RFX-mod programme workshop 2011 February 7 - 9, 2011 Wall conditioning techniques: status and perspectives S.Dal Bello, A.Canton, L.Grando, P.Innocente, S.Munaretto

RFX-mod programme workshop February 7- 9 Outline 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 February 7- 9 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 Why FW conditioning

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

RFX-mod programme workshop February 7- 9 Wall desaturation Why FW conditioning (3) 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 February 7- 9 First Wall reset Why FW conditioning (4) 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 February 7- 9 Possible control of recycling – density (performance) Why FW conditioning (5) 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 February 7- 9 FW conditioning in RFX-mod Baking Glow Discharge Cleaning Plasma discharges (i.e. He) Boronization Lithization What are the possibilities we have: RT Pellet injector (solid Li) LLL used as limiter or evaporator (liquid Li) (performed by Glow

RFX-mod programme workshop February Temperature rised to 170°C and kept for the whole sequence - Baking (4h) for outgassing pressure stabilization - H 2 GDC (30-40h) to extract O 2, N 2 (CH 4 also formed and removed) - He GDC (6-10h) to remove H 2 - Baking to favorite the removal of He (15h) RF-assisted Glow Discharges in continuous flow Executed at Room Temperature (during experimental sessions as desaturation technique) or at high temperature

RFX-mod programme workshop February 7- 9 precursor gas: B2H6 (Diborane) 10% - He 90% RF-assisted Glow Discharges in continuous flow In 2010 (from Jan to Oct) we performed 4+1 boronizations: 30’, 120’, 240’, 240’ + 360’ (to possibly reduce the Li 2 CO 3 formation/effects) by

RFX-mod programme workshop February 7- 9 Pellet feature: V = 8.8 mm 3 4x10 20 atoms ∆n e ~10 20 m -3 In both the cases, we deposited in or on (?) the wall approximately 2g of Lithium Lithization: during pulses (pellets and LLL) or between pulses (LLL used as evaporator)

RFX-mod programme workshop February 7- 9 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 February 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 February 7- 9 H discharge post boronization Filled+Puffed atoms = 8.2*10 20 Desorbed atoms = 6.9*10 19 Flat top electron density versus 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 Increased wall gas adsorbing capacity... on H2 recycling...

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

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

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

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

RFX-mod programme workshop February 7- 9 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 February 7- 9 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] Effect of BORONIZATION

RFX-mod programme workshop February 7- 9 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 (1992), 236] TEXTOR: yes [Winter et al., J. Nucl. Mat (1989), 713] TFTR: no significant change in Hydrogenic recycling properties [Dylla, J. Nucl. Mat (1990), 337] Effect of BORONIZATION

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

RFX-mod programme workshop February 7- 9 FTU  Similar to Boronization but with better results  Peaking of electron density radial profile  Strong pumping effect of Lithium modificates the SOL T e [Apicella et al Journal of Nuclear Materials ] TJ-II  Improved density control by external puffing  Suppression of impurities [Tabarès et al PPCF ] Effect of LITHIZATION

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

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

RFX-mod programme workshop February 7- 9 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 February 7- 9 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 February 7- 9 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 February 7- 9 CDU Proto-Liter evaporator mg/min evaporated lithium Cos   Lithium distribution Horizontal set-up... just for example...

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

RFX-mod programme workshop February 7- 9 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 February 7- 9 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 !!