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1US PFC Meeting, UCLA, August 3-6, 2010
PFC Activities in Alcator C-Mod
G.M. Wright, H. Barnard, B. Lipschultz, D.G. Whyte, S. Wukitch
Plasma Science & Fusion Center, MIT, Cambridge USA
2US PFC Meeting, UCLA, August 3-6, 2010
Outline
• Tungsten migration in the Alcator C-Mod divertor Sputtering scenario Melting scenario Discussion/Comparison
• Evolution of 100 um boron coatings on C-Mod upper divertor shelf and antenna limiters Deposition/creation Evolution Discussion
3US PFC Meeting, UCLA, August 3-6, 2010
W migration erosion scenarios
• Strike point parameters for sputtering scenario.
• Failed W tile leading to significant W melting.
4US PFC Meeting, UCLA, August 3-6, 2010
W migration PIXE set-up
• 1-3 MeV protons yield probing depths of up to 10 μm
• External beam allows for individual tiles or full divertor cassettes to be manipulated and analyzed.
5US PFC Meeting, UCLA, August 3-6, 2010
Sputtering case: dome and outer divertor
• Outer divertor shows high levels of prompt re-deposition
• Inner divertor shows neutral transport and ion transport through the private flux region.
6US PFC Meeting, UCLA, August 3-6, 2010
Sputtering case inner divertor
• Deposition on the inner wall limiter shows significant deposition through a combination of ion transport through the private flux region and ion transport of the SOL and core W impurities to the inner strike point.
7US PFC Meeting, UCLA, August 3-6, 2010
Mechanical failure of a W tile lead to a leading edge and significant melting
• Strike-point could not be operated on the W row after the melting event.
Melted W tile
9US PFC Meeting, UCLA, August 3-6, 2010
Melting case: Outer divertor cassettes
• Large deposition on the outer divertor upper shelf (row 1-2).
• Non-uniform toroidally• Generally higher deposition than in
sputtering scenario.• High deposition in row 7 (below W)
W
10US PFC Meeting, UCLA, August 3-6, 2010
Melting case: Inner divertor B-tear cassette
• Deposition is not in excess of sputtering scenario
• Strike points were often run “above” melt layer or in the upper divertor
11US PFC Meeting, UCLA, August 3-6, 2010
Comparison of W deposition from sputtering and melting scenarios
• Regions of very high deposition on boron-coated tiles on outer divertor upper shelf. Effect of porous, thick B coating?
• Non-uniform toroidally in melting scenario. Transport is much less clear in the melting scenario
W
A-B cassette
8
87654321
7 56
12US PFC Meeting, UCLA, August 3-6, 2010
Thick Boron coating applied to reduce Mo impurities from ICRH-powered discharges
• Boron film is estimated to be about 50% theoretical density of boron.
• Eddy current measurements confirm thickness between 75-150 m (porosity was variable)
Boron coated RF limiter tile
13US PFC Meeting, UCLA, August 3-6, 2010
And it works!
• Mo line brightness is decreased and no longer scales with ICRF power
• Confinement no longer degrades with accumulated ICRF power
14US PFC Meeting, UCLA, August 3-6, 2010
Post-campaign analysis
• Boron layer has spalled and peeled in some locations.
• Eddy currents show no large changes in layer thickness.
15US PFC Meeting, UCLA, August 3-6, 2010
PIXE analysis show presence of both Mo and W on the B-coated tiles
• W is due to deposition from marker tiles.
• Mo could also be deposition or shine-through from the bulk.
16US PFC Meeting, UCLA, August 3-6, 2010
Summary of B layer results
• ~100 μm boron layers on the outer wall limiter and outer divertor shelf have reduced core Mo impurities in ICRF heated discharges.
• Some tiles showed sections of spalling and peeling although this appeared to be on a tile-to-tile basis.
• Eddy current measurements show no large changes in layer thickness but PIXE analysis could be showing some shine-through from the bulk (depositon or pores?)
• PIXE shows presence of Mo and W that were not on the initial coating.
17US PFC Meeting, UCLA, August 3-6, 2010
Back-up slides
• Fine spatial resolution across leading edge of G-H divertor cassette