Performance near operational boundaries

The performance of ELMy H-mode operation in ASDEX Upgrade andJET is compared. Special attention is paid to variations (usuallyreductions) in this performance near the operational limits whichwill need to be approached in a next-step device. In JET it isfound that input powers substantially above the H-mode thresholdpower are required to obtain discharges with energy confinementenhancement factors at or above the usual ELMy H-mode scalings.Such a margin (as much as a factor of two in JET) is not observedin ASDEX Upgrade. It is proposed that this difference may be dueto the higher edge collisionality in ASDEX and the results arecompared to a recent theory based on interchange instabilitiesand magnetic flutter. In ASDEX Upgrade, the confinement in type IELMy discharges degrades as the density is raised due to astiffness of the temperature profiles which leads to adegradation of the core confinement. This type of stiffness isobserved in JET only at relatively high edge densities. In JET, theedge confinement degrades as the density is increased by externalgas fuelling, consistent with a constant edge pressure gradientand an edge barrier width which reduces in proportion to the edgeion poloidal Larmor radius. In both machines, H-mode performanceis limited at high density by a transition first to the type III ELMregime and then to the L-mode. The confinement penalty, relativeto good type I ELM discharges, of operating with type III ELMs isabout 25-30%. The maximum densities for operation withtype I or type III ELMs can be substantially increased byincreasing the plasma triangularity in both machines.