The effect of divertor geometry on divertor and core plasma performance in JET

Abstract

JET has completed a series of experiments in the Mk I and Mk IIA divertors on the effects of increased geometrical closure and target orientation. The potential benefits from closure were expected to be enhanced volumetric energy loss in the divertor (detachment), increased divertor neutral pressure for better pumping and He exhaust, and reduced main chamber neutral pressure for reduced sputtering. The expected effects on neutral pressures were observed. In ohmic and L-modes this led to detachment at lower upstream density and reduced density limits, in qualitative agreement with code calculations. The pumping speed was increased by about a factor of three. Z eff did not reduce, despite the reduced main chamber neutral pressure. In ELMy H-modes the effects of closure were less distinct, which may have been due in part to ELMs striking the upper surfaces of the divertor and main chamber limiting surfaces. The density limit and confinement quality were unaffected by changes in divertor geometry. Increasing triangularity increased the density limit, but also raised Z eff. Confinement was degraded by either deuterium puffing or nitrogen puffing. Detachment occurred at the inner target between ELMs, but not at the outer target until confinement was strongly degraded. Vertical target ELMy H-modes have thinner SOL's and lower midplane separatrix densities than those run on horizontal targets in Mk IIA. Given the JET observations on the lack of sensitivity of core plasma ELMy H-mode performance to divertor geometry, it appears appropriate to review the possibility of simpler, lower cost divertor options than the deep divertor design currently proposed for ITER.