H-mode power threshold reduction in a slot-divertor configuration on the Alcator C-Mod tokamak

Abstract

H-mode power thresholds (Pth) are examined in Alcator C-Mod lower single null plasmas with standard vertical-plate and slot-type divertor configuration (BT = 5.4 T, Ip = 0.9 MA, q95 ∼ 4.0 and 〈ne〉 = (1.3–1.6) × 1020 m−3). The required power to access H-mode is found to be significantly reduced when the plasma is operated with a slot divertor. The lowest Pth achieved is ∼0.7 MW, about 1/3 of the normal level and 40% of the multi-machine scaling law prediction. Pth does not explicitly correlate with plasma shaping parameters (X-point locations, elongation, triangularities, etc), but somehow shows an overall inverse correlation with outer divertor leg length (the poloidal distance between X-point and outer strike point) and/or the outer divertor connection length. This implies that another, still unknown parameter hidden in the vicinity of the separatrix outer leg might be responsible for the observed difference in H-mode access power. Despite the large differences in Pth, the edge electron temperature (Te) and density (ne) profiles prior to L–H transition are not strongly affected by the divertor configuration. Values of Te,95 (i.e. Te at 95% normalized poloidal flux surface) at the L–H transition are similar for the two divertor configurations.