1. 1Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Th Loarer with contributions from C. Brosset 1, J. Bucalossi 1, P Coad 2, G Esser 3, J. Hogan 4, J Likonen 5, M Mayer 6, Ph Morgan 2, V Philipps 3, V. Rohde 6, J Roth 6, M Rubel 7, E Tsitrone 1, A Widdowson 2, EU TF on PWI and JET EFDA contributors Gas balance and fuel retention in Fusion Devices 1) Association EURATOM-CEA, CEA-Cadarache,13108 St Paul lez Durance, France. 2) Culham Science Centre, EURATOM-UKAEA Fusion Association, OX14 3DB, UK 3) Institute of Plasma Physics, Association EURATOM-FZJ, 52425 Jülich, Germany 4) Oak Ridge National Laboratory, Fusion Energy Division, TN37831-8072, USA 5) Association EURATOM-TEKES, VTT Processes, PO Box 1608, 02044 VTT Espoo, Finland. 6) Max-Planck IPP-EURATOM Association, Garching, Germany 7) Alfven Laboratory, Royal Institute of Technology, Association EURATOM-VR, Stockholm, Sweeden Outline: Gas balance and fuel retention  During a pulse, after/between pulses  Integrated over a day, a week and a full campaign  Fuel retention mechanisms Summary

2. 2Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom - Evaluation of the hydrogenic retention in present tokamaks is of crucial importance for the long discharges foreseen in ITER (400 sec ~ 7min). A retention of 5% of the T injected would lead to the limit of 350g ( working guideline for initial operation) in 70 pulses. INTRODUCTION Results from different tokamaks Limiter machine Divertor machine TEXTOR Tore Supra Full carbon Full Carbon Actively cooled Long discharges ASDEX Upgrade JET First wall: WHigh performances Divertor: C Carbon, Berylium - In the frame of the EU TF on PWI, efforts are underway to investigate the gas balance and fuel retention during discharges and integrated over experimental campaign. The aim is to assess the dominant processes of the fuel retention and to extrapolate to ITER.

3. 3Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Retention during pulse Significant retention unless : Low fuelling rate (Long L mode in JET) No influence of W observed between 2003 and 2005 in AUG (45 to 80% of W coverage) No influence of ELMs observed so far (W and/or C) Phase 2 : ~ constant retention rate Always a significant fraction of the injected flux (20-50%), but small fraction of the recycling flux (1-5%) Phase 2 Low fuelling AUG Common features on all devices : Phase 1 : decreasing retention rate ~ 1 to 50 s Machine (Limiter/Divertor), Scenario Conditioning and Material (Be - C – W)… Phase 1 TS

4. 4Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Recovery after/between pulses Small fraction recovered after shot, but > plasma content (C, C-W and Be) Independent of inventory cumulated during the pulse (TS, JET, AUG) Except for disruptions, this amount is independent of I p, B T, density, input power, fuelling method. [V. Mertens et al., EPS 2003] AUG JET  wall Retention Short pulse ~ 10-30% Long pulse/Strong injection ~ 50% t  Recovery ~ retention in phase 1  Transient mechanism

5. 5Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Integrated balance - Day --- Total Injected --- Total exhausted --- Outgased between pulses TS Short discharges Recovery between pulses is significant Cumulated inventory can be ~ recovered by conditionning (GDC…):  Overall balance ~0 Long discharges Same recovery between pulses but negligible compared to the overall balance  Significant inventory built up proportional to discharge duration

6. 6Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Steady state retention – Saturation ? - “Wall saturation” is a “local” de-saturation of overheated PFCs. - BUT does not prevent and/or cancel retention (layers, gaps, below divertor…) - Wall saturation in the sense of “no retention” has not been observed yet. TS before “before upgrade”, “only” 80% actively cooled and no pumping Time (s) Central Line Density (10 19 m -2 ) 26776 4.5MW 19249 2.5-3MW 19621 1.8MW 19622 1.8MW 19980 2.4MW 19976 2.4MW - Result of overheated PFCs and as T surf increases  outgassing Eventually, Outgassing > Exhaust  loss of density control (also observed on JET w/o pumping and JT-60U w div. pumping) - Uncontrolled outgassing is no more observed in “fully” actively cooled devices (TS); the source is constant. - Same plasma  same retention rate, no “history effect” observed. C Grisolia et al., PSI 1999 TS

7. 7Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Integrated gas balance – Day - Week Accuracy in gas balance studies likely limited by the requirement to substract pairs of large numbers, with inherent accuracy. For integrated balance of the order of week the accuracy strongly depends on - the “time” for the integration (pulse~10 sec, day~10 5 sec), - evaluation of the outgassing flux, D and C x H y released (disruptions)  Gas balance is an upper limit of the retention For integrated gas balance and fuel retention over periods longer than a day or a week, complementary methods are required:  Post-mortem analysis of samples from divertor/limiters, main chamber, deposition in gaps in between tiles, below the limiter/divertor… But this analysis cannot include all PFCs.  Post mortem analysis is a lower limit of the retention

8. 8Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom D/C 0.12 0.090.05 0.32 0.31 0.28 0.13 10.050.30.38 Fuel retention in JET (MKII GB) (NRA: D/C ratio, SIMS: layer thicknesses) Only plasma facing surfaces at divertor included (not tile gaps, inner limiters...) MkIIGB Divertor time: 57500 sec (16 hours) D injection: 766g Inner ion flux: 1.3x10 27 C deposition: 400g Rate: 3.4x10 20 Cs -1 Inner Divertor: D/C~0.2 Retention of 3% (25g) J Likonen, P Coad et al., - D retention in the divertor: 3% (Mk-IIGB), 2.4% (MKII-SRP).

9. 9Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom 2002/2003 campaign: Mainly carbon machine (45% W)  Retention governed by trapping on inner tile surface (70% inner divertor tiles, 20% in remote ares (below roof baffle,...)  Total retention ~4% of input (10-20% from gas balance) 2004/2005 campaign: Full W machine except the divertor (Carbon)  No significant difference in retention between 2002/2003 and 2004/2005 AUG: 2002/2003: Deposition of D and C M Mayer et al., PSI 2004

10. 10Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Retention mechanism Adsorption : phase 1 AUG, JET, TEXTOR, TS Implantation (saturates, sensitive to  T surf ) : TS, JET and JT60U Bulk diffusion (long pulse / high flux, high Te) Suspected to play a dominant role in long pulse in TS Codeposition (low Te, cold shadowed areas in direct line of sight of C source) : supposed to be the dominant process (AUG and JET) Density control Detritiation (depth in C) Detritiation (remote areas) ITER Limited (released after shot) Limited (reservoir >> plasma)  (fluence) 0.5 for CFC (Lab exp) (not for graphite)  (fluence) Fuel retention mechanisms (in C) Main open issue : Dominant retention mechanism with mixed materials (C/Be/W) ? Courtesy E Tsitrone

11. 11Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Summary Gas balance and fuel retention: Large data base with carbon showing common features for the retention (AUG, JET, TEXTOR, Tore Supra) - During pulse: significant retention unless low fuelling - Long term: ~0 for short pulse, significant for long discharges (TS) - No “wall saturation” (sense of no retention) is observed for actively cooled devices - Recovery after pulse independent of the cumulated inventory Retention in carbon dominated devices: 10-20% (Gas balance: upper limit) 3-4% (Post-mortem: lower limit) Still no influence of W (AUG: 80%) on the retention (ELMs ? AUG & JET) Co-deposition dominant process (AUG and JET) ITER: 200 Pam 3 s -1, D-T 50% (5 10 22 Ts -1, 400s), assuming retention similar to carbon devices 5%  limit to 70 pulses before reaching 350g  detritiation New results without C as PFC: Full W (AUG) and W-Be (JET)  Co-deposition cancelled with full metallic machine and therefore should significantly reduce the retention

12. 12Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom

13. 13Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom 38 g 73 g 55g 63g 300g 5g Total inner: 603 gTotal outer 380g Fuel retention in JET (MKII-SRP) - D retention in the divertor: 2.4% (MKII-SRP), 3% (Mk-IIGB), consistent with DTE1 results ~2% (Mk IIA, 0.2 g in tiles 0.5 g in 150 g flakes). - Lower limit: analysis does not include all PFCs (SRP, main chamber…) - Flakes in subdivertor after DTE1 ~1 kg : “seen” but not quantified ~ 3g MkII-SRP D injection: 1800g C dep: inner (outer): 603g (380g) C dep rate: 3.7 10 20 s -1 (2.2 10 20 s -1 ) Inner (outer) divertor D/C~0.3 (0.2) D retention inner: 1.6% (30g) D retention outer: 0.8% (12.6g) Total D retention 2.4% (42g), no SRP, no main chamber P Coad, A Windowson et al.,

14. 14Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom GAS BALANCE JET TEXTOR Tore Supra AUG (Integrated over the pulse duration) Balance verified at any time during and between pulses Particle Injection Gas, NBI, Pellets INJECTIONPLASMA Scenario EXHAUST (Vessel and Divertor) WALL (Retention), Scenario, PFCs,… “WALL” PLASMA “Mid-plane” Particle Exhaust Divertor

15. 15Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom W-coverage in ASDEX-Upgrade 2002/2003 2004/2005 Increasing coverage with W Regular boronizations about 8 per discharge period  Mainly effective in main chamber 6370 s 75.4 g D 3864 s 43.9 g D B-concentration in main chamber deposits 200280% 200574 – 98%

16. 16Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Comparison of 2002/2003 and 2004/2005 2002/2003 2004/2005 No significant change in D retention despite replacement of C by W in main chamber

17. 17Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Status of knowledge on D retention in carbon materials Retention of implanted D in graphite saturates at about 10 21 D/m 2 depending on energy →G. Staudenmaier, J. Roth et al., JNM 84 (1979) 149 No complete saturation for fine grain graphites and CFC, depending on porosity →A.A. Haasz et al., JNM 209 (1994) 155 →B. Emmoth et al., Nucl. Fusion 30 (1990),1140 →M. Balden et al., Phys. Scripta T103 (2003) 38

18. 18Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom DTE1 experiments in JET

19. 19Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Poloidal distribution of T in JET JET T (DTE1) : 6.1 g left (17%) before ”Venting” 2.4 g removed with H 2 O from air 3.7 g left (10%) [N. Bekris et al., JNM 2005]  3 g remaining in subdivertor flakes (~1 kg : seen but not quantified) 0.2 g in tiles 0.5 g in 150 g flakes (D/C~1 in cold deposits) 0.7 g found (2 %)

20. 20Th LoarerGas balance and fuel retention – IAEA Chengdu – 18 October 2006 TEC Euratom Sputtering of C by D : Temperature (K) Sputtering yield (atoms/ion) 1 keV D on C Implications of T surf  cte To be kept in mind when interpreteting experiments with evolving Tsurf [Nuc. Fus special issue 1, 1991] (°C) Temperature (°C) Saturated concentration of D in C : Fuel retention : implantation / desorption Net wall pumping  outgassing Fuel retention : codeposition T surf : key parameter for “chemistry” of carbon Chemical erosion Phys. Sputt RES Thermal sublimation 200 450 700 950 1200 1450 1700 T(°C)