Adequate helium enrichment and exhaust have beenachieved in reactor relevant ELMy H mode plasmas in JETperformed in the Mark II divertor geometry. These quantities,describing the retention of impurities in divertors, have beenexperimentally inferred from CXRS measurements in the coreplasma, and from a spectroscopic analysis of a Penning gaugedischarge in the exhaust gas. The retention of helium was foundto be sufficient with respect to the requirements in a next stepdevice, with helium enrichment factors exceeding 0.1 in highdensity ELMy H mode discharges. With increasing core plasmadensity the helium partial pressure in the exhaust channelincreases. While in L mode plasmas the helium enrichmentdecreases with increasing core plasma density, it remains almostconstant in ELMy H mode plasma. The noble gas neon is betterenriched in the divertor at high core plasma densities in bothconfinement regimes. These experimental results can be explainedby the significant differences between the penetration depths ofthe impurity neutrals and by their subsequent different impuritytransport mechanisms. Analytical and numerical analyses of theseplasmas using the impurity code package DIVIMP/NIMBUS supportthe proposition that, owing to their much longer ionization meanfree path, helium particles can escape from the divertor chamberas neutrals, while neon escapes by means of ion leakage.Consequently, the divertor plasma conditions strongly influencethe noble gas compression and enrichment. Variations of thedivertor plasma configuration and modifications to the divertorgeometry have enhanced the pumping capabilities of the tokamak,but have been found not to affect the helium enrichment.