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
1987 Nuclear Fusion  
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
more » ... ient 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.
doi:10.1088/0029-5515/27/8/003 fatcat:pevrwcwgjvhlllxkwrzlrvyyg4