Confinement and gas fueling in LHD limiter discharges

Plasma discharges in the Large Helical Device are normally open helical divertor discharges. To compare limiter discharges with open divertor discharges and to examine the role of the peripheral region, a radial movable limiter, whose head was made of carbon with high heat conductivity, was inserted into the plasma from the high field side (near the helical coil). The electron temperature was bounded well by the limiter. A high temperature gradient at the edge region was observed in both open divertor and limiter discharges. Formation of such a high temperature gradient led to good energy confinement even in the limiter discharges and an enhancement factor of 1.1±0.3 for International Stellarator Scaling 95 (ISS95) scaling was observed at every limiter position (0.75<ρ<1). Serious degradation of the plasma confinement by increase of recycling and/or impurities was not observed in the limiter discharges. Within these experimental conditions, the dependence of the plasma minor radius on the energy confinement time was same as that of the ISS95 scaling law. A thick ergodic layer with Rax=3.75 m prevents gas fueling by puffing