Evolution of particle clouds around ablating pellets in magnetically confined hot plasmas

Cryogenic hydrogen isotope pellets are being used for introducing fuel particles into the plasma interior in magnetic confinement fusion experiments. The spatial and the time evolution of the initially low-temperature, high-density particle clouds forming around such pellets are considered, with particular attention given to such physical processes as heating of the clouds by the energy fluxes carried by incident plasma particles, gas-dynamic expansion with j× B-produced deceleration in the transverse direction, finite-rate ionization and recombination processes, and magnetic field convection and diffusion. While the dynamic processes associated with the ionization and radial confinement processes are characterized by the relatively short Alfven time scale (μs range), the subsequent phase of axial expansion is associated with a notably larger hydrodynamic time scale defined by the heat input and gas-dynamic expansion rates (ms range). Data stemming from experimental measurements in toroidal confinement machines are compared with results of model calculations. Some similarities with extraterrestrial plasma scenarios, such as the earlier magnetospheric barium release experiments, are discussed