Helium and neon enrichment studies in the JET MKII AP and MKII GB divertor

Adequate helium enrichment and exhaust have been achieved in reactor-relevant ELMy H-mode plasmas in JET performed in the MKII divertor geometry. These quantities, describing the retention of impurities in divertors, have been experimentally inferred from Charge Exchange Recombination Spectroscopy (CXRS) measurements in the core plasma, and from a spectroscopic analysis of a Penning gauge discharge in the exhaust gas. The retention of helium was found to be sufficient with respect to the requirements in a next-step device, with helium enrichment factors exceeding 0.1 in high-density, ELMy H-mode discharges. With increasing core plasma density the helium partial pressure in the exhaust channel increases. While in L-mode plasmas the helium enrichment decreases with increasing core plasma density, it remains almost constant in ELMy H-mode plasma. The noble gas neon is better enriched in the divertor at high core plasma densities in both confinement regimes. These experimental results can be explained by the significant difference of the penetration depth of the impurity neutrals and their subsequent different impurity transport mechanism. Analytical and numerical analyses of these plasmas using the impurity code package DIVIMP/ NIMBUS support the proposition that, due to their much longer ionisation mean free path, helium particles can escape from the divertor chamber as neutrals, while neon escapes by means of ion leakage. Consequently, the divertor plasma conditions strongly influence the noble gas compression and enrichment. Variations of the divertor plasma configuration and modifications to the divertor geometry have enhanced the pumping capabilities of the tokamak, but found not to affect the helium enrichment. (author)