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Th Loarer with contributions from

Gas balance and fuel retention in Fusion Devices. Th Loarer with contributions from

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Th Loarer with contributions from

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  1. Gas balance and fuel retention in Fusion Devices Th Loarer with contributions from C. Brosset1, J. Bucalossi1, P Coad2, G Esser3, J. Hogan4, J Likonen5, M Mayer6, Ph Morgan2, V Philipps3, V. Rohde6, J Roth6, M Rubel7, E Tsitrone1 , A Widdowson2, EU TF on PWI and JET EFDA contributors • 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 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

  2. Results from different tokamaks Divertor machine Limiter machine TEXTOR Tore Supra Full carbon Full Carbon Actively cooled Long discharges ASDEX Upgrade JET First wall: W High performances Divertor: C Carbon, Berylium INTRODUCTION • 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. - 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. TS Common features on all devices : Phase 1 Phase 1 : decreasing retention rate ~ 1 to 50 s Machine (Limiter/Divertor), Scenario Conditioning and Material (Be - C – W)… Phase 2 Phase 2 : ~ constant retention rate Always a significant fraction of the injected flux (20-50%), but small fraction of the recycling flux (1-5%) AUG Low fuelling 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)

  4. 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 Ip, BT, density, input power, fuelling method. JET Gwall • Recovery • ~ retention in phase 1 •  Transient mechanism  t Retention Short pulse ~ 10-30% Long pulse/Strong injection ~50% AUG [V. Mertens et al., EPS 2003] Recovery after/between pulses

  5. --- Total Injected --- Total exhausted --- Outgased between pulses TS Integrated balance - Day • Short discharges • Recovery between pulses is significant • Cumulated inventory can be ~ recovered by • conditionning (GDC…): •  Overall balance ~0 • Long discharges • Samerecovery between pulses but negligible compared to the overall balance •  Significant inventory built up proportional to discharge duration

  6. TS before “before upgrade”, “only” 80% actively cooled and no pumping - Result of overheated PFCs and as Tsurf increases  outgassing Eventually, Outgassing > Exhaust  loss of density control (also observed on JET w/o pumping and JT-60U w div. pumping) 26776 4.5MW 19622 1.8MW 19249 2.5-3MW 19976 2.4MW Central Line Density (1019 m-2) 19621 1.8MW 19980 2.4MW Time (s) Steady state retention – Saturation ? C Grisolia et al., PSI 1999 • 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. TS • - “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.

  7. 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~105 sec), - evaluation of the outgassing flux, D and CxHy 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. D/C 0.12 0.32 0.31 0.28 0.13 1 0.05 0.3 0.05 0.09 0.38 Fuel retention in JET (MKII GB) J Likonen, P Coad et al., MkIIGB Divertor time: 57500 sec (16 hours) D injection: 766g Inner ion flux: 1.3x1027 C deposition: 400g Rate: 3.4x1020Cs-1 Inner Divertor: D/C~0.2 Retention of 3% (25g) (NRA: D/C ratio, SIMS: layer thicknesses) Only plasma facing surfaces at divertor included (not tile gaps, inner limiters...) • D retention in the divertor: 3% (Mk-IIGB),2.4% (MKII-SRP).

  9. AUG: 2002/2003: Deposition of D and C M Mayer et al., PSI 2004 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

  10. Fuel retention mechanisms (in C) Density control Detritiation (depth in C) Detritiation (remote areas) Courtesy E Tsitrone Main open issue : Dominant retention mechanism with mixed materials (C/Be/W) ?

  11. 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 Pam3s-1, D-T 50% (5 1022Ts-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. 73 g 38 g 55g 63g 5g 300g Total inner: 603 g Total outer 380g Fuel retention in JET (MKII-SRP) P Coad, A Windowson et al., MkII-SRP D injection: 1800g C dep: inner (outer): 603g (380g) C dep rate: 3.7 1020s-1(2.2 1020s-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 • - 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

  13. GAS BALANCE “WALL” JET TEXTOR Tore Supra AUG (Integrated over the pulse duration) Particle Injection Gas, NBI, Pellets PLASMA Balance verified at any time during and between pulses “Mid-plane” Particle Exhaust Divertor WALL (Retention), Scenario, PFCs,… EXHAUST (Vessel and Divertor) INJECTION PLASMA Scenario

  14. 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 B-concentration in main chamber deposits 2002 80% 2005 74 – 98% 6370 s 75.4 g D 3864 s 43.9 g D

  15. Comparison of 2002/2003 and 2004/2005 2004/2005 2002/2003 No significant change in D retention despite replacement of C by W in main chamber

  16. Status of knowledge on D retention in carbon materials 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 Retention of implanted D in graphite saturates at about 1021 D/m2 depending on energy →G. Staudenmaier, J. Roth et al., JNM 84 (1979) 149

  17. DTE1 experiments in JET

  18. 3 g remaining in subdivertor flakes (~1 kg : seen but not quantified) [N. Bekris et al., JNM 2005] Poloidal distribution of T in JET JET T (DTE1) : 6.1 g left (17%) before ”Venting” 2.4 g removed with H2O from air 3.7 g left (10%) 0.2 g in tiles 0.5 g in 150 g flakes (D/C~1 in cold deposits) 0.7 g found (2 %)

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

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