Importance of the divertor configuration for attaining the H-regime in ASDEX

Abstract

The good confinement regime of neutral-beam-heated plasmas (H-mode) is restricted in ASDEX to operation in divertor configuration. Neutral injection into limiter discharges invariably leads to degraded confinement properties (L-mode). Investigations of the boundary layer and divertor plasma parameters do not yield any evidence for specific boundary properties giving rise to the H-mode. On the other hand, a strong rise in electron temperature in the peripheral zone of the main plasma is observed, and can be correlated to the H-transition. For example, a sawtooth event can trigger the transition into the H-mode due to the thermal wave which propagates into the outer plasma zone. Impurities, puffed into the discharge, cool the plasma edge and quench the H-mode. Similarly, a poloidal limiter, rotated to the plasma surface, reduces the edge temperature and, at a limiter separation ⩽ 2.5 cm, the discharge loses its good confinement properties. In particular, the edge electron temperature of limiter discharges is largely independent of injection power, whereas in divertor discharges it increases linearly with beam power reaching ∼ 800 eV at a position 5 cm inside the separatrix. In addition to impurity cooling, molecular recycling and refuelling in the main plasma chamber also seem to be unfavourable. From these observations we conclude that neutral injection into divertor discharges shapes the plasma profiles to such an extent that transport is affected and confinement is improved. A decisive parameter seems to be the electron temperature or resistivity at the edge. With sufficient injection power, the negative effects of cooling at the plasma periphery (radiation, convection, thermal conduction) are overcome and beyond a certain temperature (conductivity) threshold, the H-transition occurs. In the H-mode, the plasma is characterized by the favourable aspects of broad pressure and current density profiles and by q 0 > 1.