Methods for the analysis of low-Z plasma facing surfaces in magnetic fusion applications
A number of advances in the design and operating performance of magnetic fusion devices have been made in recent decades. While the optimization of reactor components and magnetic topologies have led to considerable improvements in plasma density, temperature, and confinement time, research also suggests that materials selection in plasma-facing components (PFCs) can also have a substantial impact on plasma performance. Within the plasma-material interaction (PMI) community, the application of low-Z materials on wall components is of particular interest. In order to better understand the behavior of various low-Z systems, two PFC studies are described herein. The goals of these studies were to: (1) better understand material migration in boronized graphite wall tiles via the ex-vessel characterization of the B-O-C layer in exposed graphite samples, and (2) explore issues relating to the impacts of handling and processing on a liquid phase Sn-Li binary alloy. The National Spherical Torus Experiment Upgrade (NSTX-U) has been used to investigate the effect of wall tile surface conditioning on plasma performance during operation. Previous campaigns have demonstrated the enhanced suppression of edge-localized modes (ELMs) and reduced divertor recycling when wall tile conditioning via boronization was performed, and high confinement (H-mode) operating conditions were routinely achieved during operation after conditioning. In order to better understand the impact of surface conditioning and subsequent plasma exposure on wall materials, cored sample of the exposed NSTX-U wall tiles were obtained and their surface chemistry was analyzed. The boronized NSTX-U samples were analyzed using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and Rutherford backscattering spectrometry (RBS). In addition, variations in surface morphology in each tile were mapped using a 3D laser scanning confocal microscope. Similar to the pure low-Z coatings used in the NSTX-U device, tin-lithium is a low melting-point eutectic that has been identified as a material with favorable performance in plasma material interaction (PMI) studies. The Sn-Li eutectic is a more stable alternative to pure lithium that provides a lower rate of evaporative flux due to the presence of tin while experiencing the bulk segregation of lithium to the surface of the material in the liquid phase. This surface segregation prevents the high-Z tin from entering the plasma. While the alloy is of considerable interest to the PMI community, few studies have directly documented the presence of a pure lithium surface in a tin-lithium melt. In order to expand the existing body of knowledge in this area, samples of an 80% Sn—20% Li eutectic were pre-cast and subsequently melted and analyzed in the Angle-Resolved Ion Energy Spectrometer (ARIES) at Sandia National Laboratories.
Nuclear fusion, plasma-material interactions
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign