Evolution of high-density particle clouds in magnetically confined plasmas.

Abstract

The subject of this study is the spatial and time evolution of initially low-temperature high-density particle clouds in magnetically confined hot plasmas, such as those produced by ablating cryogenic hydrogen pellets in fusion machines. Particular attention is given to such physical processes as heating of the cloud by the energy fluxes carried by incident plasma particles (classical flux-limited energy transport by thermal electrons along the magnetic field lines, anomalous heat conduction across them), gas dynamic expansion with $\vec j \times \vec B$-produced deceleration in the transverse direction, finite-rate ionization and recombination (collisional and radiative) processes, and magnetic field convection and diffusion. The Lagrangian approximation used allows one to take into account all relevant physical processes that affect the radial expansion and deceleration of the cloud particles, including the change of the magnetic field topology. The results show the existence of a distinct structure in the ablatant cloud surrounding an ablating pellet: a hollow temperature profile coupled to a peaked density profile in the plane normal to the magnetic field direction. The lifetime of this structure is measured on hydrodynamic time scales. The basic properties of the cloud, such as its radial extent, average temperature, bulk density, etc., are complex functions of the pellet ablation rate, the parameters of the background plasma, and the magnetic field strength applied. Simple analytical or ad hoc description of this functionality, as done in present ablation models, does not seem to be possible.