Addressing the challenges of plasma-surface interactions in NSTX-U

Abstract

Summary form only given. The importance of conditioning plasma-facing components (PFCs) has long been recognized as a critical element in obtaining high-performance plasmas in magnetic confinement devices. Lithium coatings, for example, have been used for decades for conditioning PFCs. On the National Spherical Torus Experiment (NSTX), the amount of lithium introduced into the plasma chamber has been correlated with the achievement and persistence of discharges with good confinement and stability. This empirical relationship, however, does not reflect the detailed interaction between plasma-facing surfaces and the dynamic conditions that occur during discharges and also between shots.A first step developing an understanding such complexity will be taken in the upgrade to NSTX (NSTX-U) that is nearing completion. New measurement capabilities include the Materials Analysis and Particle Probe (MAPP), that has already demonstrated the dependence of plasma performance with PFC conditions. In situ surface analysis has been performed on samples exposed to plasmas in the Lithium Tokamak Experiment (LTX), but also during glow discharge cleaning between operational periods. Increased spatial coverage of the divertor region with Langmuir probes will provide more detailed information on the edge plasma. The data from existing and enhanced diagnostics will be interpreted with new modeling capabilities. Post-run observations of the NSTX PFCs indicate a distribution of lithium could not be solely explained by effluence of the lithium evaporators, and suggest that erosion/redeposition played a key role. The OEDGE suite of codes, for example, will provide a new way to model the underlying mechanisms for such material migration in NSTX-U. This will lead to a better understanding of how plasma-facing surfaces evolve during a shot, and how the composition of the plasma facing surface influences the discharge performance we observe. This presentation will provide an overview of these capabilities, and highlight their importance for NSTX-U plans to transition from carbon to high-Z PFCs.