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

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. Since the initial studies on the Tokamak Fusion Test Reactor (TFTR), experiments on devices with different aspect ratios and magnetic geometries like the National Spherical Torus Experiment (NSTX) continue to show the relationship between the lithium PFCs and good confinement and stability. While such results are promising, their empirical nature do not reflect the detailed relationship between the PFCs and the dynamic conditions that occur in the tokamak environment. A first step developing an understanding such complexity will be taken in the upgrade to NSTX, or the National Spherical Torus Experiment-Upgrade (NSTX-U) that is nearing completion. New measurement capabilities include the materials analysis and particle probe for in situ surface analysis of samples exposed to tokamak plasmas. The onion-skin modeling for edge analysis (OEDGE) suite of codes, for example, will be used 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 paper will provide an overview of these capabilities, and highlight their importance for NSTX-U plans to transition from carbon to high- {Z} PFCs.