Author: Yanzeng Zhang
Requested Type: Consider for Invited
Submitted: 2022-03-04 19:24:38
Co-authors: Jun Li, Xianzhu Tang
Los Alamos National Laboratory
Bikini Atoll Rd., SM 30
Los Alamos, 87545
ITER's standard approach of mitigating the thermal load of an incoming thermal quench in the event of a major disruption is to inject neon pellets and force a core thermal collapse through radiation. This is conceptually similar to structure formation in clusters of galaxies in which local accumulation of gas produces a radiative cooling spot for the surrounding intra-cluster hot gas. An even more striking similarity is that both plasmas are nearly collisionless, so the plasma cooling due to the presence of a radiative cooling spot is in an exotic regime in which one would normally assume that electron thermal conduction, even with free-streaming flux limiting, is the dominant mechanism for the thermal quench. In the aforementioned astrophysics problem, one instead observes a robust cooling flow into the radiative cooling spot. Ways to inhibit thermal conduction, for example, by tangled magnetic field lines in the transverse direction, have been an active area of research for the astrophysics community. Here we use first-principles kinetic simulation and analytical theory to show that for a nearly collisionless plasma, its thermal collapse due to the interaction with a radiative cooling spot is not conduction-dominated, but instead relies on convective energy transport for cooling. As the result, there is a robust cooling flow towards the radiative cooling spot. The fundamental physics is the constraint of ambipolar transport, which in the case of a nearly collisionless plasma, enforces a particularly simple and robust form of electron energy transport that favors convection over conduction. The thermal collapse now takes the form of four propagating fronts that originate from the radiative cooling spot, along the magnetic field in the case of a tokamak. The slowest one, which is responsible for deep cooling, is a shock front. Similar physics can occur for an unmitigated thermal quench where 3D field lines connect the core plasma directly to the first wall.