PFC Activities in Alcator C-Mod

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


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


W migration erosion scenarios

• Strike point parameters for sputtering scenario.

• Failed W tile leading to significant W melting.


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.


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.


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.


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


Cassette tile position labeling

12

345678


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


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


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


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


And it works!

• Mo line brightness is decreased and no longer scales with ICRF power

• Confinement no longer degrades with accumulated ICRF power


Post-campaign analysis

• Boron layer has spalled and peeled in some locations.

• Eddy currents show no large changes in layer thickness.


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.


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.


Back-up slides

• Fine spatial resolution across leading edge of G-H divertor cassette


Back-up slides

• Mo and W deposition on adjacent rows on the G-H divertor cassette.

PFC Activities in Alcator C-Mod

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