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|Title: ||Quantification of Chemical Erosion in the Divertor of the DIII-D Tokamak|
|Authors: ||McLean, Adam Gordon|
|Advisor: ||Stangeby, Peter C.|
|Department: ||Aerospace Science and Engineering|
|Issue Date: ||13-Apr-2010|
|Abstract: ||The International Thermonuclear Experimental Reactor (ITER) is currently designed to use graphite targets in the divertor for power handling and impurity control. Understanding and quantifying chemical sputtering is therefore key to the success of fusion as a clean energy source. The principal goal of this thesis is to design and carry out experiments, then analyze and interpret the results in order to elucidate the role of chemical sputtering in carbon sources in the DIII-D tokamak.
A self-contained gas puff system has been designed, constructed, and employed for in-situ study of chemical erosion. The porous plug injector (PPI) releases methane through a porous graphite surface into the divertor plasma at a precisely calibrated rate, minimizing perturbation to local plasma while replicating the immediate environment of methane molecules released from a solid graphite surface more accurately than done previously. For the first time in a tokamak environment, the methane flow rate used in a puffing experiment was the same order of magnitude as that expected from laboratory experiments for intrinsic chemical sputtering.
Effective photon efficiencies for injection are reported; results are found to have significant dependencies on surface conditions and the divertor operating regime. The contribution of sputtering processes to sources of C0 and C+ are assessed through measurement of background and incremental spectroscopic emissions of both physically and chemically-released sputtering products and by CI, 910 nm line profile fitting. Comparison of background and incremental emissions of chemically-released products demonstrate a dramatic drop in production of CH in cold and detached conditions. Finally, the chemical erosion yield is calculated in both attached and cold-divertor conditions and found to be much closer to that measured ex-situ in ion beam experiments than previously determined in DII-D.
These observations represent a positive result for ITER which will operate at all times with a detached divertor, i.e., a low chemical sputtering yield. Results and analysis techniques presented here point the direction for future experiments with the PPI for study of chemical sputtering in the tokamak edge environment.|
|Appears in Collections:||Doctoral|
Institute for Aerospace Studies - Doctoral theses
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