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

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 [Formula: see text] although [Formula: see text] is difficult to diagnose in actual experiments. Instead direct experimental measurements can readily track the perpendicular electron temperature [Formula: see text] via electron cyclotron emission. The physics underlying the observed fast drop in [Formula: see text] 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 [Formula: see text] cooling that closely tracks the mostly collisionless crash of [Formula: see text] These findings motivate both experimental validation and reexamination of a broad class of plasma cooling problems in laboratory, space, and astrophysical settings.