Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000. / Includes bibliographical references (p. 187-191). / In this thesis a study of molybdenum sources and screening in Alcator C-Mod is presented. This work contributes in characterizing the performance of molybdenum, a high Z material, as a tokamak first wall surface. Understanding the molybdenum production mechanism is crucial if one wants to minimize it. A simple physical sputtering model has been developed that calculates the molybdenum source at the divertor, providing satisfactory agreement with the spectroscopic results. The effect of deuterons, boron ions, and redeposited molybdenum incident on the target is included in the calculation which shows that the boron ions dominate the sputtering. It is also found that the probability of molybdenum being "promptly" redeposited (within a gyration after having been sputtered) can be as high as 80%. High probability of redeposition is favorable because it reduces the net erosion. Specifically, it has been found that although the molybdenum gross erosion peaks close to the separatrix, the net erosion peaks further away in the target plate. Three surfaces have been identified spectroscopically in C-Mod to be significant sources of molybdenum: the inner wall, the outer divertor and the antenna protection tiles. The inner wall is the only important source during limited plasma operation, while, in diverted discharges, the molybdenum generated there is very well screened by the plasma. In RF-heated diverted plasmas, it is believed that the antenna protection tiles are the source of most of the molybdenum that ends up in the core. The outer divertor can not be excluded as a contributor to the core molybdenum density but there are indications that it is often not the dominant source during RF heating. This result is significant since it is expected that the divertor target in the next generation fusion devices will be made primarily with a high-Z material. The study of boronization as a surface conditioning method which reduces the molybdenum source rates and core concentration has shown a varying effectiveness dependent on first wall surface location. The beneficial effects of boronization disappear rather fast for the outer divertor, last longer for the inner wall, with the antennas and plasma core benefiting the most. / by Dimitrios Pappas. / Ph.D.
| Identifer | oai:union.ndltd.org:MIT/oai:dspace.mit.edu:1721.1/8838 |
| Date | January 2000 |
| Creators | Pappas, Dimitrios |
| Contributors | Bruce Lipschultz., Massachusetts Institute of Technology. Dept. of Nuclear Engineering., Massachusetts Institute of Technology. Department of Nuclear Engineering, Massachusetts Institute of Technology. Department of Nuclear Science and Engineering |
| Publisher | Massachusetts Institute of Technology |
| Source Sets | M.I.T. Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 191 p., 12844737 bytes, 12844494 bytes, application/pdf, application/pdf, application/pdf |
| Rights | M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission., http://dspace.mit.edu/handle/1721.1/7582 |
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