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

Abstract

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 (PLOSS), plasma current (Ip), and magnetic flux expansion (fexp), 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 Ip⩽1.2MA. First we note that the mid-plane heat flux width mapped to the mid-plane, λqmid, is largely independent of PLOSS for PLOSS⩾4MW. λqmid is also found to be relatively independent of fexp; peak heat flux is strongly reduced as fexp is increased, as expected. Finally, λqmid is shown to strongly contract with increasing Ip such that λqmid∝Ip-1.6 with a peak divertor heat flux of qdiv, peak∼15MW/m2 when Ip=1.2MA and PLOSS∼6MW. These relationships are then used to predict the divertor heat flux for the planned NSTX-Upgrade, with heating power between 10 and 15MW, Bt=1.0T and Ip=2.0MA for 5s.