Conditioning of the graphite bumper limiter for enhanced confinement discharges in TFTR

Abstract

A strong pumpi.ig effect has been observed with plasma operation on the toroidal graphite bumper limiter on TFTR. The pumping effect was induced by conditioning the limiter with a short series (10-20) of low density deuteriumor heliura-iniciated discharges. The density decay constant (T) for gasfueled ohmic discharges was reduced from T > 10 s before conditioning to a minimum value of t = 0.15 s after conditioning, corresponding to a reduction in the global recycling coefficient from-1002 to less thin 502. Coincident with the low recycling conditions, low current neutral-beam-fueled discharges show global energy confinement times which are enhanced by a factor of two over results with an unconditioned limiter. Two models are proposed for the observed pumping effects: (1) a depletion model based on pumping of hydrogenic species in the near-surface region of the limiter after depletion of the normally saturated surface layer by (carbon and helium) ion-induced desorption} and (2) a codeposition model based on pumping of hydrogenic species in carbon films sputtered from the limiter by the conditioning process. MASTER DISTRIBUTION UF THIS liLC'i '^":? IS l 1 '•'-'-•

Experiments on TFTR supershot plasmas

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