Particle balance in the Tokamak Fusion Test Reactor

Plasma particle pumping effects with carbon limiters have been seen inTokamak Fusion Test Reactor (TFTR), Joint European Torus (JET), and TorusExperiment for Technology Oriented Research (TEXTOR). The pumping effects inTFTR were induced by conditioning the large-area axisymmetric bumper limiter with low-density, helium initiated discharges. As a result of the conditioning ∼150 Torr l of gas are removed from the carbon limiter tiles. The conditioning has reduced the particle recycling coefficient from near unity to values as low as 0.5. The low recycling conditions are not permanent. A pumping capacity of ∼100 Torr l has been determined from wall loading experiments. The total particle input has been compared to the total outgassing after a discharge for Ohmic plasmas in both helium and deuterium, and for deuterium neutral beam heated discharges. Ohmic deuterium plasmas have less outgassing than input. Ohmic helium plasmas have equal or greater outgassing compared to gas input. Neutral beam heated discharges have a slight deficit in outgassing, but it is much less than that for Ohmic shots. The low recycling conditions last much longer with neutral beams than in Ohmic discharges. The limiter pumping effect and the reduction in recycling is a prerequisite for the enhanced confinement neutral beam heated discharges observed on TFTR