$$ although $$T_{e\parallel }$$ is difficult to diagnose in actual experiments. Instead direct experimental measurements can readily track the perpendicular electron temperature $$T_{e\perp }$$ via electron cyclotron emission. The physics underlying the observed fast drop in $$T_{e\perp }$$ requires a resolution. Here two collisionless mechanisms, dilutional cooling by infalling cold electrons and wave-particle interaction by two families of whistler instabilities, are shown to enable fast $$T_{e\perp }$$ cooling that closely tracks the mostly collisionless crash of $$T_{e\parallel }.$$ These findings motivate both experimental validation and reexamination of a broad class of plasma cooling problems in laboratory, space, and astrophysical settings.