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Wall conditioning techniques: status and perspectives

Wall conditioning techniques: status and perspectives. S.Dal Bello, A.Canton, L.Grando, P.Innocente, S.Munaretto. Why wall conditioning is needed/desired First Wall conditioning in RFX-mod Effects of present wall conditioning techinques on plasma properties in RFX-mod and in other Experiments

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Wall conditioning techniques: status and perspectives

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  1. Wall conditioning techniques: status and perspectives S.Dal Bello, A.Canton, L.Grando, P.Innocente, S.Munaretto

  2. Why wall conditioning is needed/desired First Wall conditioning in RFX-mod Effects of present wall conditioning techinques on plasma properties in RFX-mod and in other Experiments Open points and activities aimed at improving the present wall conditionig effectiveness Conclusions Outline RFX-mod programme workshop 2011 - February 7- 9

  3. Why FW conditioning FW cleaning (after vessel venting) graphite is a good absorber of humidity (after the production, tiles were degassed at 2000°C) even the vessel is continuously vented by dry N2 (overpressure of hundreds of mbar), during the shutdown some air retro diffuses through the open porthole RFX-mod programme workshop 2011 - February 7- 9

  4. Why FW conditioning (2) Impurities control (performance) the graphite wall prevents the presence of high-Z impurities main impurities in RFX-mod: C, O2, B or Li (when conditioning procedures applied) Zeff in the range 2-4 RFX-mod programme workshop 2011 - February 7- 9

  5. Why FW conditioning (3) Wall desaturation at a certain time, reached the “saturation level” (that is not a parameter, yet) the removal of Hydrogen from the wall is necessary to operate the machine at “allowed” density values RFX-mod programme workshop 2011 - February 7- 9

  6. Why FW conditioning (4) First Wall reset Between to (very) different experimetal campaigns the status of the wall is considered needy of a reset. The resetting wall treatment should remove the memory of different PW interactions RFX-mod programme workshop 2011 - February 7- 9

  7. Why FW conditioning (5) Possible control of recycling – density (performance) We would try to modify the behaviour of the wall (recycling factor  1) increasing the operational range we have to modify/obtain the “desired” density, hopefully at different plasma currents. RFX-mod programme workshop 2011 - February 7- 9

  8. FW conditioning in RFX-mod What are the possibilities we have: BakingGlow Discharge Cleaning Plasma discharges (i.e. He)BoronizationLithization (performed by Glow Discharge @170°C) RT Pellet injector (solid Li) LLL used as limiter or evaporator (liquid Li) RFX-mod programme workshop 2011 - February 7- 9

  9. RF-assisted Glow Discharges in continuous flow Executed at Room Temperature (during experimental sessions as desaturation technique) or at high temperature - Temperature rised to 170°C and kept for the whole sequence - Baking (4h) for outgassing pressure stabilization - H2 GDC (30-40h) to extractO2, N2 (CH4 also formed and removed) - He GDC (6-10h) to remove H2 - Baking to favorite the removal of He (15h) RFX-mod programme workshop 2011 - February 7- 9

  10. Boronization @170°C by RF-assisted Glow Discharges in continuous flow precursor gas: B2H6 (Diborane) 10% - He 90% In 2010 (from Jan to Oct) we performed 4+1 boronizations: 30’, 120’, 240’, 240’ + 360’ (to possibly reduce the Li2CO3 formation/effects) RFX-mod programme workshop 2011 - February 7- 9

  11. 4x1020 atoms Pellet feature: V = 8.8 mm3 ∆ne~1020 m-3 Lithization: during pulses (pellets and LLL) or between pulses (LLL used as evaporator) In both the cases, we deposited in or on(?) the wall approximately 2g of Lithium RFX-mod programme workshop 2011 - February 7- 9

  12. Why wall conditioning is needed/desired • First Wall conditioning in RFX-mod • Effects of present wall conditioning techinques on plasma properties in RFX-mod and in other Experiments • Open points and activities aimed at improving the present wall conditionig effectiveness • Conclusions RFX-mod programme workshop 2011 - February 7- 9

  13. ... on impurities ... Impurity reduction • The main sources of impurity at RFX-mod are C and O • For both after Lithization the influx is lower than with clean graphite RFX-mod programme workshop 2011 - February 7- 9

  14. ... on H2 recycling... Increased wall gas adsorbing capacity Flat top electron densityversus total filled particles between two He-GDC performed to remove H from the wall After Lithization a greater quantity of filled particles does not lead losing density control H discharge post boronization Filled+Puffed atoms = 8.2*1020 Desorbed atoms = 6.9*1019 RFX-mod programme workshop 2011 - February 7- 9

  15. ... on density radial profile modification... • Different density radial profile • Statistically it can be verified by the Peaking Factor: PF = n(0)/<n> • On RFX-mod PFis a function of plasma current and average density • After the Lithization it is clearly higher than with a clean graphite first wall RFX-mod programme workshop 2011 - February 7- 9

  16. reference post Li ... on density at the edge ... • Edge density with Thermal Helium Beam diagnostic • AfterLithization edge density is lower than on cleangraphite • It has been confirmed by the cut off layer position measurements of a fixed frequency reflectometer • Measured cut-offlayer position is averaged over similar density discharges • In Li discharges cut-off layer position moves inward by about 1-2 cm RFX-mod programme workshop 2011 - February 7- 9

  17. ... on edge transport ... Gas Puffing Imaging diagnostic raw signals: measurement of edge density fluctuations Reference After Li to quantify the differences Autocorrelation time Packingfraction • larger edge blobs • less plasma volume occupied • different turbulent particles transport at the edge C Li C Li RFX-mod programme workshop 2011 - February 7- 9

  18. Lower Hαemission at the edge at the same density with a Li wall Improvement of the particles confinement time ... on global properties ... No difference on the energy confinement time with respect to the Graphite wall RFX-mod programme workshop 2011 - February 7- 9

  19. Why wall conditioning is needed/desired • First Wall conditioning in RFX-mod • Effects of present wall conditioning techinques on plasma properties in RFX-modand in other Experiments • Open points and activities aimed at improving the present wall conditionig effectiveness • Conclusions RFX-mod programme workshop 2011 - February 7- 9

  20. Effect of BORONIZATION Suppression of Oxygen contamiantion of plasma this is the main effect on all experiments (TEXTOR, FTU, TORE SUPRA, DIII-D...) Boron is a getter for Oxygen ! Suppression of contamination by wall material In the case of high-Z wall materials, the advantage is the reduction of Zeff (Alcator C-Mod, Asdex-U with W divertor...) In the case of carbon wall, boronization was seen to reduce the chemical sputtering of carbon due to Oxygen impurities (via formation of carbon-dioxide) [Hino et al. Thin Solid Films 253 (1994) 518] RFX-mod programme workshop 2011 - February 7- 9

  21. Effect of BORONIZATION Reduction in Hydrogen recycling DIII-D: They observe that particle fueling from plasma facing surfaces is reduced after boronization. They speculate that B binds H more effectively than C [Jackson et al., J. Nucl. Mat. 196-198 (1992), 236] TEXTOR: yes [Winter et al., J. Nucl. Mat. 162-164 (1989), 713] TFTR: no significant change in Hydrogenic recycling properties [Dylla, J. Nucl. Mat. 176-177 (1990), 337] RFX-mod programme workshop 2011 - February 7- 9

  22. TFTR Enhanced energy confinement time Reduction of the H recycling Peaking of plasma parameters Suppression of impurities [Mansfield et al. 1996 Phys. Plasmas 3 1892] NSTX Enhanced energy confinement time Increase in the total power radiated from the plasma Reduction in the edge electron density [Bell et al. 2009 PPCF 51 124054] Effect of LITHIZATION RFX-mod programme workshop 2011 - February 7- 9

  23. FTU Similar to Boronization but with better results Peaking of electron density radial profile Strong pumping effect of Lithium modificates the SOL Te [Apicella et al. 2007 Journal of Nuclear Materials363-365 1346] TJ-II Improved density control by external puffing Suppression of impurities [Tabarès et al. 2008 PPCF 50 124051] Effect of LITHIZATION RFX-mod programme workshop 2011 - February 7- 9

  24. Why wall conditioning is needed/desired • First Wall conditioning in RFX-mod • Effects of present wall conditioning techinques on plasma properties in RFX-mod and in other Experiments • Open points and activities aimed at improving the present wall conditionig effectiveness • Conclusions RFX-mod programme workshop 2011 - February 7- 9

  25. Are the (small) effects of the wall conditioning we see strongly affected by DISUNIFORMITY (non uniform deposition/interaction with the FW during treatment) and QUANTITY ? (thickness of Boron and Lithium layer deposited on the wall) RFX-mod programme workshop 2011 - February 7- 9

  26. Glow Discharge disuniformity in toroidal direction from ISIS measurements Confirmed also by analyses of samples (SIMS at IENI institute) RFX-mod programme workshop 2011 - February 7- 9

  27. By adding GDC anodes we can improve the uniformity, or better, the number of peaks but a good uniform density can only be reached with an number of GDC anodes that cannot be proposed ! Tests with a toroidal field (in the order of some mT) applied during the Glow Discharge process will be done in the near future (next P17 campaign) We could (hopefully) discover that only one anode is necessary to have a very good and uniform glow discharge ... In this case “Thanks toroidal filed ! ” RFX-mod programme workshop 2011 - February 7- 9

  28. If the GDC plant will be able to sustain the discharge with the toroidal field necessary to obtain a good uniformity the test will be done also during the boronizazion RFX-mod programme workshop 2011 - February 7- 9

  29. If possible,we could try to modify the position of our LLL (as evaporator or limiter in equatorial ports) or install new evaporator(s) RFX-mod programme workshop 2011 - February 7- 9

  30. ... just for example ... • CDU Proto-Liter evaporator • 100-200 mg/min evaporated lithium • Cos(Q)2 Lithium distribution • Horizontal set-up RFX-mod programme workshop 2011 - February 7- 9

  31. An idea could be the centrifugal Lithium multi-pellet injector ... RFX-mod programme workshop 2011 - February 7- 9

  32. Just questions ... Can we obtain the same plasma performances we obtained so far “simply” performing 5’He GDC shot by shot (or every two shots)? What about to try with daily i.e. 20’H2 + 40’He to maintain impurities at low values and a good wall absorbing capability far from boronization? RFX-mod programme workshop 2011 - February 7- 9

  33. if, at the end, to change the wall conditioning technique will not lead to a good improvement of the First Wall behaviour … … try to change the First Wall !! RFX-mod programme workshop 2011 - February 7- 9

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