A new edge-based energy scaling law for W7-AS and its implications for boundary-island divertor operation

Within this paper it is established that the total plasma energy W for Wendelstein 7-AS plasmas is uniquely related to the density near the separatrix nes, for an extremely wide range of conditions, in the fashion W=5.47nes0.7[kJ,1019m-3]. This expression holds for peaked core density profiles (Normal Confinement, NC) as well as those with an edge transport barrier (High Density H-mode, HDH). It encompasses absorbed powers P = 0.26-2.4 MW over a line-averaged density range n¯e=1-40×1019m-3 (Pecrh = 0.26-1.5 MW: n¯e=1-10×1019m-3; Pnbi = 0.7-2.4 MW: n¯e=7-40×1019m-3), including even attached/detached conditions at the target plates. A consequence of this scaling for a boundary-island divertor - which is an inherent feature of the Wendelstein stellarator line, requiring a large nes to effect detachment - is that detachment on W7-AS can be expected and is only found for high-confinement situations, i.e. the HDH-mode.