Time dependent simulation of pellet evaporation in tokamak plasmas

The evaporation of frozen deuterium pellets injected into Alcator C plasmas is examined using a specialized time dependent plasma hydrodynamic code which includes flux limited thermal conduction and neglects magnetic field effects. Ablatant characteristics (density, temperature, ionization) are temporally and spatially resolved and are in good agreement with experimental measurements and with values predicted by other models. The hydrogen Balmer-alpha (Hα) line radiation measured experimentally from the ablating pellet is quantitatively reproduced. A thin neutral region of millimetre radius and a partially ionized region of centimetre radius are calculated to evolve. The neutral region does not play a dominant role in determining the pellet lifetime. Flux limited thermal conduction in the cold, dense ablatant naturally 'shields' the pellet from the hot plasma, relatively independently of the degree of ionization. For the flux limit parameter an estimate of f approx. 0.2 is obtained, which is close to the 'expected' value of 0.32. Finally, it is found that the time dependence of the Hα signal is not directly proportional to the mass ablation rate, but is more nearly proportional to the thermal flux from the tokamak plasma to the ablatant. (author). Letter-to-the-editor. 30 refs, 4 figs