Trace tritium and the H-mode density limit

Trace amounts of tritium gas have been injected in short puffs into JET ELM y H-modes with a wide range of deuterium gas-fuelling rates. Analysis of th e subsequent time evolution of the neutron profile and extraction of the pa rticle transport coefficients have allowed us to distinguish between broad classes of mechanism which have been suggested as explanations for the H-mo de density limit. The high penetration probability (similar to 20%) and rap id transport (similar to tau(E)) Of fuel ions are shown to be only weakly i nfluenced by strong gas fuelling - hence mixture control is possible even w hen the total electron content is clamped. (C) 1999 JET Joint Undertaking, published by Elsevier Science B.V. All rights reserved.