PFC Activities in Alcator C-Mod

1. 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

2. 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

3. W migration erosion scenarios • Strike point parameters for sputtering scenario. • Failed W tile leading to significant W melting.

4. 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.

5. 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.

6. 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.

7. 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

8. Cassette tile position labeling 1 2 3 4 5 6 7 8

9. 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

10. 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

11. 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 8 7 6 5 A-B cassette W 3 7 8 1 2 4 5 6

12. 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 mm (porosity was variable) Boron coated RF limiter tile

13. And it works! • Mo line brightness is decreased and no longer scales with ICRF power • Confinement no longer degrades with accumulated ICRF power

14. Post-campaign analysis • Boron layer has spalled and peeled in some locations. • Eddy currents show no large changes in layer thickness.

15. 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.

16. 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.

17. Back-up slides • Fine spatial resolution across leading edge of G-H divertor cassette

18. Back-up slides • Mo and W deposition on adjacent rows on the G-H divertor cassette.