Neutral and plasma shielding model for pellet ablation

Abstract

The neutral gas shielding model for ablation of frozen hydrogenic pellets is extended to include the effects of (1) an initial Maxwellian distribution of incident electron energies; (2) a cold plasma shield outside the neutral shield and extended along the magnetic field; (3) energetic neutral beam ions and alpha particles; and (4) self-limiting electron ablation in the collisionless plasma limit. Including the full electron distribution increases ablation, but adding the cold ionized shield reduces ablation; the net effect is a modest reduction in pellet penetration compared with the monoenergetic electron neutral shielding model with no plasma shield. Unlike electrons, fast ions can enter the neutral shield directly without passing through the cold ionized shield because their gyro-orbits are typically larger than the diameter of the cold plasma tube. Fast alpha particles should not enhance the ablation rate unless their population exceeds that expected from local classical thermalization. Fast beam ions, however, may enhance ablation in the plasma periphery if their population is high enough. Self-limiting ablation in the collisionless limit leads to a temporary distortion of the original plasma electron Maxwellian distribution function through preferential depopulation of the higher energy electrons.