The effect of divertor geometry on divertor and core plasma performance inJET

JET has completed a series of experiments in the Mk I and Mk IIA diverters on the effects of increased geometrical closure and target orientation. The potential benefits from closure were expected to be enhanced volumetric en ergy loss in the divertor (detachment), increased divertor neutral pressure for better pumping and He exhaust, and reduced main chamber neutral pressu re for reduced sputtering. The expected effects on neutral pressures were o bserved. In ohmic and L-modes this led to detachment at lower upstream dens ity and reduced density limits, in qualitative agreement with code calculat ions. The pumping speed was increased by about a factor of three. Z(eff) di d 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 unaffecte d by changes in divertor geometry. Increasing triangularity increased the d ensity limit, but also raised Z(eff). Confinement was degraded by either de uterium puffing or nitrogen puffing. Detachment occurred at the inner targe t between ELMs, but not at the outer target until confinement was strongly degraded. Vertical target ELMy I-I-modes have thinner SOL's and lower midpl ane separatrix densities than those run on horizontal targets in Mk IIA. Gi ven the JET observations on the lack of sensitivity of core plasma ELMy I-I -mode performance to divertor geometry, it appears appropriate to review th e possibility of simpler, lower cost divertor options than the deep diverto r design currently proposed for ITER. (C) 1999 JET Joint Undertaking, publi shed by Elsevier Science B.V. All rights reserved.