HIGH-CONFINEMENT NBI DISCHARGES IN THE W7-AS STELLARATOR

In W7-AS, the longest energy confinement times were achieved in neutra l beam injection heated (NBI-heated) discharges under low wall-recycli ng conditions. Low recycling is needed to control the density at line- averaged values of (n) over bar(e) approximate to 10(20) m(-3). Under these conditions, confinement was improved by a factor of two above th e common scaling estimate. The reduction of radial transport is concen trated into a layer at about two-thirds of the plasma radius. In this region steep pressure gradients and a strong gradient in the radial el ectric field develop. Specific for the discharges is the slow transiti on to improved confinement, lasting up to three energy confinement tim es. Since the measured electric field is consistent with the neoclassi cal ambipolar field, this high-confinement mode could be an example wh ere sheared plasma flow as created by the neoclassical radial electric held leads to a suppression of anomalous transport.