RECYCLING ENHANCEMENT WITH (N)OVER-BAR(E) AND Q(EFF) IN HIGH-DENSITY DISCHARGES ON JT-60U

Quantitative studies of recycling, particle dux and divertor plasma parameters have been made in high density L mode discharges on the JT-60U tokamak, with the ion del B drift towards the divertor. The diagnostics used were H-alpha/D-alpha photon emission detectors and a Langmuir probe array. Deuterium particle influx Phi(D alpha) and particle outflux Phi(probe) measurements were in fairly good agreement in both the inboard and the outboard diverters. The relationship between the particle recycling and the divertor plasma parameters was investigated to clarify the conditions under which a cold and dense divertor plasma was formed. Phi(D alpha) was raised from 10(22) to 10(24) s(-1) with increasing ($) over bar n(e) and q(eff). Recycling in the inboard divertor was amplified substantially owing to the appearance of a third peak in the local recycling profile, displaced from the separatrix. Numerical results from an interpretative divertor code showed that the particle amplification factor G(div) in the inboard divertor was larger than that in the outboard divertor, and that both increased with increasing q(eff) and ($) over bar n(e). These results explain the observed dependence of the recycling on the parameters ($) over bar n(e) and q(eff). The maximum electron density n(e,div) in the outboard divertor was (4-7) x 10(19) m(-3), exceeding ($) over bar ne for high q(eff) discharges. In the same discharges, the divertor electron temperature T-e,T-div was reduced to about 20 eV before MARFEs were observed. High ($) over bar n(e), and q(eff) operation was favourable for obtaining a cold and dense divertor plasma.