Dokument: Impact of boronizations on impurity sources and performance in Wendelstein 7-X
| Titel: | Impact of boronizations on impurity sources and performance in Wendelstein 7-X | |||||||
| URL für Lesezeichen: | https://docserv.uni-duesseldorf.de/servlets/DocumentServlet?id=56214 | |||||||
| URN (NBN): | urn:nbn:de:hbz:061-20210514-104501-3 | |||||||
| Kollektion: | Dissertationen | |||||||
| Sprache: | Englisch | |||||||
| Dokumententyp: | Wissenschaftliche Abschlussarbeiten » Dissertation | |||||||
| Medientyp: | Text | |||||||
| Autor: | Sereda, Stepan [Autor] | |||||||
| Dateien: |
| |||||||
| Beitragende: | Prof. Dr. Liang, Yunfeng [Gutachter] Prof. Dr. Pretzler, Georg [Gutachter] | |||||||
| Dewey Dezimal-Klassifikation: | 500 Naturwissenschaften und Mathematik » 530 Physik | |||||||
| Beschreibung: | The Wendelstein 7-X (W7-X) stellarator has recently successfully completed its experimental campaign with the passively cooled test divertor unit (TDU) made of graphite. TDU is the plasma facing component where the most intensive plasma-surface interactions take place in a controlled manner and it provides particle and heat exhaust. In addition to TDU the plasma vessel also consisted of other elements made of carbon and the stainless steel wall. Still impurity influx into the plasma due to the plasma-surface interactions is unavoidable. In addition the recessed areas can be also sources of impurities. Specifically, the low-Z oxygen and carbon were the main plasma impurities at W7-X.
To tackle this issue boronization was applied – plasma-chemical in-situ deposition of amorphous boron-containing hydrogen films on all plasma-facing components in a helium glow discharge with 10$\%$ B$_2$H$_6$ [1]. The application of boronization has lead to one of the main achievements of the campaign: plasma operation at high core densities of more than 10$^{20}$ m$^{-3}$ due to the reduced radiation-induced density limit. This work mainly focuses on full characterization of the change of impurity influx at TDU, the corresponding effects of boronization on the performance of W7-X and the underlying mechanism. Photon influx spectroscopy of line emission of the atoms and ions of carbon, oxygen, boron and hydrogen allowed to deduce normalized (divided by hydrogen flux) influx of oxygen, carbon and boron. Those measurements were conducted with the overview spectrometer system during the complete experimental campaign. The photon emission spectroscopy data was supported by a number of other diagnostics provided at W7-X. In total three boronizations were applied during the second part of the TDU campaign. After the first boronization the oxygen to hydrogen flux ratio (normalized influx of oxygen) at the divertor substantially decreased by a factor of 10 and the carbon to hydrogen flux ratio (normalized influx of carbon) decreased by a factor of 4 as obtained from spectroscopy. In the same time, boron emission appeared in the spectra. Between the boronizations oxygen and carbon normalized influxes increased but never reached the pre-boronization values. With each subsequent boronization the oxygen level decreased even more, reaching the lowest values after the third boronization which were more than a factor of 100 lower than before the first boronization and the boron level was increasing simultaneously. Such a decrease in low-Z impurity concentration significantly extended the operation window of W7-X in terms of line-integrated electron density (from 4$\cdot$10$^{19}$ m$^{-2}$ to more than 1$\cdot$ 10$^{20}$ m$^{-2}$) and diamagnetic energy (from 330 kJ up to 510 kJ). Z$_{\text{eff}}$ decreased from 4.5 down to values close to 1.2 as obtained from bremsstrahlung measurements. The above mentioned changes in the line-integrated electron density, diamagnetic energy and Z$_{\text{eff}}$ are given for the two reference discharges before and after boronization. Plasma conditions did not significantly deteriorate between boronizations allowing device operation without glow-discharge cleaning. The decrease in impurity influx can be explained in terms of effective oxygen gettering by boron in the remote plasma-facing components of W7-X, while at the strike line area it is quickly eroded. Not only the freshly available boron plays an important role in this processes, but also redeposited layers provide newly available boron atoms. This thesis demonstrates boronization effects on the high performance of W7-X and answers question about the underlying mechanisms. These findings are discussed in the view of the future steady-state experimental campaign of W7-X, when steady-state discharges of up-to 30 min duration are planned. [1] J. Winter et al. “Boronization in Textor”. In:Journal of Nuclear Materials 162-164(1989), pp. 713–723. | |||||||
| Lizenz: |  Urheberrechtsschutz | |||||||
| Fachbereich / Einrichtung: | Mathematisch- Naturwissenschaftliche Fakultät » WE Physik » Laser- und Plasmaphysik | |||||||
| Dokument erstellt am: | 14.05.2021 | |||||||
| Dateien geändert am: | 14.05.2021 | |||||||
| Promotionsantrag am: | 02.03.2021 | |||||||
| Datum der Promotion: | 28.04.2021 |
