1. Laboratorio Nacional de Fusión 1/13 CLEANING EFFICIENCY OF CARBON FILMS BY OXYGEN PLASMAS IN THE PRESENCE OF METALLIC GETTERS Francisco L. Tabarés, J.A. Ferreira, D. Tafalla 1, I. Tanarro, V. Herrero 2, C. Gómez-Aleixandre and J.M. Albella 3 1 Laboratorio Nacional de Fusión (CIEMAT) 2 IEM. CSIC 3 ICMM. CSIC

2. Laboratorio Nacional de Fusión 2/13 MOTIVATION Tritium retention in PFM in ITER is a concern for: – Safety issues: maximum T allowed in vessel 350 g – Tritium global inventory in plant – Operation under carbon-dominated PFC scenarios Urgent need of in-situ de-tritiation techniques – Full oxidation of co-deposits by plasma techniques suited for cleaning areas exposed to the plasma: CO, CO 2 and T 2 O generated/pumped. – O and O + species generated by GD,ECR and ICR plasmas in He/O 2 mixtures But: Unknown effect of mixed materials (Be, W..)

3. Laboratorio Nacional de Fusión 3/13 Previous works: Examples Silicon: Thermo-oxidation: Higher T required for C/Si. SiO 2 remains in sample. Ref. Balden and Mayer. JNM(2000) O + irradiation: Enhanced O retention. Lower erosion yield. Ref. A. Refke et al. JNM(1997) Tungsten: Expected to catalyze oxidation of C. Deposited on top of a:C-H film and thermo-oxidized: Inhibition of C removal (surface effect?) Ref. Davis et al. JNM(2002) Lithium: Etching of C/Li films by O 2 + ions: At low E(<500): lower etching rate vs C At high E: same rate. Surface segregation of Li during oxidation. Ref. J.U. Thiele and P. Oelhafen, U. Basel Boron: Ample experience in boronized Tokamaks ( AUG, Textor, lab. experiments…) Inhibition of etching in O 2 /He plasmas. Ref. C. Hopf et al. JNM(2007)

4. Laboratorio Nacional de Fusión 4/13 C/B layers in He/O 2 Glow Discharge

5. Laboratorio Nacional de Fusión 5/13 Experimental set-up

6. Laboratorio Nacional de Fusión 6/13 In situ thickness monitoring Laser Interferometry: He/Ne or Diode laser (633,670 nm) Cross-checked with: – Profilometry – XPS – C balance

7. Laboratorio Nacional de Fusión 7/13 Carbon deposition He/CH 4 followed by Ar/CH 4 (80:20) 1Pa, 100 mA – Total deposited …270 nm Deposition in HeDeposition in Ar

8. Laboratorio Nacional de Fusión 8/13 a:C/H erosion Good balance of O atoms: O removed = O in CO,CO 2 CO/CO 2 =10 High erosion rate >10 nm/min Strong H 2 release End point by RGA=IF Higher rate over He produced films, but O balance? Lower erosion in the presence of released H??

9. Laboratorio Nacional de Fusión 9/13 Sequential deposition with Li C deposition in He/CH 4 +Li evaporation+Ar/CH 4 deposition:270 nm (C) He/O 2 GD removal of the film: decaying RGA signals – 50% of cracked O 2 missing!! Li layer?

10. Laboratorio Nacional de Fusión 10/13 C/Li mixing by simultaneous deposition O2O2 CO Constant erosion rate, ≈ pure C film. 50% of cracked O 2 missing!! 10% at. Li

11. Laboratorio Nacional de Fusión 11/13 Mg/C mixed films Why Mg?.. BeO MgO Hform(kJ/mol)-609 -601 Density (g/cm3) 3.01 3.58 M.P. (K) 2200 3073 Etching by He/O 2 Plasma: – Constant erosion rate – O balance: Not matched – Similar rate as in pure C 9% at. Mg

12. Laboratorio Nacional de Fusión 12/13 Particle balance Film RGA CC/Li layerC/Li mixedC/Mg mixed O2O2 1.8e-61.1e-63.3e-61.2e-6 O2O2 1.06e-63.8e-72.7e-69.13e-7 CO9.06e-71.5e-71.1e-66.08e-7 CO 2 1.09e-72.9e-82.0e-78.64e-8 H2H2 3.5e-61.4e-77.0e-64.07e-6 Erosion rate (nm/s) 0.180.060.230.19 CO+2CO 2 /  O 2 10.55 0.85

13. Laboratorio Nacional de Fusión 13/13 Conclusions Erosion of a-(C+M)/H films (M = Li, Mg) by He/O 2 GD monitored by laser IF and RGA: High erosion rates achieved, but not yet optimized. R>10 nm/min, similar to deposition rates in He/CH 4 plasmas, seen in pure C and mixed with Li and Mg No major changes in product distribution by M doping (but CO/CO 2 ?) Layered deposition (Li) slows down erosion, but recovers later. Results stress the impact of film structure on oxidation behavior More structural characterization required (in progress)