Divertor heat flux scaling with heating power and plasma current in H-mode discharges in the national spherical torus experiment

We present the first results of lower divertor heat flux scaling with input power (P-NBI) and plasma current (I-p) in H-mode discharges in the National Spherical Torus Experiment. These experiments were conducted with 0.6 <= I-p <= 0.9 MA, 1 < P-NBI < 6 MW, on-axis toroidal field B-t = 0.45 T, in a lower-single null discharge shape with elongation K similar to 2.0, triangularity delta similar to 0.45, and the ion grad-B drift toward the X-point. As P-NBI was raised from 1 MW to 6 MW, the outer divertor peak heat flux was observed to increase up to 6 MW/m(2). A break in the slope of peak heat flux vs. power was observed at heating power similar to 3 MW, which we interpret as the transition of the divertor toward the high recycling regime and well away from the detachment threshold. The peak heat flux was also observed to increase strongly with Ip. (c) 2007 Elsevier B.V. All rights reserved.