1. 1 US 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

2. 2 US 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

3. 3 US 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.

4. 4 US 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.

5. 5 US 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.

6. 6 US 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.

7. 7 US 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

8. 8 US PFC Meeting, UCLA, August 3-6, 2010 Cassette tile position labeling 1 2 3 4 5 6 7 8

9. 9 US 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

10. 10 US 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

11. 11 US 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 87 654 3 21 756

12. 12 US 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

13. 13 US 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

14. 14 US 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.

15. 15 US 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.

16. 16 US 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.

17. 17 US PFC Meeting, UCLA, August 3-6, 2010 Back-up slides Fine spatial resolution across leading edge of G-H divertor cassette

18. 18 US PFC Meeting, UCLA, August 3-6, 2010 Back-up slides Mo and W deposition on adjacent rows on the G-H divertor cassette.