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Title:Plasma Facing Components conditioning techniques and their correlation with plasma performance in the National Spherical Torus eXperiment Upgrade (NSTX-U)
Author(s):Bedoya Arroyave, Carlos Felipe
Director of Research:Allain, Jean P.
Doctoral Committee Chair(s):Allain, Jean P.
Doctoral Committee Member(s):Kaita, Robert; Ruzic, David M.; Goddard, Lynford L.; Zhang, Yang
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
Nuclear Fusion
X-ray Photoelectron Spectroscopy
Surface Science
Materials Diagnostic, Plasmas
Abstract:The effect of Plasma Facing Components (PFC) conditioning with boronization on plasma performance in the National Spherical Torus eXperiment Up- grade (NSTX-U) has been investigated in this work. A novel PFC diagnostic, the Materials Analysis Particle Probe (MAPP) was commissioned in NSTX-U and was used as the main characterization tool. MAPP can expose a set of four samples to the plasmas NSTX-U and interrogated their surface using X-ray Photoelectron Spectroscopy (XPS) in-situ. Several ATJ graphite samples were exposed to boronization and multiple plasma discharges and MAPP was used to monitor their surface chemistry at the end of each day of plasma operations (around 10 to 20 plasma shots). According to the XPS data, boronization with deuterated-Trimethylboron (d-TMB) results in a coating of B4C, with thick- ness greater than 10 nm. The concentrations of the deposits were measured following several boronizations, and in average, the composition was around 33% B, 5%O and 64%C. Good plasma performance was in general observed shortly after a boronization and a reduction in the loop voltage, and increase in the electron density and routine access to H-mode were common following the surface conditioning. It was found that repeated exposure to D+ plasmas drives the oxidation of the samples, since oxygen concentrations close to 20% were measured days after boronization and following multiple plasma discharges. The increase in the concentration of oxygen on the sample seems to be correlated with the also increasing oxygen concentration in the plasma and its consequential decrease in performance due to the rise in power losses through line radiation. MAPP was also used in NSTX-U as a mean to com- pare boronization methodologies, in this way full (9.1 g d-TMB) and mini (1.8 g d-TMB) boronizations were evaluated from a surface science perspective. Mini boronizations were found to deposit less than 2 nm coatings, with concentrations of boron lower than 10%. Post-mortem analysis of cored samples, extracted from the tokamaks’ tiles and controlled laboratory experiments on boronized samples were performed as a complement to the data collected with MAPP. Controlled irradiations with D+ ions in the Ion-Gas and Neutrals Interactions with Surfaces (IGNIS) facility showed the removal via sputtering of thin oxidized boron coatings, a similar behavior was seen with Ar+ irradiations. XPS depth profiles per- formed on cored samples from different locations of NSTX-U revealed similar surface chemistry near the plasma striking points. The surface of these samples was dominated by oxides and they showed a close resemblance to the chemistry of the samples analyzed with MAPP. In contrast, one sample extracted from the inner lower divertor, the usual location of the private flux region (PFR) in diverted plasmas, revealed XPS spectra dissimilar to those of the other samples. In that case, the B1s region exhibit two XPS envelopes, while the remaining samples showed only one. We hypothesized that the additional peak is related to high energy ions or deuterium neutrals, common in the PFR. An additional experiment was carried out with one of the cores following the Ar+ irradiation done with the depth profile. Two D+ irradiations were performed and were followed by additional argon exposures. The irradiations with deuterium increased the amount of oxygen on the previously ”clean” sample, a similar phenomenon to that observed in NSTX-U with MAPP. Interestingly, the sequential Ar+ irradiation removed the oxides completely, leaving the surface as before the deuterium exposure. The emission of impurities from the samples under irradiation was monitored as well. Low emission was observed during the controlled irradiations with D+ and the XPS depth profiles as the mass spectra were dominated by the gas species used during the particular plasma exposure i.e. Ar or D. How- ever, the effect of water dangling bonds post Ar+ irradiation was observed during the XPS depth profiles exposures in addition to some impurities during the removal of the passivated layers, the latter in the early stages of the profiles. In addition, retention of deuterium was observed during the D+ irradiation of one of the core tiles, this was evidenced by the clear dependence of the partial pressure of deuterium on the ion flux during the Ar+ that followed the D+ exposures. The results presented in this work show that the improvements in plasma performance associated with boronization in NSTX-U are correlated with the reduction of impurities in the vacuum vessel. Oxygen related impurities are retained on the surface of the boronized PFC, and the binding process is driven by the presence of D+ plasmas. However, the improvements are only temporal, and according to the results in this document that is due to the enhanced sputtering yield of the oxidized boron carbide surfaces which turns the walls that previously acted as a sink on impurities in a source of those.
Issue Date:2017-08-22
Rights Information:© 2017 Carlos Felipe Bedoya Arroyave. All rights reserved.
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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