Dependence of divertor heat flux widths on heating power, flux expansion, and plasma current in the NSTX

We report the dependence of the lower divertor surface heat flux profiles, measured from infrared thermography and mapped magnetically to the mid-plane on loss power into the scrape-off layer (P_LOSS), plasma current (I _p), and magnetic flux expansion (f_exp), as well as initial results with lithium wall conditioning in NSTX. Here we extend previous studies [R. Maingi et al., J. Nucl. Mater. 363-365 (2007) 196-200] to higher triangularity ∼0.7 and higher I_p ≤ 1.2 MA. First we note that the mid-plane heat flux width mapped to the mid-plane, λqmid, is largely independent of P_LOSS for P_LOSS ≥ 4 MW. λqmid is also found to be relatively independent of f_exp; peak heat flux is strongly reduced as f_exp is increased, as expected. Finally, λqmid is shown to strongly contract with increasing I_p such that λqmid∝Ip-1.6 with a peak divertor heat flux of q _{div, peak} ∼ 15 MW/m² when I_p = 1.2 MA and P_LOSS ∼ 6 MW. These relationships are then used to predict the divertor heat flux for the planned NSTX-Upgrade, with heating power between 10 and 15 MW, B_t = 1.0 T and I_p = 2.0 MA for 5 s.