1. CIPS SEWG FR, JET 2008C. Hopf O 2 /He glow discharge cleaning: Experience at IPP Christian Hopf, Volker Rohde, Wolfgang Jacob Max-Planck-Institut für Plasmaphysik

2. CIPS SEWG FR, JET 2008C. Hopf Experience at IPP ASDEX Upgrade (2005) He/O 2 DC glow discharge 5.4 A, ca. 600 V 2% O 2 + 98% He 6 x 10 -3 mbar 49 h total O 2 /He glow time AUG + laboratory experiments: (E)Efficiency of He/O 2 mixtures as function of the mixture ratio (A)Accessible locations (B)Effect of boron in carbon redeposits (C)Collateral damage: Physical sputtering of various materials and oxidation of tungsten

3. CIPS SEWG FR, JET 2008C. Hopf (E) Efficiency of He/O 2 mixtures

4. CIPS SEWG FR, JET 2008C. Hopf (E) Why He/O 2 mixtures? Use of He/O 2 mixtures rather than pure O 2 because of safety hazards (oil-sealed rotary pumps, dust). He has lower physical sputtering yields. pure O 2 DC glow discharges are hard to ignite/sustain. How does the erosion rate depend on the mixture ratio?

5. CIPS SEWG FR, JET 2008C. Hopf (E) Mass spectrometry during discharge in AUG I = 5.4 AO 2 /(O 2 +He) = 0.6 % erosion limited by ion flux density, not O 2 supply

6. CIPS SEWG FR, JET 2008C. Hopf (E) Dependence of rate on O 2 /(He + O 2 ) ratio Rate saturation already at 10 % O 2 ECR He/O 2 discharge Substrate bias: - 400 V Power constant: 150 W Total pressure constant: 0.5 Pa O 2 /He ratio changed

7. CIPS SEWG FR, JET 2008C. Hopf (E) Ion + O 2 synergism Particle beam experiment: Ar + (20 to 800 eV) plus thermal O 2 (background gas) C. Hopf,a M. Schlüter, and W. Jacob, Appl. Phys. Lett. 90 (2007) 224106 C. Hopf, M. Schlüter, T. Schwarz-Selinger, and W. Jacob, New. J. Phys., submitted

8. CIPS SEWG FR, JET 2008C. Hopf (E) Ion + O 2 synergism Ar + + O 2 : O 2 flux dependence

9. CIPS SEWG FR, JET 2008C. Hopf (E) Rate Saturation Saturation at Ion energy j ion (cm -2 s - 1 ) p (Pa)j O2 (cm -2 s -1 )% O 2 AUG300 eV 1 10 14 0.6 < 7 10 15 < 0.5 ECR400 eV 5 10 15 0.5 1.2 10 17 10 The required oxygen flux density scales roughly like the ion (energ y) flux density.

10. CIPS SEWG FR, JET 2008C. Hopf (A) Accessible Locations

11. CIPS SEWG FR, JET 2008C. Hopf (A) a-C:H samples in AUG sectors 6, 7, 8 sectors 3, 11, 15 C removed No C removed Glow discharge accesses large surface area does not access shielded places, such as tile gaps, behind first wall, and deep in the divertor See T. Schwarz- Selinger, tile gap experients

12. CIPS SEWG FR, JET 2008C. Hopf (B) Effect of Boron in Redeposited Layers

13. CIPS SEWG FR, JET 2008C. Hopf (B) Real layers a-C:HTokamak redeposits Laboratory glow discharge: Layer removed AUG: No signs of erosion Ion beam analysis of 2005 layers: Dominated by boron B/C >> 1

14. CIPS SEWG FR, JET 2008C. Hopf (B) Effect of boron on a-C:H erosion a-B:C:H films (varying B content) ECR oxygen discharge RF substrate bias -60 eV

15. CIPS SEWG FR, JET 2008C. Hopf (B) Effect of boron on a-C:H erosion a-B:C:H films (varying B content) ECR oxygen discharge RF substrate bias -60 eV Erosion rate drops by factor of 50. 20 % B factor 10 reduction

16. CIPS SEWG FR, JET 2008C. Hopf (B) Effect of boron on a-C:H erosion a-B:C:H films (varying B content) ECR oxygen discharge RF substrate bias -60 eV Erosion rate drops by factor of 40. 20 % B factor 10 reduction

17. CIPS SEWG FR, JET 2008C. Hopf (B) Effect of boron on a-C:H erosion a-B:C:H films (varying B content) ECR oxygen discharge RF substrate bias -60 V Boron erosion rate constant: Enrichment of boron at surface B eroded by physical sputtering Carbon eroded according to stoichiometry Erosion rate drops by factor of 40. 20 % B factor 10 reduction

18. CIPS SEWG FR, JET 2008C. Hopf (B) Arcing during DC glow discharge Layers conductivity decreased during venting prior to AUG GDC cleaning. Surface charged positively. Local arcs burnt holes through layer, tungsten, and graphite. No energetic ion bombardment on major parts of the films. Main reason for finding no erosion of the layers.

19. CIPS SEWG FR, JET 2008C. Hopf (B) Conclusion: High B concentrations in AUG layers would have significantly reduced their erosion rates but as they had become insulating they experienced no energetic ion flux and were not eroded at all.

20. CIPS SEWG FR, JET 2008C. Hopf (C) Collateral Damage

21. CIPS SEWG FR, JET 2008C. Hopf (C) Sputtering of C, Al, Fe, and W by He + and O + Reactivity of oxygen can increase (carbon) or decrease (Al, W) the sputtering yield.

22. CIPS SEWG FR, JET 2008C. Hopf (C) Collateral damage: Selectivity Erosion yields for 300 eV ion bombardment: to be compared to a yield of 1 C/ion for a-C:H or 0.1 C/ion for B-impurified films Selectivity: Keep ion energy low!

23. CIPS SEWG FR, JET 2008C. Hopf (C) Redeposition of sputtered metals O 2 /He DC GDC in stainless steel vessel: Analysis (chemical, XRF, XPS): Fe, Cr, Ni, O redeposited and oxidized SS

24. CIPS SEWG FR, JET 2008C. Hopf (C) Collateral damage II: Oxidation of W Oxidation of W saturates Rapid oxygen removal in H discharge

25. CIPS SEWG FR, JET 2008C. Hopf Summary Accessible locations: Critical issue: gaps O 2 /He mixtures: Required oxygen concentrations depend on ion energy flux density (understood by modelling) Typically 1–10% O 2 required Effect of impurities: Boron: Huge reduction of erosion rate even at low concentrations Other impurities that do not form volatile oxides: Experiments under well defined laboratory conditions needed Collateral damage: Selectivity: Keep ion energy low (problem for DC discharges) Oxidation: rapidly reversible, no problems reported after tokamak experiments

26. CIPS SEWG FR, JET 2008C. Hopf

27. CIPS SEWG FR, JET 2008C. Hopf (E) Ion + O 2 synergism

28. CIPS SEWG FR, JET 2008C. Hopf (E) Ion + O 2 synergism

29. CIPS SEWG FR, JET 2008C. Hopf (E) Ion + O 2 synergism Particle beam experiment: Ar+ (20 to 800 eV) plus thermal O 2 (background gas) C. Hopf,a M. Schlüter, and W. Jacob, Appl. Phys. Lett. 90 (2007) 224106 C. Hopf, M. Schlüter, T. Schwarz-Selinger, and W. Jacob, New. J. Phys., submitted

30. CIPS SEWG FR, JET 2008C. Hopf (3) a-C:H erosion samples: toroidal scan 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 No a-C:H erosion where arcing occurred C removed No C removed

31. CIPS SEWG FR, JET 2008C. Hopf a-C:H erosion samples: toroidal scan Anodes: Roof Baffle Outer Divertor Film thickness left of initial 350 nm No a-C:H erosion where arcing occurred

32. CIPS SEWG FR, JET 2008C. Hopf (B) Composition of AUG layers in 2005 Layers on all investigated tiles are boron and oxygen dominated No significant erosion No significant reduction of C or D concentration minimum and maximum concentrations on tiles

33. CIPS SEWG FR, JET 2008C. Hopf (E) Ion + O 2 synergism Ar + + O 2 : O 2 flux dependence

34. CIPS SEWG FR, JET 2008C. Hopf Where do the removed 25 g C come from? Eroded carbon: 25 g = 1.2 x 10 24 C atoms Ions:50 h x 5.4 A = 6 x 10 24 Ions Yield:0.2 (in lab experiment: 0.1) Arc pits:40 holes / cm 2 1 mm x 0.1 mm x 7 µm assumption: 20 m 2 affected 8 g C eroded Not W-covered surface:34 % Carbon dust Not analyzed tiles with C-rich layers