Recycling enhancement with ne and qeff in high density discharges on JT-60U

Abstract

Quantitative studies of recycling, particle flux and divertor plasma parameters have been made in high density L mode discharges on the JT-60U tokamak, with the ion ∇B drift towards the divertor. The diagnostics used were Hα/Dα photon emission detectors and a Langmuir probe array. Deuterium particle influx Φ(Dα) and particle outflux Ψprobe measurements were in fairly good agreement in both the inboard and the outboard divertors. 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. Ψ(Dα) was raised from 1022 to 1024 s-1 with increasing n̄e and qeff. 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 Gdiv in the inboard divertor was larger than that in the outboard divertor, and that both increased with increasing qeff and n̄e. These results explain the observed dependence of the recycling on the parameters ne and qeff. The maximum electron density n̄e,div in the outboard divertor was (4-7) × 1019 m-3, exceeding ne for high qeff discharges. In the same discharges, the divertor electron temperature Te,div was reduced to about 20 eV before MARFEs were observed. High n̄e and qeff operation was favourable for obtaining a cold and dense divertor plasma