Neutral gas and plasma shielding scaling law for pellet ablation in Maxwellian plasmas

Pellet penetrations are generally well reproduced by the neutral gas shielding (NGS) scaling law when neutral gas and plasma shielding (NGPS) models are less successful, although their description of the ablation physics is much more realistic. The solution of this apparent contradiction is to be found in the structure of the NGPS models. Indeed, since they are generally not fully self-consistent, several choices are possible in the description of the elementary shielding processes. The problem thus reduces to finding a set of compatible assumptions defining an NGPS model that can fit the experimental measurements on a database as large as possible. The model presented here reproduces the penetrations of approximate to 120 pellets from 5 different machines: Tore Supra, JET, FTU, T-10 and RTP. An NGPS scaling law has been constructed for the ablation rate that exhibits dependences on n(e), T-e and r(p) close to those of the NGS model, in addition to weak dependences on the magnetic field and tokamak major radius. For reactor grade plasmas, the penetrations calculated by the NGPS model are approximate to 15% smaller than the NGS predictions.