Behaviour of upstream separatrix density in JET H-mode plasmas

Abstract The variation of n Sep e / n e in JET H-modes is described. The upstream separatrix density, n Sep e , was determined using total pressure balance, in conjunction with `onion-skin' modelling and reciprocating Langmuir probe measurements. It was found that the ratio n Sep e / n e was strongly influenced by ELM frequency increasing, proportional to the rate of particle loss to the scrape-off layer (SOL) from ELMs, towards the pedestal density and then saturating. The initial n Sep e / n e ratio and rate of increase were different for horizontal and vertical target configurations with the latter having an initial n Sep e / n e ratio of 0.12. Related information on the behaviour of the density SOL width and the variation of n Sep e / n e with confinement are also presented.