JET pellet ablation studies and projections for CIT and ITER

Pellet penetration in JET under a wide variety of plasma conditions is analyzed and compared with the predictions of the neutral and plasma shielding (NGPS) model for pellet ablation. Specific issues addressed in the context of the NPGS model that impact pellet ablation are the effective ionization radius of the ablatant normal to the magnetic field and the role of non-Maxwellian ions and electrons in the ablation process. The model is then used to evaluate pellet penetration in CIT and ITER. Nearly all non-ohmic heating techniques produce small populations of energetic electrons or ions. Slowing-down distributions of hydrogenic and helium ions can be used to evaluate thresholds for neutral beam injection and fusion alpha enhanced ablation respectively. Pellet ablation in neutral beam heated plasmas agrees with the fast ion treatment in the NGPS model. Classical, local thermalization of alphas from thermal reactions is shown to lead to no significant pellet ablation enhancement. Energetic distributions without a sharp energy cutoff, as produced with RF heating or runaway discharges, are far more difficult to analyze with simplifying approximations. Projections for CIT and ITER show that active fueling of the outer half of the plasma volume is most probable for the pellet sizes and velocities considered. Penetration to the plasma center is highly unlikely at the nominal design operating conditions. Thus, it is important to understand transient particle transport under conditions of partial pellet penetration. (author). 13 refs, 2 figs, 2 tabs