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

H-mode power thresholds ( P th ) are examined in Alcator C-Mod lower single null plasmas with standard vertical-plate and slot-type divertor configuration ( B T = 5.4 T, I p = 0.9 MA, q 95 ∼ 4.0 and 〈 n e 〉 = (1.3–1.6) × 10 20 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 P th achieved is ∼0.7 MW, about 1/3 of the normal level and 40% of the multi-machine scaling law prediction. P th 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 P th , the edge electron temperature ( T e ) and density ( n e ) profiles prior to L–H transition are not strongly affected by the divertor configuration. Values of T e,95 (i.e. T e at 95% normalized poloidal flux surface) at the L–H transition are similar for the two divertor configurations.