On the way to divertor detachment in the W7-AS stellarator

3D modeling on W7-AS suggests that to obtain divertor plasma detachment, upstream separatrix densities nes in the range of 1020 m−3 will be necessary. This paper investigates the ratio of nes to the line-averaged density n̄e under a variety of conditions – Pheat∼0.4–2 MW, n̄e∼0.2–2.7×1020 m−3 – for inner-limiter plasma configurations in preparation for the upcoming divertor phase. nes/n̄e ranges from 0.05 for the quiescent H-mode H* (Pecrh∼0.4 MW) up to ∼0.35 for high-power (Pnbi∼2 MW), heavily fueled discharges where nes approaches 1020 m−3 within a density ramp. For Pnbi<1 MW, nes/n̄e declines with increasing n̄e as the H-mode threshold density n̄ethr approaches. L–H–L switching transitions cause nes/n̄e to alternate between higher and lower values. The attainment of H* implies reduced nes/n̄e and incipient radiation collapse of the core plasma. n̄ethr and nes/n̄e both increase with heating power by which means detachment-relevant nes can be attained, albeit at n̄e close to the density limit. These phenomena in detail exhibit a sensitive dependence on ιa – even a 1% change results in evident and reproducible plasma behavior.