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

H. F. Dylla, P. H. Lamarche, M. Ulrickson, R. J. Goldston, D. B. Heifetz, K. W. Hill, A. T. Ramsey

Research output: Contribution to journalReview article

52 Scopus citations

Abstract

A strong pumping effect was observed with plasma operation on the toroidal graphite bumper limiter in TFTR. The pumping effect was induced by conditioning the limiter with a short series (10-20) of low density deuterium or helium initiated discharges. The decay constant for gas fuelled Ohmic discharges was reduced from before conditioning to a minimum of after conditioning, which corresponds to a reduction in the global recycling coefficient from about 100% to less than 50%. Coincident with the low recycling conditions, low current neutral beam fuelled discharges showed global energy confinement times which were enhanced by a factor of two over the values obtained 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 co-deposition model, based on adsorption of hydrogenic species in carbon films formed by material sputtered from the limiter during conditioning.

Original languageEnglish (US)
Pages (from-to)1221-1230
Number of pages10
JournalNuclear Fusion
Volume27
Issue number8
DOIs
StatePublished - Aug 1987

All Science Journal Classification (ASJC) codes

  • Nuclear and High Energy Physics
  • Condensed Matter Physics

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