Recent results from divertor and scrape-off layer studies at JET

Recent progress in the study of divertor and scrape-off layer plasma phenomena in JET is reviewed. Three pumped divertors (Mark I, Mark IIA/AP and Mark IIGB) have been installed and exploited under reactor relevant conditions. With increased divertor closure, the particle exhaust rate has increased and neutral compression factors of > 100 have been obtained with the Mark IIGB divertor. Helium enrichment factors of > 0.2 have been measured under a wide range of conditions and satisfy the minimum requirements for ITER. Excellent power handling has been demonstrated in all divertor configurations. Fast infrared camera measurements exhibited broad deposition profiles during type I ELMs and energy densities of ~ 0.12 MJ/m2, which extrapolate to unacceptable levels for ITER. During the recent DT experiments, the co-deposition of tritium on cold shadowed surfaces in the inner divertor was identified as an important form of long term tritium retention. This has serious implications for the divertor design and tritium inventory in a next step tokamak. Core plasma purity has not improved with enhanced divertor closure or decreased main chamber neutral pressure. Studies of the chemical sputtering yield showed a dependence on surface temperature and hydrogen isotope. This accounts for the observation of increased carbon impurity production in Mark II (at 500 K) relative to Mark I (at 300 K). The variation of the disruptive (L mode) density limit appears to be more strongly dependent on the local impurity production rate than on the divertor geometry. Significant progress has been made in the study of divertor detachment, and volume recombination has been spectroscopically identified. With increasing isotope mass, detachment and the disruptive density limit occur at lower main plasma density, as predicted by the EDGE2D/NIMBUS codes. Using differential gas fuelling in the Mark IIGB divertor, it was possible to modify the in-out asymmetry of the divertor plasma parameters and energy deposition for the first time.