Divertor experiment on particle and energy control in neutral beam heated JT-60 discharges

Abstract

The divertor characteristics in particle and energy control in neutral beam (NB) heated discharges on JT-60 have been studied with injection powers of up to 20 MW. The essential divertor functions are achieved successfully. In ohmically heated discharges, the minimum clearances between the separatrix magnetic surface and the fixed limiter for sufficient divertor action are 1.5 cm for n̄e = 1.5 × 1019 m−3 and 2.5 to 3 cm for n̄e = 4 × 1019 m−3. Global power balance studies show that, in NB heated divertor discharges, about 5% to 10% of the total absorbed power, PABS, is radiated from the main plasma, while 50% to 60% is radiated in the limiter discharge. For n̄e = 6 × 1019 m−3, 50% of PABS flows to the divertor plate. The radiation loss in the divertor chamber is 15% of PABS. According to the spatial distribution of the temperature rise on the divertor plate, the half-width of the heat load is less than 1 cm for n̄e = 1.5 to 4.4 × 1019 m−3. The neutral pressures in the divertor chamber and around the main plasma increase in proportion to n̄e2. The compression ratio is about 45. The effectiveness of the divertor pumping system in particle control is demonstrated for NB pulses of, at least, 1 s. Reduction of evaporation by separatrix swing is also shown.