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Title:In situ and post mortem surface chemistry measurements of boronization in the national spherical torus experiment upgrade
Author(s):Schamis, Hanna
Advisor(s):Allain, Jean Paul
Contributor(s):Andruczyk, Daniel
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear, Plasma, Radiolgc Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):plasma facing components
PFC conditioning
X-ray photoelectron spectroscopy
Abstract:Plasma facing component (PFC) conditioning has been shown to improve plasma performance in tokamaks by reducing impurities in the core. The method of PFC conditioning used in the National Spherical Torus eXperiment Upgrade (NSTX-U) during the 2015-2016 experimental campaign was boron conditioning. This method deposits ~10 nm of boron on the surface of the PFCs. The boron layer then acts as an oxygen getter, binding oxygen to the surface and preventing it from contaminating the plasma. The Materials Analysis and Particle Probe (MAPP) is an in situ materials characterization facility installed on NSTX-U and used during the 2015-2016 experimental campaign. MAPP is designed as a PFC diagnostic and can expose and analyze samples on a shot-to-shot basis, without exposing materials to atmosphere. During the campaign, one of the objectives of MAPP was studying high-Z candidate PFC materials and their surface chemistry. In this work, the results from a boronized molybdenum alloys exposed to NSTX-U plasmas are shown, along with post mortem work on those same substrates. In the high Z case, the data taken with MAPP is primarily composed of X-ray Photoelectron Spectroscopy (XPS) taken on a day-to-day basis. The online MAPP results show that throughout the course of several months, boron is deposited on the surface, oxidized with plasma operations, and then eroded after further plasma exposure. This behavior is expected and parallels the behavior observed in MAPP for the graphite substrates. The post mortem work includes XPS depth profile analysis, which shows that boron remains on the surface. Some low-energy ion scattering spectroscopy (LEISS) analysis was also done. Post mortem analysis on graphite tiles removed from NSTX-U is crucial to studying PFC surface chemistry at more than one location than the MAPP probe location. In addition, it gives us information about the distribution of boron redeposited layers and the evenness of that distribution throughout the tokamak. Post mortem XPS depth profile was carried out on five samples from four different tiles. The results show a strong radial dependence on the atomic concentration of boron in the outer strike point location, as well as variation in the inner strike point region and private flux region.
Issue Date:2019-12-09
Rights Information:Copyright 2019 Hanna Schamis
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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