Behaviour of upstream separatrix density in JET H-mode plasmas

The variation of n(e)(Sep)/(n) over bar(e) in JET PT-modes is described. The upstream separatrix density, n(e)(Sep), was determined using total pressure balance, in conjunction with 'onion-skin' modelling and reciprocating Langmuir probe measurements. It was found that the ratio n(e)(Sep)/(n) over bar(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(e)(Sep)/(n) over bar(e) ratio and rate of increase were different for horizontal and vertical target configurations with the latter having an initial n(e)(Sep)/(n) over bar(e) ratio of 0.12. Related information on the behaviour of the density SOL width and the variation of n(e)(Sep)/(n) over bar(e) with confinement are also presented. (C) 1999 JET Joint Undertaking, published by Elsevier Science B.V. All rights reserved.