Evidence for volume recombination in JET detached divertor plasmas

Evidence for volume recombination in the JET divertor has been obtained from the population ratios of the excited states of neutral hydrogen. The ratio of the Balmer series lines Dγ/Dα has been used to identify recombination at both the inner and the outer divertor target. At the outer target the onset of recombination is well correlated with the plasma detachment as observed with Langmuir probes in the divertor target. While Te obtained from the target probe characteristics falls to ∼2 eV, Te from the population distribution of the excited states is 0.8±0.1 eV. The peak plasma density viewing across the outer divertor leg just above the target exceeds 2.5 × 1020 m-3. When the plasma becomes partially detached from the outer target, the radial distribution of recombination shows a maximum close to the outer divertor strike point.