Evolution of pellet clouds and cloud structures in magnetically confined plasmas

Abstract

The subject of this study is the space 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 and anomalous heat conduction across them), gas dynamic expansion with j⃗ × B⃗ produced deceleration in the transverse direction, finite rate ionization and recombination (collisional and radiative) processes, and magnetic field convection and diffusion. The results show the existence of a distinct structure in the ablatant cloud surrounding and ablating pellet: a hollow temperature profile coupled to a peaked density profile in the plane normal to the magnetic field direction. The separation distance between the high and low temperature and density layers is typically the ionization or confinement radius. Also the flutes developing preferentially at the cloud surface have, at a certain phase of their development, the same wavelength. The temperature and density variations from the cloud interior to the cloud periphery may exceed two orders of magnitude. The lifetime of this structure is measured on hydrodynamic time-scales.