Pellet penetration experiments on JET

The measured penetration depth lambda of deuterium pellets injected into the Joint European Torus (JET) confirms some features of the neutral gas shielding (NGS) model, but not others. The scaling of lambda with plasma and pellet parameters agrees with the NGS model, as in earlier ASDEX studies. Pellet velocity was varied over the range 0.46-1.35 km/s in the JET experiments to test specifically the scaling of lambda with velocity. This scaling also agrees with the NGS model. However, the penetration is deeper in JET than in ASDEX when it is corrected for the expected machine size dependence. Furthermore, the penetration depths measured in JET are greater (by nearly a factor of two) than those predicted by local ablation calculations using the NGS model with an incident Maxwellian distribution of electrons. Plasma shielding used in previous modelling of the JET penetration data can account for the additional shielding, but it also removes the observed velocity dependence. The implications of both the scaling observations and the penetration depths for improvements in ablation theory and in the models are discussed.