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Impact of lithium-coated walls on plasma performance in the TJ-II Stellarator

Impact of lithium-coated walls on plasma performance in the TJ-II Stellarator. J. Sánchez and the TJ-II Team Laboratorio Nacional de Fusión. CIEMAT 28040 Madrid, Spain. Outlook. Introduction Why Lithium? Plasma Wall Interaction in TJ-II Li coating technique Results

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Impact of lithium-coated walls on plasma performance in the TJ-II Stellarator

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  1. Impact of lithium-coated walls on plasma performance in the TJ-II Stellarator J. Sánchez and the TJ-II Team Laboratorio Nacional de Fusión. CIEMAT 28040 Madrid, Spain J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  2. Outlook • Introduction • Why Lithium? • Plasma Wall Interaction in TJ-II • Li coating technique • Results • Particle recycling and confinement • Edge parameters and impurity generation • Plasma profiles • Radiation and energy confinement • ELMs and Enhanced Confinement • Conclusions J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  3. TJ-II stellarator 4.6 m Vertical field coils Vacuum vessel Plasma Toroidal field coils Central coil system J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  4. Plasmas in TJ-II Heliac Stellarator 4 periods R=1.5 m <a>= 15-25 cm BT=1 T ECH : 2x300kW,53.6 GHz NBI:2x500 kW, 40 KeV Vol Plasma ~ 1m3 P0=5.10-8 mbar Low Z scenarios : - 2 Graphite Limiters - First Wall Boronization Scientific goals: Scan in magnetic configuration high operation J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  5. Why Li? - Very low Z - High impurity getter (O2,N2,CO, H2O,CO2…) - Strong H retention (LiH) - Low melting point: Liquid PFC - Effect on C sputtering OK (H. Sugai, JNM 1998) Very good results in Tokamaks: TFTR, CDX-U, FTU, T-10, T-11M…. Different ways of deposition; Liquid tray, pellets, LLL, CPS, evaporation…… But : problems in reproduce beneficial effect: Total coverage?? Lithium in Tokamaks TJ-II: first stellarator operated under Li walls J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  6. 0.12 0 -0.12 -0.24 -0.36 -0.2 0 0.2 P-W Interaction in TJ-II Central Coil Z (m) R-R (m) 0 J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  7. 0.12 0 -0.12 -0.24 -0.36 -0.2 0 0.2 P-W Interaction in TJ-II H a Central Coil Z (m) gass-puff hole R-R (m) 2 Instrumented Limiters, 180 º 0 But:no limiter effect for <2.5 cm insertion in ECRH plasmas J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  8. PW Interaction and Edge Diagnostics PWI mainly on Toroidal limiter (VV) Edge diagnostics:-Supersonic He beam (ne, Te, Ti)1,2 - Recip. Langmuir Probes - Reflectometry - Fast Cameras3 - LIF…. See P 1-18 Tabarés et al See P 2-43 Guzmán et al See P 3-39 Carralero et al J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  9. a t 4.5x1020 . o m s H Uncontrolled density GD He 1 2 2 8 0 1 2 2 8 5 1 2 2 9 0 1 2 2 9 5 1 2 3 0 S h o t ( # ) 10-15 dis. Density control under wall saturation (ECRH) Wall Conditioning: • Metal walls +He GD+Ar GD: He release, Enhanced Particle Confinement transition • (EPC, F.L. Tabarés et al PPCF 2001) • -Boronization: O-carborane+He GD, 4 Ovens:1g/oven, 80ºC • ….. wall saturation ( <20 discharges)+ EPC Particle balance in B Saturation @ 4.1020 Interaction area~ 1-2 m2 J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  10. Density control under wall saturation (ECRH) J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  11. Lithium coating in TJ-II How? • - 4 ovens, symmetric,tangential LOS • 4 g deposited each time (600ºC) • Role of background pressure: - HV: line of sight - 10-3-10-5mbar: diffusion - Ne GD+ ovens As deposited (HV) Deposition on top of B-coated walls Re-distribution by plasma: Improving with operation time!! After 2 days J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  12. Li Coating Technique 4 Lithium ovens: 2 fixed (side windows) and 2 in retractable manipulators (top windows) Emission of Li injected from a retractable oven 1 gr of Li per oven, heated to ~600 ºC during Ne GD J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  13. 3,5 3 2,5 2 1,5 1 0,5 0 1 2 3 4 5 Li-wall experimental campaigns in TJ-II • May-June 2007: 4 g fully evaporated under vacuum. Li-wall plasmas: ECRH/NBI H plasmas • Nov-Dec 2007: B wall reference discharges ECRH/NBI H Plasmas + Improvement of NBI and ECRH heating systems • Feb-June 2008: New Li-W Campaign: Refreshing of Li layer by repetitive evaporation: H/He/ECRH/NBI Plasmas Diamagnetic energy vs. plasma density Operational Window: B vs Li • wall / plasma • B / H • Li / H • Li / He Diamagnetic energy (kJ) line density (1019 m-3) J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  14. Li-wall experimental campaigns in TJ-II • May-June 2007: 4 g fully evaporated under vacuum. Li-wall plasmas: ECRH/NBI H plasmas • Nov-Dec 2007: B wall reference discharges ECRH/NBI H Plasmas + Improvement of NBI and ECRH heating systems • Feb-June 2008: New Li-W Campaign: Refreshing of Li layer by repetitive evaporation: H/He/ECRH/NBI Plasmas * Li/He (NBI 1+2) Operational Window: B vs Li • wall / plasma • B / H • Li / H • Li / He J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  15. Impact on plasmas ECRH B wall Li wall J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  16. Particle Control Li vs B Total wall inventory > 3 times, no sign of saturation J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  17. Dynamic particle balance dN/dt= f. Qin-N/(p/1-R) He plasmas: R<1!!, enhanced contamination For ECRH plasmas: f ~1, t peff ~8 ms, R<0.2 ne ne gas gas J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  18. Shot 17813 m2 Density control improving with plasma operation: m2=dne/dt approaching nominal beam fuelling rate! (~1020 e-/s) t ms Shot 17825 12 shots later m2 J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  19. J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  20. Edge Parameters-1 He beam at three different times: evolution of edge profiles J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  21. Edge Parameters-2 B wall Te t3 ne t1 t2 Li wall Te:Similar edge values but steeper profiles. Transient cooling in Li wall Ne:Transient rise of density gradient, but recovers in density-controlled discharges Te ne t2 t1 t3 J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  22. Impurity composition/generation Impurity (Li) generation SXR profiles • From spectroscopy, Li/ H~0.44% • for R=20%, Li/ H~9.10-4 8 µm Be But…expected?: 33 µm Be • Li/H= SH/1-S’ss, S’ss= Sss-(1-Rn) • Li/Hsputt~3.10-2 • Reduction>30x!! - No contribution of O radiation J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  23. Shot # 17941 5 250 Prad LITHIUM 4 200 ), Wdia (kJ), SRX (a.u.) 3 150 Prad (kW) ne Wdia 2 100 -3 m SXR -19 1 50 ne (10 Pnbi 0 0 1050 1100 1150 1200 time(ms) Profile shape: impurity & ne behavior same global parameters but... noncollapsing collapsing BELL DOME central impurity peaking 0.25 Wcm-3 0.14 Wcm-3 edge thermal instability J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  24. 4 0,6 3 0,4 2 0,2 1 0 0 0,2 0,4 0,6 0,8 1 Profile shape: impurity & ne behavior same global parameters but... noncollapsing collapsing BELL DOME ne DOMEBELL ne ne (10 Zeff eff Z 20 m -3 ) Zeff r eff J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  25. Energy Confinement Boron Li (dome) Li (bell) J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  26. ELM activity and transitions ELM J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  27. Coating System Upgrade • Searching for homogeneity: • 8 ovens, loaded for repetitive, in situ evaporation ( 6-8 cycles) • Diffusion can help: F dif.= 1/3 l.v.n/a, l @10-5 mbar He ~80cm x a He Li beam F parallel. = 1/2 n.v • Free Parameters: • Type of gas • Bkgnd pressure • Distance between ovens n (x)~1/x2.exp(-x/l) Fdiff (x)＝ n0exp(-x/l)/l.2pa, l~1/PHe Long term: LLL in TJ-II(?) J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  28. Conclusions • Li coating by evaporation was performed in TJ-II. • Only a partial coverage initially achieved, but evolved with plasma interaction • Machine operation more reliable and reproducible • Extended operational window • Density control highly improved, long lasting effect • Strong change in particle recycling • Good impurity control, but still C dominated (?) • Strong confinement improvement in NBI plasmas. • ELM activity observed at critical densities ~1.1.1013 cm-3 • Change in plasma profiles depending on fuelling rate/edge cooling • Improvement of technique still possible J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  29. Thanks for your attention! J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  30. Comments onBoronization • Wall Conditioning • Boronization: highly recommended! • How: In TJ-II: o-carborane sublimated at < 80ºC in He GD • 4 ovens, 4 anodes, 4 Turbopumps(4000l/s nominal), 1/2 h+ GD He • afterwards for H depletion: Good homogeneity…50 nm aprox. • but…it can be performed more easily • Parameter of merit: degree of dissociation o-carb < ~80% • 1 Turbo, 2 ovens, 1 anode ? • res= V/S~1-2 s in TJ-II, ne He GD ~109 cm-3, Te~3-5 eV dN/dt=Q/V-Kd.ne.N-N/r =0 Degree of cracking= N/N0=1/(1+Kd.ne.r) For high r, lower ne (Ip) required (but more film contamination?) J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

  31. H/He NBI • Similar Ti • values for H • and He target • plasmas! • Ti~Te at • high ne (up • to 140 eV in H • targets) H plasma He plasma J. Sánchez and the TJ-II Team, 18th PSI. Toledo,26-30 May 2008

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