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

3D modeling on W7-AS suggests that to obtain divertor plasma detachment, up stream separatrix densities n(es) in the range of 10(20) m(-3) will be nece ssary. This paper investigates the ratio of n(es) to the line-averaged dens ity (n) over bar (e) under a variety of conditions - P-heat similar to 0.4- 2 MW, (n) over bar (e) similar to 0.2-2.7 x 10(20) m(-3) - for inner-limite r plasma configurations in preparation for the upcoming divertor phase. n(e s)/(n) over bar (e) ranges from 0.05 for the quiescent H-made H* (P-eerh si milar to 0.4 MW) up to similar to0.35 for high-power (P-nbi similar to 2 MW ), heavily fueled discharges where n(es) approaches 10(20) m(-3) within a d ensity ramp. For P-nbi < 1 MW, n(es)/<(n)over bar>(e) declines with increas ing (n) over bar (e) as the H-mode threshold density (n) over bar (thr)(e) approaches. L-H-L switching transitions cause n(es)/(n) over bar (e) to alt ernate between higher and lower values. The attainment of H* implies reduce d n(es)/(n) over bar (e) and incipient radiation collapse of the core plasm a, (n) over bar (thr)(e) and n(es)/(n) over bar (e) both increase with heat ing power by which means detachment-relevant n(es) can be attained, albeit at (n) over bar (e) close to the density limit. These phenomena in detail e xhibit a sensitive dependence on l(a) - even a 1% change results in evident and reproducible plasma behavior. (C) 2001 Elsevier Science B.V. All right s reserved.