Spectroscopic study of impurities in neutral beam heated and ohmically heated JT-60 discharges

Abstract

Impurities in JT-60 were studied by visible and VUV spectroscopy over two periods of operation. The two periods were distinguished by the use of different first wall materials: from April 1985 to March 1987, TiC coated molybdenum was used as limiter material; from June to October 1987, graphite was used. Quantitative spectroscopic measurements of Zeff, impurity concentrations and radiated power losses were made for ohmically heated and neutral beam heated discharges with limiter and divertor configurations. In the first phase with metallic first wall material, oxygen, carbon and titanium were identified as the main plasma impurities. In neutral beam heated, diverted discharges, Zeff was 1.6 at n̄e = 4 × 1019 m−3. The concentrations of oxygen, carbon and titanium were 1%, 0.1% and 0.006% of ne, respectively. In the second phase with graphite material, the metallic impurities were reduced, and the contribution of metallic impurities to the radiated power loss was less than 1%. However, Zeff increased up to 3 in neutral beam heated discharges. In limited plasmas, the concentrations of oxygen and carbon were 1% and 5%, respectively, at n̄e = 4 × 1019 m−3; in diverted plasmas, these concentrations were 2% and 0.4% at the same n̄e. The radiated power loss from the main plasma was 20–40% of the input power in neutral beam heated, limited discharges, and 7–25% in diverted discharges. The contributions of oxygen and carbon to the radiated power in limited discharges were comparable, and in diverted discharges the contribution of oxygen was dominant.