Confinement and fueling studies during additional heating phase in the JFT-2M tokamak

Abstract Increments of peripheral hydrogen/deuterium neutral gas pressures ( P H 2 / P D 2 ) during the additional heating phase (neutral beam and RF heatings) have been observed in a D 2 gas-puff fueled JFT-2M tokamak with H 2 -absorbed graphite limiters/divertor plates. In the beam heating phase, a large increment of P H 2 raises the plasma density 2 times or more without the degrading energy confinement time. The D 2 gas-puff valve is closed in this phase. This improvement is interpreted as being due to “wall fueling”, i.e., fueling by desorbed hydrogen from the graphite wall by energetic particles. The first observation of the H-mode in the INTOR-type stubby open divertor with a short divertor channel (1–8 cm) at the high density regime (4–7 × 10 13 cm−3) enhanced by the wall fueling is presented. The improved energy confinement time is comparable to or higher than that of ohmically heated discharges.