Dependence of the scrape-off layer heat flux widths on external parameters in the NSTX

Introduction Spherical tori (ST) face the prospect of high heat flux onto the plasma facing components (PFC), owing to their compact nature and design as high power density systems. Power balance mandates a straightforward relationship between the peak heat flux and its characteristic footprint. The divertor heat flux profile and its characteristic scale length, ?q are determined by the balance of parallel and radial thermal transport in the scrape-off layer (SOL), along with volumetric losses on the open field lines. Clearly ?q is related to the upstream mid-plane density and temperature widths. Previously, National Spherical Torus Experiment [1] has performed scaling experiments to determine the dependence of the peak heat flux on controllable engineering parameters such as plasma current, heating power[2][3] and magnetic flux expansion [1]; in those experiments, a preliminary assessment of the divertor footprint was made. Understanding how those quantities scale [4] is a necessity for building larger, higher power spherical tori. The proposed NSTX-upgrade will stress the thermal limits of the existing graphite plasma facing components, with 10-12 MW of neutral beam input power, up to 6 MW of RF heating power, toroidal magnetic field up to 1T and plasma current of up to 2 MA with pulse lengths up to 5 sec [5]. Viable divertor designs may therefore require some form of heat flux mitigation techniques, including a detached or radiative divertor, magnetic flux expansion [6][7], and/or a snowflake divertor [8][9].