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EU-PWI TF meeting, Warsaw, 4-6 November 2009 Fuel Removal SEWG report 2009 J. P. Coad On behalf of all those involved in Tasks for the SEWG Introduction.

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Presentation on theme: "EU-PWI TF meeting, Warsaw, 4-6 November 2009 Fuel Removal SEWG report 2009 J. P. Coad On behalf of all those involved in Tasks for the SEWG Introduction."— Presentation transcript:

1 EU-PWI TF meeting, Warsaw, 4-6 November 2009 Fuel Removal SEWG report 2009 J. P. Coad On behalf of all those involved in Tasks for the SEWG Introduction Chemical cleaning/inhibition using Oxygen and Nitrogen Tokamak cleaning and conditioning techniques Cleaning gaps and castellations Laser removal and in-situ application Conclusions

2 2/28 Chemical cleaning/inhibition using Oxygen and Nitrogen

3 3/28 Scavenging effect of ammonia and nitrogen injected at two positions: in the plasmas (blue) and in in front of the deposition sample (red). Note the higher values of mass 28 required for the same inhibition effect CONCLUSIONS - Better inhibition of carbon deposits by ammonia injection due to volatile HCN formation - NO difference between in-plasma/afterglow injection for ammonia PILOT PSI Experiments F Tabares, CIEMAT, in conjunction with FOM and MHEST

4 4/28 Radical/radical reaction Reaction 2: Delta NH3 ~ NH2 cc Reaction 1: Delta CH4 ~ CH3 cc Correlation of HCN formation from NH 3 /CH 4 in RF ICP M Mozetic, MHEST F Tabares, CIEMAT Best correlation for HCN formation with Reaction 1

5 10CH4/8N2/2H2 no scavenging effect has been detected complete suppression of deposition on powered electrode is attributed to direct sputtering of the growing film by N 2 + ions charge exchange reactions with molecules (methyl radicals) as well decrease the sticking probability At high rf power, in high fragmentation condition of molecules, a complete suppression of the deposition has been observed above 300 W. The inhibition of deposition was attributed to the gas phase reaction between the film precursors, like CxHy, and atomic nitrogen. These results lead to conclusion that the scavenger effect produces volatile molecules such as hydrocarbons and hydrogen cyanide which are pumped away. E Vassallo et al, ENEA-CRN Inhibition of a-C:H films by N 2 dilution in RF plasmas

6 Mass spectra of gas mixture passing through the discharge chamber (M 16, M 17 – NH 3 M 27 – HCN M 28 – N 2 ) At low RF powers, the NH 3 molecules are destroyed and N 2 molecules are formed, causing a rise of partial pressure of N 2. At generator powers above 600 W the nitrogen in the discharge interacts with a-C:H films deposited on the walls of the vessel, resulting in a production of HCN. Reaction of ammonia with a-C:H films in RF plasma NH 3 is injected into a discharge created in a CH 4 – H 2 mixture. Prior to introduction of NH 3, a-C:H film is formed on the walls of the discharge and experimental chamber. M Mozetic, MHEST

7 Experimental set-up: 1 – discharge chamber, 2 – radiofrequency generator, 4 – narrow separating tube, 5 – experimental chamber, 6 – retractable catalytic probe, 7 – stationary probe. Shaded area represents the plasma. Side-arm configuration: 1 – probe housing, 2 – kovar finishing part, 3 – glass tube, 4 – probe tip, 5 – inserted aluminium foil covered by a-C:H Measuring oxygen atom loss coeffs on a-C:H covered surfaces Material Roughness (nm) Hydrogen content (%) Recombination coefficient a-C:H40 ( ) a-C105 ( ) Al foil45N/A ( ) Results High reactivity => efficient a-C:H removal No electric charge => unaffected by magnetic fields Good selectivity => minimal substrate damage Neutral oxygen atoms: M Mozetic, MHEST

8 P. Panjan, MHEST Deposition of a-C(W/Mg):H films by triode sputtering target substrate holder weak plasma thermoionic arc hot filament inert gas, Ar anode reactive gas, C 2 H 2, N 2 heater Balzers sputtering system with thermionic arc Target: graphite, W / Mg Ar/C 2 H 2 atmosphere Surface temperature below 120 C Schematic of film AES Profile

9 AES profile of untreated sample AES profile after 5 min of plasma AES profile after 10 min of plasma Interaction of a-C/W:H with H plasma radicals Experiments with H 2 microwave plasma and concentrated sunlight were performed at the PROMES solar facility in Font-Romeu, France Pheat = 6 kW Tmax = 850 K Pmw = 1 kW natom = 2.5×1021 m 3 P Panjan, MHEST

10 Section summary Oxygen atoms can remove hydrocarbons effectively in several different experiments All the evidence indicates that NH 3 is more effective for removing hydrocarbons than N 2 HCN is the predominant volatile species resulting from removing carbon films Films containing substitutes for Be for ITER-like trials have been successfully developed

11 Tokamak cleaning and conditioning techniques

12 D2D2 H2H2 HD D 2 -GDC H 2 -GDC isotope exchange of D saturated walls by H-GDC formation of HD 6A H GDC : 1 x H + /s 1.2 x HD release (initial) depletion of D increase of H 2 release decrease of HD Quasi saturation after 22 min = H/cm 2 2 x D-atoms released = 6 x D/cm ,0 4,0x ,0x ,2x ,6x10 20 HD release( HD molecules/sec) Time (sec) HDproduction isotope exchange by GDC Lost H Released HD V Philipps, TEXTOR team

13 107826: first RF He/H2 after D 2 -GDC : third shot : first shot after D 2 -GDC with Bv :second shot with with B v + B r Rf pulse Smaller initial HD release compared with GDC at the beginning But HD release increasing shot by shot Shot to shot variation depends on a complex parameter field: Bt, gas composition, magnetic fields, wall history,.... Comparison: isotope exchange by RF He/H 2 ICWC on D saturated walls All at 2.3T HD increases in 6 consecutive shots V Philipps, TEXTOR team

14 Overall daily particle balance in He/H2 RF shots on D saturated walls. Hydrogen injection (left scale) is about 10 times hydrogen release from walls (H2 and HD) (right scale). D 2 GDC was done at the red arrow marked points. As can be seen, after fresh GDC, the walls tends to saturate shot by shot after, but no saturation is reached in the (3-5 RF) shots. RF shots at low Bt ( blue points) show more hydrogen consumption, since larger areas are wetted by RF D 2 GDC injection Low Bt (0.23T) V Philipps, TEXTOR team Overview of 2 days ICWC experiments on TEXTOR

15 D removal and H implantation during ICWC on Tore Supra During ~ 850 sec. of CW ICWC in He-H 2 : Total D desorbed : 3, D 2 monolayers Total H implanted : 3, H H implanted /D pumped = 9.4 After 15 min ICWC in He-H 2 : n H /(n H +n D ) from 5 50% Recovery from disruption after 3 mins pulsed He-ICWC D Douai, Tore Supra team

16 Pulsed He-ICWC discharge, duty cycle = 2 sec. ON / 8 sec. OFF Increase due to summation of aftershot pressure level Decrease due to wall desaturation (approach to p(H 2 )= 0) Duty cycle can be decreased 2:20 or more Arcing traces on antenna straps: unipolar arcs, plasma between FS and straps -too high RF voltage when operating RF antenna Pulsed He-ICWC discharges TS#43532 P RF ~60 kW, ~0,1 Pa D Douai, Tore Supra team

17 Both TEXTOR and Tore Supra have devoted campaign days to investigating ICWC during 2009 Both have concentrated on D/H isotope exchange using H 2 /He ICWC Resulting wall loading with H has then meant that He discharges have been required for plasma recovery (though relatively easy) Section summary

18 Cleaning gaps and castellations

19 CLEANING BY A PLASMA TORCH FROM INSIDE GAPS WP09-PWI-02-05/MEdC: CLEANING BY A PLASMA TORCH FROM INSIDE GAPS (influence of the geometric aspect ratio of castellated surfaces) Samples preparationScanning procedure Removal conditions Nitrogen flow = 8200 sccm RF power = 350 W Distance from top face of the built castellation = 2mm Scanning speed = 5mm/s Gap width = 0.5 – 1.5 mm Results: from profilometry Stainless steel cubes a-C:H layers Cubes coated with carbon inside gaps (20mm x23mmx20mm) A stripe of un-deposited layer is defined from top to bottom Conclusions: Removal of a-C:H layers from inside gaps demonstrated for gap widths mm - Narrower the gap, higher the removal rate - Higher removal rate at the gap entrance - Carbon removal is efficient even on the bottom of the gap (down to 23 mm) Profile of the remained layer at various scan numbers for gap width 500 microns Profilometry: film thickness: 2.2 microns 1 scan (4 sec)46 scans (184 sec)101 scan (404 s) Exemple: gap width 500 microns

20 20/28 tile gaps decay length: 1.5 times gap width Erosion of gap structures with remote oxygen GDC at 470 K: decay length: several times gap width at 270 K: plasma deposited a-C:H Si while at 270 K the decay length is too small to be effective it is largely increased at 470 K (larger than for tokamak deposition profiles) T Schwarz-Selinger and W Jacob, IPP

21 21/28 accessible to ITER direct plasma exposure: Å/s – nm/s Erosion of flat substrates with remote oxygen plasmas erosion of 70 nm hard a-C:H effective activation energy: 0.25 eV compared to 1.3 eV for thermo-oxidation erosion rate increases nearly exponential with surface temperature T Schwarz-Selinger and W Jacob, IPP

22 22/28 Cavity technique: surface loss probabilities β of neutral species Si substrates (reminder: = 1- reflection) at 270 K: = 0.5 Si substrates at 540 K: 0.1 explanation: two different species dominate erosion T Schwarz-Selinger and W Jacob, IPP

23 Summary of section Oxygen can only effectively clean gaps and castellations at elevated temperatures The challenge is still to find a method that may work on a practical timescale in ITER The challenge is still to find a method that may work on a practical timescale in ITER

24 Laser removal and in-situ application

25 25/28

26 26/28

27 27/28

28 Rolling from carrier LASK under vacuum : main issues LASK Carrier Environment : Very low pressure (10 -6 Pa) LASK very exigent with the carrier (Rolling/Pitching & Positioning) Scanned area reduction Fluence reduction Coverage has to be increased Limited dust collection efficiency Design constraints (laser collimation system size) An innovative system with limited efficiency LASK => Laser beam inclination required and dust collection by adhesion C Hernandez, CEA

29 LASK V1 P=10 -6 Pa T= 200°C max Environment : Dust collection : adhesion LASK V2 P=Atm T= 50°C Environment : Dust collection : aspiration Improvements to efficiency and vacuum compatibility Advantages No vacuum break Efficient dust collection Optimal laser ablation throughput Limited dust collection efficiency Limited laser ablation throughput (tilt effect) Vacuum break Disadvantages C Hernandez, CEA Diagnostic system Cleaning system

30 ASDEX sample 041 (graphite with 4 m layer of tungsten) Experimental set-up Nd:YAG laser: 1.063mm, 3.5 ns, 300 mJ, for crater size mm 13 GW/cm2 Quartz fibre collimator t delay = 100 ns, t exp = 500 ns, Movable holder Vacuum: 5x10 -5 Torr CII WI P Gasoir, IPPLM

31 ASDEX sample 041 (graphite with 4 m layer of tungsten) Line intensity dependence on number of laser pulses after 4 laser shots Carbon appears, which suggests that during 1 laser pulse 1 m of surface layer is removed continued presence of tungsten for a longer time than expected could be explained by the melting of the metallic tungsten P Gasoir, IPPLM

32 Images of dust generation Acquisition time: 20 us Acquisition time: 50 us Acquisition time: 10 us Delay between laser shot and start of acquisition ~35 us. Best acquisition is with 19 us frame. Most of dust particles are released ~40 us after laser shot. P Gasoir, IPPLM

33 Target: Pure graphite plate slots for TEM nets holder for a SEM sticker 4 cm Before exposure After exposure Dust generation under laser light impact Collector: Aluminum plate with TEM nets and SEM sticker Experiment 1Experiment 2 Number of shots30100 Deposited energy/shot 0.76 J0.33 J Crater depth17 µm24 µm Crater volume0.017 mm mm -3 Ablated material1.6·10 18 atoms1.28·10 18 atoms M Rubel, VR

34 Microscopy after J TEM nets located near the irradiated spot are destroyed 200 µm 2 µm 500 nm Fine dust (0.5 – 3 m) is generated during graphite irradiation even with laser pulses of moderate power. Next step: Irradiation of PFC and probes (incl. NB41) with co-deposits. M Rubel, VR

35 Collection of Dust in TEXTOR Vacuum cleaning using a cascade set of filters. Collection on carbon stickers (adhesive) Collection on nets for TEM Scraping-off co-deposits from various locations Separation of non-magnetic and magnetic fraction Deposition (1) and erosion (2) zones on ALT; bottom of the liner (3); main poloidal limiters (4); DED bottom shield (5); inner bumper (6) M Rubel, VR

36 Fuel in TEXTOR ALT-II tile In TEXTOR fuel is mainly retained in flaking co-deposits on the ALT-II limiter tiles. Therefore, flakes from ALT-II were taken for long-term outgasing: 70 hours at 573 K Fast temperature increase to h at 1273 K M Rubel, VR

37 Long-term Fuel Desorption at 623 K To determine efficiency of release at maximum baking temperature of ITER divertor ALT-II: final stage: jump to 1273 KALT-II: start-up stage 623 K Release of fuel during the long-term desorption HeatingH 2 [%]HD [%]D 2 [%] 350 ºC for 3 days ºC (final stage) Summary: Only ~10% of D released at 623 K. Efficient thermal fuel removal requires baking at high temperature. M Rubel, VR

38 Laser cleaning has been demonstrated in situ in Tore Supra Laser cleaning has to be accompanied by dust collection. This is only efficient at atmospheric pressure and low temperature. Removal of W films on graphite demonstrated. Dust from tokamaks heated to the ITER vessel temperature will only release ~10% of the contained tritium Section summary

39 SEWG – Fuel Removal Conclusions: i. All the evidence shows that NH 3 is a more effective scavenger than N 2, and HCN is the important volatile species ii. TEXTOR and Tore Supra have each devoted campaign days to development of ICWC iii. Standard films that simulate likely deposition in ITER (with a replacement for beryllium) have been developed iv. Consideration is now being given to applicability of chemical and photonic cleaning methods on ITER v. Removal rates for co-deposit trapped in tile gaps/castellations still require improvement to be applicable vi. The impact of repetitive oxidising plasmas (GDC/RF) on beryllium bulk, and removal of beryllium oxide have yet to be explored

40 Baseline Support Summary Table TA: Fuel removal *)Completed, Partially done, Not done Task id.Associations involved Manpower (PPY) Status*)Short description with milestones/deliverables WP09-PWI-02-05/CEA/BS a)Optimisation of Wall Conditioning Techniques in presence of a permanent magnetic field. b)Characterization of carbon erosion during O2-glow discharges CEA programme completed Not done Two days of experimental campaign time on Tore Supra devoted to ICWC Vessel tank not available WP09-PWI-02-06/CEA/BS a)Improve the understanding of the film break-up processes in laser cleaning b)Test of in situ laser layer removal technique in Tore Supra c)Studies of diags/detri demonstration with remote handling tools (LASK Project) d)In vessel tritium diagnostic optimisation CEA Not done Completed 2009 programme completed No effort available Integrated with DITS project Designs of diagnostic and cleaning versions made Unit can be vacuum, temperature and magnetic field compatible

41 Baseline Support Summary Table TA: Fuel Removal *)Completed, Partially done, Not done Task id.Associatio ns involved Manpower (PPY) Status*)Short description with milestones/deliverables WP09-PWI-02-02/CIEMAT/BS CIEMAT programme completed Scavenger experiments WP09-PWI-02-05/CIEMAT/BS CIEMAT0.5Not doneRemoval of films in gaps and castellations WP09-PWI-02-02/CNR/BS ENEA-CRN programme completed Scavenger experiments under RF fields WP09-PWI-02-02/FOM/BS FOM0.1CompletedScavenger experiments on PILOT-PSI WP09-PWI-02-05/FZJ/BS FZJ programme completed 2 days ICWC experiments carried out on TEXTOR IPP0.5Not doneProblems with Majestix system WP09-PWI-02-01/IPPLM/BS IPPLM1.0CompletedRemoval of C layers with Al, W WP09-PWI-02-06/IPPLM/BS IPPLM programme completed Film break-up under laser WP09-PWI-02-02/IPP/BS

42 Baseline Support Summary Table TA: Fuel Removal *)Completed, Partially done, Not done Task id.Associations involved Manpower (PPY)Status*)Short description with milestones/deliverables WP09-PWI-02-05/MEdC/BS MEdC programme completed Removal of films in gaps by plasma torch WP09-PWI-02-01/MHEST/BS MHEST0.2CompletedDeposition of C(W,Mg) films WP09-PWI-02-02/MHEST/BS MHEST programme completed WP09-PWI-02-05/MHEST/BS MHEST programme completed Removal of a-C:H films with neutral oxygen WP09-PWI-02-01/VR/BS VR programme completed Surface analysis before and after fuel removal

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