1 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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 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, Jülich, Germany 4) Oak Ridge National Laboratory, Fusion Energy Division, TN , USA 5) Association EURATOM-TEKES, VTT Processes, PO Box 1608, 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 and further plans TEC Euratom

2 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 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 (1% ~1g). - In the EU TF on PWI, SWEG to study gas balance and fuel retention, to assess the processes of the fuel retention and to extrapolate to ITER. - SWEG meeting on gas balance and fuel retention at JET 11 and 12 July 2006 INTRODUCTION Results from different test beds and tokamaks Limiter and Divertor devices in EU: ASDEX Upgrade, JET, TEXTOR, Tore Supra, but also from Alcator-C, JT-60U, Triam

3 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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 s Machine (Limiter/Divertor), Scenario Conditioning and Material (Be - C – W)… Phase 1 TS

4 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Also observed on JT-60U… n e ~0.65 n GW Low fueling (low n e ) ~ no retention High fueling (high n e ) significant retention Kubo et al., IAEA 2006

5 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Strong retention in Alcator C-mod - Pulse duration of ~ 2 sec, but very high plasma density. -16 repeated discharges (~ 30 s plasma exposure w/o disruptions) - Retained D fluence remains linear with incident D ion to the wall at an average rate of 0.75% D Whyte et al., IAEA 2006 Metallic device (Mo, room temperature) Co-deposition ?

6 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Recovery after/between pulses Retention Short pulse ~ 10-30% Long pulse/Strong injection ~ 50% 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 t wall Recovery ~ retention in phase 1 Transient mechanism

7 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 D Whyte et al., IAEA 2006 Strong retention…and recovery in Alcator C-mod Net depletion of D fuel from the wall is observed Cummulative effect of planned disruption H/D recovery over a C-Mod run day.

8 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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 (at least in limiter machine) Phase 1 Phase 2

9 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Steady state retention – Saturation ? - Wall saturation is a local de-saturation of overheated PFCs. - BUT does not prevent and/or cancel retention in other areas (layers, gaps, below divertor…) - No wall saturation in the sense of no retention observed. - Uncontrolled outgassing is no more observed in fully actively cooled devices (TS); the source is constant. - Retention rate is also constant and for the same plasma, no history effect is observed. TS before before upgrade, only 80% actively cooled and no pumping Time (s) Central Line Density (10 19 m -2 ) MW MW MW MW MW MW - 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) C Grisolia et al., PSI 1999 TS T Nakano et al., IAEA 2004

10 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Integrated gas balance – Day - Week Integration over a campaign : long term retention Retention = N inj – N recovery - N disruptions - N cleaning Gas balance accuracy limited by the requirement to substract pairs of large numbers. 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 Fuel retention over period ~ day/week complementary method required: Post-mortem analysis of samples from limiters, main chamber, deposition in gaps in between tiles, below the limiter/divertor… But this analysis cannot include all PFCs and air (H 2 0) exposure during transfert. Post mortem analysis is a lower limit of the retention Recovery Disruption Cleaning …

11 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November JET pulse # ISP at horizontal tile integral erosion ISP at vertical tile integral deposition ISP at hori- zontal tile integral erosion ISP at hori- zontal tile integral erosion ISP at vertical tile integral deposition Quartz Micro Balance Integral deposition when inner strike point at vertical tile 3 Integral erosion when inner strike point at horizontal tile 4 QMB4 (LBSRP) integral behaviour for restart / commissioning phase frequency [Hz] G Esser et al.,

12 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 D/C 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: 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 J Likonen, P Coad et al., -D retention in the divertor: 3% (Mk-IIGB)

13 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 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: s -1 ( 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 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 W-coverage in ASDEX-Upgrade 2002/ /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 % – 98% M Mayer

15 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November /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

16 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Hydrogen retention and carbon deposition in JT-60U K Masaki et al., IAEA 2006 Highest (D+H) retention ~16x10 22 m -2 on layer on outer dome wing and highest concentration (D+H)/C ~13% In plasma-shadowed area underneath the dome, ~2 m layers found (8x10 19 Cs -1 ) and a very high concentration (D+H)/C ~80%

17 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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

18 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 M Sakamoto al., IAEA 2006 Real time measurement of Co deposition in TRIAM - In situ and real time measurement of erosion/deposition based on interference of a thin semi-transparent layer. - Located 7.5cm from the LCFS and viewing a poloidal limiter Growing rate ~2.3x10 -4 nm s -1 (~1.5x10 16 Mo m -2 s -1 ) Retention ~ Hs -1 (8x10 21 H after 5h25 of plasma) Similar to Alcator-C (11x10 21 D in 30 sec) …lower flux but longer duration! Constant increase of wall inventory and growth of deposited layer of Mo

19 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 In these samples the D is trapped in the 3.7 m deposited layer (~40%) - D located in depth (up to 10 m) >> the ion implantation (few nm) CFC samples (Sepcarb® N11) exposed in the SOL of TS 3.7 m No saturation observed with fluence

20 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Open porosity at the matrix/fibre interface significant role in D migration ? Analysis of these CFC samples (Sepcarb® N11) No modification of the C hybridization in both the CFC matrix and the fibers observed with Electron Energy Loss Spectroscopy (EELS). No C-D chemical bonding Transport mechanism and D migration in the bulk (8 m) to be investigated

21 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Summary Gas balance and fuel retention: Large data base with carbon showing common features for the retention (AUG, JET, TEXTOR, Tore Supra, but also JT-60U, LHD) - 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) New results w/o C as PFC: Full W (AUG) and W-Be (JET) (Alcator-C, Triam) Co-deposition cancelled with full metallic machine and therefore should significantly reduce the retention compare to Mo ! Future exp in AUG (series of experiments on gas balance proposed), JET (2 gas balance experiments late 2006 and early 2007) and TS (Sector of TPL removed for analysis) ITER: 200 Pam 3 s -1, D-T 50% ( Ts -1 for 400sec), assuming retention similar to carbon devices ~70 (5%) before reaching 350g detritiation

22 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Retention characterization In vessel deuterium not very sensitive to plasma scenarios, in particular higher recycling scenarios does not exhibit higher retention flux co-deposition could not be dominant LHCD power seems to have an influence power losses in the SOL might increase ion and neutral (CX) energies nl = m -2 nl = m -2 ICRH =2-4 MW D retention rate [Pa.m 3 /s] LHCD power [MW] He

23 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Different edge parameters … T e [ eV ] distance from LCFS [ mm ] Comparison of reciprocating probe measurements (upper port) nl = m -2 nl = m -2 2 MW ICRH Determination of deuterium and carbon fluxes from D and CII spectral line brightnesss - Low n e, LHCD only (GJ): D ~ D/m 2 /s C ~ C/m 2 /s - High n e, LHCD+ICRH: D ~ D/m 2 /s C ~ C/m 2 /s Recycling increased by a factor ~2 D emission (6563 Å) TPL n e [10 18 m -3 ] distance from LCFS [ mm ] (with 3 MW of LHCD)

24 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 What about active pumping? no effect on the dynamic in vessel D retention

25 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006

26 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Status of knowledge on D retention in carbon materials Retention of implanted D in graphite saturates at about 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

27 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 DTE1 experiments in JET

28 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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 %)

29 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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 T(°C)

30 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Comparison of 2002/2003 and 2004/ / /2005 No significant change in D retention despite replacement of C by W in main chamber M Mayer et al., PSI 2004

31 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 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

32 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Adsorption : candidate for phase 1 Outgassing after shot ~ phase 1 duration (~ 100s) : ok with filling / emptying the porosity reservoir Good candidate for phase 1 BUT : extrapolation from lab to tokamak environment (temperature, pressure …) Adsorption : weak bond ( chemical bond) ok for release after shot, identical shot to shot behaviour, recovery independent of cumulated inventory Recovery ~ retention in phase 1 : transient retention mechanism x [E. Tsitrone et al., IAEA 2004] Deposited layers (10 22 D/g) >> virgin CFC >> graphite : enough to account for retention in phase 1 for TS based on lab experiments [P. Roubin et al., JNM 2005] wall t

33 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Codeposition Carbon balance : roughly coherent in most machines Net erosion rate : nm / s Net redeposition rate : nm / s up to 20 nm/s No coherence between D retention rate / D/C ratio / C source Implication for codeposition : Particle balance wall << Spectroscopy + Post mortem Post mortem (1 to 10 %) codep ~ C * D/C C Divertors (JET, AUG, JT60U)Limiters (TS, Textor) Net erosionouter divertormain plasma interaction zone Net depositioninner divertor + remote areas (louvers) shadowed or far SOL areas + neutralisers NB1 : codep ok for T retention for JET DTE1 (D/C 1 in cold deposits, 1 kg of flakes in subdivertor) [N. Bekris et al., JNM 2005] NB2 : codep with Mo also observed in TRIAM-1M [H. Zushi et al., Nuc Fus 45 (2005)] M. Sakamoto, P1-26 [E. Tsitrone et al., IAEA 2004] [N. Asakura, PPCF 2004] Factor 10 for TS Factor 50 to 400 for JT60U C. Brosset, P2-79

34 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 [E. Tsitrone et al., ITPA div and SOL 2006] C Dmax = at/m 2 Bulk diffusion : explanation for TS ? Simple model for preliminary estimates : D + flux on TPL implanted up to C Dmax, Implantation Codep D/C = 1 Codep D/C = 0.1 Gas permeation through open pores Molecular diffusion (dissociation/ recombination) Trapping at edge of crystallites (90 %) Trapping at interstitial sites (10 %) [H. Atsumi et al., JNM (2003)] Pressure dependent T. Hayashi P2-85 Absorption rate Desorption rate : detrapping Transient retention (Phase 1) Long term retention (if long pulse / high flux) short pulse long pulse for fuel retention (cyclic continuous exposure) Evidenced in lab experiments at high fluence Key parameter : exposure time Retained fraction (fluence) 0.5 [J. Roth et al., ITPA div and SOL 2006] J. Roth P1-85 Tail of D at low D/C in JT60U outer div tiles (several m) [T. Hayashi et al., JNM 349 (2006)] exp Impl + bulk diffusion then retained fraction (fluence) 0.5

35 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Adsorption : porosity filled, easily recovered Candidate for phase 1 Fuel retention mechanisms (in C) Implantation + saturates / permanent Codeposition : Linked with C erosion source Bulk diffusion + trapping : negligible ? Implantation : until C Dmax reached saturates / transient Adsorption Wall saturationNo saturation Trapped fuel in plasma interaction zoneTrapped fuel in shadowed areas does not saturate / permanent Bulk diffusion + does not saturate / permanent Codeposition +

36 Th LoarerGas balance and fuel retention – EU TF on PWI – 13 November 2006 Retention mechanism Adsorption : phase 1 Implantation (saturates, sensitive to T surf ) : JT60U Bulk diffusion (long pulse / high flux, high Te) : TS Codeposition (low Te, cold shadowed areas in direct line of sight of C source) : JET Main PWI limitations for long pulses : Localised heat loads (fast particles losses) Uncontrolled density rise (outgassing from PFCs) Fuel retention : During pulse : significant retention unless low fuelling or saturated walls Long term : ~ 0 for short pulse, significant for long pulse Density control Detritiation (depth in C) Detritiation (remote areas) Active cooling Main open issue : mixed materials (C/Be/W) ? See R. Doerner, R-4 ITER Limited (released after shot) Limited (reservoir >> plasma) (fluence) 0.5 (fluence)