1. Experiments about carbon removal and codeposit inhibition J.A. Ferreira, F.L. Tabarés, W. Bohmeyer and A. Markin, I. Tanarro, V. Herrero

2. Introduction Tritium Retention Problem Removal of Codeposits Prevent the codeposition Different techniques for removal (Thermooxidation, GD cleaning, laser ablation, etc) Remove CFC´s Scavenger technique ELMs, plasma operation? Efficiency? Availability? Does it work in ITER?

3. Cleaning (Experiments carried out at CIEMAT) Cleaning chamber QMS chamber Carbon layers (~200nm) were produced using a He/CH 4 or a He/C 2 H 4 mixture by DC glow discharge Films were monitored using laser interferometry ( = 670 nm) Residual gas was sampled using a differential pumped mass spectrometer

4. Cleaning with N 2 GD Nitrogen-containing DC glow discharge plasmas have shown erosion rates ~ 3 nm/min Lower reactivity than He/O 2 plasmas (similar conditions 12 nm/min) Time (s) 01000200030004000 5000 6000 0 0.1 0 0.5 1 1.5 2 x 10 -7 16172552 182627 C 2 N 2 C 2 H 2 HCN H 2 O a b Does it depend on the chemical form?

5. Cleaning with NH 3 GD He: 18,80 sccm, NH3: 13,50 sccm (42%). 200 mA, 450V Erosion rate: 16,68 nm/min Main product: HCN Minor products: C 2 H 2, C 2 N 2 Some hydrogen of the film is released as H2 High erosion rate comparable Rate HCN/C2H2 = 4,73 18 th PSI (Toledo) J.A. Ferreira et al P1-53

6. Thermooxidation with NO 2 18 th PSI (Toledo) I. Tanarro et al P3-51 Left: Hard a-C:H film (~100 nm), deposited on a stainless steel sample. Right: After 1 hour thermal oxidation with O 2, 50 mbar, at 400ºC, the a-C:H film was nearly completely eliminated. In the case of NO 2 / N 2 (1:1), the films were completely eliminated even at 300ºC. 1 mm gap

7. Scavenger 18 th PSI (Toledo) W. Bohmeyer et al O-15 What is it? gas phase reactions of nitrogen or nitrogen containing species with the growth precursors. Formation of non-reactive species that can be pumped away CxHyCxHy NXNX + Stable non- reactive species F.L.Tabarés, V. Rohde and the AUG Team Nucl. Fusion 45 (2005) L27-L31.

8. Scavenger (PSI-2) No uncontrolled C x H y losses at cold walls The behavior of re-eroded species is studied Ions can not reach the collector plates 18 th PSI (Toledo) W. Bohmeyer et al O-15

9. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 QMS: CH 4 :C 2 H 2 :C 2 H 6 =3:4:2 High sticking species Pump Duct Deposition Profile

10. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Strong influence of nitrogen during methane injection No influence of nitrogen for clean plasma ( not enhanced erosion by nitrogen ) Scavenger effect: yes, it works ! Pump duct / nitrogen

11. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Pump duct: Collector temperature 310 and 330 K Nitrogen effect: Reduction of deposition (310 K) Shift to from deposition to erosion (330 K)

12. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Pump duct : deposition profile Ex situ thickness measurement (T coll not fixed, about 320 K) Strong influence of nitrogen

13. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 1.Into the pump duct: Weak effect 2.Into the hot tube: Stronger effect Nitrogen injection at different positions

14. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Neon injection Neon has no influence ! 8000120001600020000240002800032000 time / s 60 62 64 66 68 70 72 74 f i l m t h i c k n e s s / n m Plasma ON Plasma OFF 310 K 330 K 350 K 2 sccm CH 4 1.4 sccm Ne collector temperature:

15. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Protection of the collector area against ions by an additional limiter Ions are not included in the deposition process Main chamber experimental set-up

16. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Main chamber. He plasma No atomic hydrogen no HCN and NH x Deposition is not influenced no volume process No erosion no surface effect

17. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Main chamber. H 2 discharge, methane, nitrogen Enhanced erosion Surface Reaction Reduced deposition Surface and volume process

18. Scavenger (PSI-2) 18 th PSI (Toledo) W. Bohmeyer et al O-15 Nitrogen molecules are stable and dont influence film formation, but they are decomposed in the plasma. Close to the plasma (where energetic neutrals are included) enhanced erosion was detected (surface process) as well as a reduction of the deposition rate by a volume process Far away from the plasma was detected only a volume effect, the conversion of (high sticking) hydrocarbons to inert ones, the so called scavenger effect. Enhanced erosion was not found.

19. Scavengers (CIEMAT) Residual gas analysis and CTAMS spectra show a strong production of hydrocarbons (HCN/C 2 =0,1 C 2 H 2 /C 2 H 4 =3) A second peak of acetylene is also observed. Could not be explain as temperature is too high for being acetylene (unstable species???) Contribution of acetonitrile Carbon walls

20. Scavengers (CIEMAT) Scavenger mixtures (H 2 :N 2 :CH 4 ) on metallic walls show a very different ratio of products (ammonia and acetonitrile are the main products!) In our small set-up surface chemistry prevails Desorption of condensible products at liquid nitrogen temperature (CTAMS)

21. Conclusions Some alternatives based on nitrogen compounds could be of interest not only for tritium recovery (codeposition cleaning) but also for inhibition of the formation of codeposits Experiments carried out in PSI-2 have shown that nitrogen containing mixtures are able to stop codeposition in hidden areas (not reached by ions) Nitrogen reacts to form ammonia in hydrogen plasmas. Last results suggest that ammonia related molecules (NH x ) could be responsible of scavenger effect