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Title: Fueling Studies on the Lithium Tokamak Experiment
Authors: Lundberg, Daniel
Advisors: Majeski, Richard
Kaita, Robert
Contributors: Plasma Physics Department
Keywords: Density profiles
Electron beam
Lithium walls
Molecular clusters
Plasma fueling
Subjects: Plasma physics
Issue Date: 2012
Publisher: Princeton, NJ : Princeton University
Abstract: Lithium plasma facing components reduce the flux of "recycled" particles en- tering the plasma edge from the plasma facing components. This results in increased external fueling requirements and provides the opportunity to control the magnitude and distribution of the incoming particle flux. It has been pre- dicted that the plasma density profile will then be determined by the deposition profile of the external fueling, rather than dominated by the recycled particle flux. A series of experiments on the Lithium Tokamak Experiment demonstrate that lithium wall coatings facilitate control of the neutral and plasma particle in- ventories. With fresh lithium coatings and careful gas injection programming, over 90% of the injected particle inventory can be absorbed in the lithium wall during a discharge. Furthermore, dramatic changes in the fueling requirements and plasma parameters were observed when lithium coatings were applied. This is largely due to the elimination of water as an impurity on the plasma facing components. A Molecular Cluster Injector (MCI) was developed for the fueling of LTX plas- mas. The MCI uses a supersonic nozzle, cooled to liquid nitrogen temperatures, to create the conditions necessary for molecular cluster formation. It has been predicted that molecular clusters will penetrate deeper into plasmas than gas- phase molecules via a reduced ionization cross-section and by improving the collimation of the neutral jet. Using an electron beam diagnostic, the densities of the cryogenic MCI are measured to be an order of magnitude higher than in the room-temperature jets formed with the same valve pressure. This indi- cates increased collimation relative to what would be expected from ideal gas dynamics alone. A systematic study of the fueling efficiencies achieved with the LTX fueling systems is presented. The fueling efficiency of the Supersonic Gas Injector (SGI) is demonstrated to be strongly dependent on the distance between the nozzle and plasma edge. The fueling efficiency of the gas puffer was improved by the addition of a guide tube to keep the gas flow collimated and directed towards the plasma edge. Contrary to results reported on other devices, the MCI yields fueling efficiencies that are at best equal to the SGI. This suggests that any enhanced penetration from cluster formation is nullified by the reduced velocity of the cryogenic gas. Furthermore, at very high neutral fluxes, the fueling efficiency of the MCI dropped off dramatically, suggesting that the build-up of pressure in the scrape-off-layer is producing a self-shielding effect. All of these results are consistent with the idea that the best gas-based fueling source is one that brings highly directed neutrals to the plasma edge, without being scattered or ionized in the scrape-off-layer. The density profiles of LTX plasmas are measured during external fueling with the gas puffer, SGI, and cryogenic MCI. While the density profile amplitude increases with higher fueling rates, the shape of the profile remains constant. This suggests that particle transport is such that differences in the neutral deposition cannot be discerned. The density profiles are consistently hollow, suggesting a region of reduced particle transport in the core of LTX plasmas.
Alternate format: The Mudd Manuscript Library retains one bound copy of each dissertation. Search for these copies in the library's main catalog
Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Plasma Physics

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