1. with: R. Bell, D. Boyle, R. Kaita, T. Kozub, B. LeBlanc, M. Lucia, E. Merino, J. Schmitt, D. Stotler, G. Tchilingurian PPPL T. M. Biewer, T. K. Gray ORNL S. Kubota, W. A. Peebles, UCLA C. Hansen, T. Jarboe, University of Washington K. Tritz, Johns Hopkins University J. Bialek, Columbia University J. P. Allain, F. Bedoya, UIUC A.Capece, B. Koel, Princeton University K. Widman, P. Beiersdorfer, V. Soukhanovskii, F. Scotti, LLNL Supported by US DOE contracts DE-AC02-09CH11466 and DE-AC05-00OR22725 Dick Majeski Effect of solid and liquid lithium walls on confinement and equilibrium in LTX

2. Outline  LTX features –Ohmically heated tokamak, R=40, a=26, =1.6 –Hot, high Z shell construction  Lithium fill and electron beam evaporated coatings  Surface evolution of lithium coatings  Results with continuous high-field side gas fueling –Ohmic confinement with lithium PFCs –Core impurities  Electron temperature evolution when fueling is terminated –Implications for future operations  Plans for LTX-Upgrade  Summary

3. LTX features a hot, high Z, lithium compatible wall which incorporates lithium pools for evaporative coating Inner heated shell (explosively bonded SS on copper) Bottom of shells form reservoirs for up to 300 cm 3 liquid lithium Heat shielded centerstack Fast, uncased internal coil Flux loops 2-axis Mirnov coils 1.4 m

4. Lithium delivery system uses a simple weighted piston  Liquefy the lithium and it is ejected through outlet  Delivers 16 cm 3 of lithium per fill –Multiple fills required for lithium pools  Electroformed tungsten crucible with outlet –Tungsten piston  Lithium is delivered with a bellows-sealed motion stage –Dual gate valves (airlock) to prevent air exposure

5.  Electron beams are magnetically guided by low (~70 G) quasi steady-state magnetic fields  Rapid electron beam-driven evaporation from lithium pools –Simultaneous operation of both e-guns coats all four shells at once Electron gun #1 < 1.5 kW Beam trajectory in guide fields (one beam shown) Lithium pool Electron beam-based lithium evaporation system yields full lithium wall coatings Electron gun #2 < 2 kW

6. Coating sequence  Preheat shells to ~320 °C  Establish guide magnetic field  E-beam heat lithium pools  Maintain ~300 °C shell temperatue –Liquid lithium experiments  Shell heaters off for solid lithium  No performance degradation seen over a day’s run –Clean lithium requires near- elimination of residual water from vessel »Water level ~mid 10 -10 Torr since lithium deposition initiated in late 2013  Between-shots recoating possible in principle (but not in practice yet) “Spangle” pattern on solidified lithium indicative of clean metallic coating Shell temperature rise during electron beam heating

7. Successive lithium coating cycles have eliminated most water  Background water 5-9 × 10 -10 Torr  Oxygen 1-2 x 10 -10 Torr  Hydrogen dominates RGA spectrum –Total pressure 2-3 × 10 -8 Torr Now Late 2013 – After first few e-beam depositions -single e-beam coating half the shell  Background water mid × 10 -9 Torr  Oxygen 1-2 x 10 -9 Torr  Hydrogen dominates RGA spectrum –Total pressure 3-5 × 10 -8 Torr

8. Temporal evolution of lithium coatings  Li:O ratio initially high  Ratios asymptotes to 2:1 –Indicates Li 2 O, not LiOH is formed in LTX  Timescale of surface evolution suggests between-shots coating capability might produce more metallic coatings  Analysis with the UIUC MAPP probe –MAPP has been moved to NSTX-U –Surface evolution of lithium- coated graphite, in NSTX-U vacuum, can be compared

9. Ohmic discharges were used to estimate confinement with e-beam evaporated lithium coatings Centerstack gas puffs Plasma current βpβp Line average density Loop voltage βNβN Stored energy  “Continuous” gas fueling with a centerstack gas nozzle

10. LTX exhibits improved ohmic confinement with solid and liquefied lithium PFCs  Only power input is Ohmic heating of electrons  t e-i ~ 5 msec – ions weakly heated by electrons ➯ Confinement improvement in electron channel  Effects of surface aging (passivation) clear  Any lithium coating improves performance relative to bare high-Z wall  Improvements in coating quality produce performance improvements 2 hot shells 4 hot shells  Full liquid lithium wall Cold shells Passivated lithium H98P(y,2)  Good performance with 4 m 2 liquefied lithium wall  Covers 80% of plasma LCFS

11. All impurities, including lithium very low, even with liquid lithium walls at 270 °C during gas fueling emissivity peak Li 2+  Lithium core concentration < 0.5% - Estimate from concentration at peak in Li 2+ emissivity - O < 0.05%, Carbon <0.1%  No significant core impurity accumulation for Z > lithium –Total Z eff from O, C, Li ~ 1.04  No other impurity lines detected  Edge lithium concentration strongly depends on edge T e, n e - Large uncertainties with core Thomson - Edge Thomson would greatly improve accuracy  Gas puffing reduces edge T e  Lithium concentration significantly higher without gas puffing (~1-5%)

12. Flat electron temperature profile develops if edge gas load is removed  T e profile initially hollow, with strong fueling  Peaked profile develops  T e profile evolves to flat or hollow, to LCFS  Longer discharges with new OH programming  All fueling (from centerstack) terminated at 462 msec ~3-4 msec required to clear gas from duct LCFS  Edge electron temperature increases to 200 – 250 eV 464 msec 467 msec471 msec

13. Hot edge implies that ion impact energy can exceed the peak in the sputtering yield in LTX  Self-sputtering of Li on D-treated Li also drops with energy: –24.5% at 700 eV –15.8% at 1 keV  Probability of direct reflection of incident H from lithium PFC also drops to 500 eV  Li sputtering yield for D incident on deuterated Li, calculations and IIAX measurements (Allain and Ruzic, Nucl. Fusion 42(2002)202). Angle of incidence 45°  At 700 eV the yield is 9% –Yield rises to slightly above 10%, just above the melting point –Yield is similar for D, H

14. Upgrades are proposed for LTX in 2016  Double toroidal field (0.17 T to 0.32 T)  Double energy in ohmic power supply –I p ~ 150 kA –Longer flattop  Improve bakeout –Shell systems bakes to >300C, but vacuum chamber to 85 C –Improve chamber bakeout to 120 C  Increase vacuum pumping speed, address minor vacuum leaks  Add between-shots lithium coating capability  Expand diagnostic set  And add neutral beam injection for heating and fueling

15. 2016 – Adding neutral beam injection to LTX-U system loaned to LTX by Tri-Alpha Energy  17 – 23 kV, 35A, pulse length 30 – 50 msec  P aux significantly larger than P ohmic  Core fueling source  700 kW beam will also provide large toroidal momentum input  Beam system is now onsite  Tangency radius 23 cm as shown –R0 = 40 cm

16. Summary  Experiments on LTX have now demonstrated good tokamak performance with full liquefied lithium walls –Lithium surface evolves to Li 2 O  Core impurity content low during gas puffing, even with hot wall  Edge electron temperature strongly increases with removal of edge gas load –Very flat T e profile develops –Discharge impurity content increases at most to 5%  Upgrades to LTX will extend tests to higher toroidal field, higher plasma current  Neutral beam adds auxiliary heating, modest core fueling capability  Upgrades to be complete in ~ 1 year

17. Backup

18. Recycling via direct reflection from lithium Reflection Coefficient  Lithium has the lowest probability of direct reflection of any candidate PFC material  For an average incident angle of 45º, the reflection coefficient at low energy is ~20% (edge T e ~30 eV)  Drops to <10% for edge T e ~ 300 eV ITER School 2009 22-26 June 2009

19.  “Recycling” is typically thought of as an ion process  Electrons are also “recycled” via secondary electron emission –Secondaries cool the edge plasma. Power flow from the edge electron population to the wall/limiter/divertor: –For  0 = -  (kT e /e), where  0 is the sheath potential at the wall/limiter/divertor, and typically  ~3 Secondary electron effects  Lithium has the lowest secondary electron emission coefficient of any metal  Effect of secondaries in a magnetic field, in the edge plasma, is very difficult to model  Secondary electron emission is very sensitive to very thin (10s of Å) layers of surface impurities Li Sn Pt  A. J. Dekker, in Solid State Physics, Advances in Research and Applications, Vol. 6 (Academic Press, New York, 1958) pp. 251-311

20. Ohmic discharge evolution without late fueling Jumps due to OH IGBT control algorithm Gas valve HV drive signal

21. Electron temperature profile evolution in LTX. Gas puffing is terminated at 462 msec. - Gas injection nozzle is emptied 3-5 msec after termination. outboard plasma edge (LCFS) Thomson scattering T e (R)

22. New equilibrium code (C. Hansen, U. Wash.) yields edge position in reasonable agreement with magnetics analysis  Magnetics estimate of plasma edge position uses two independent sets of diagnostic coils t=463 msect=472 msec Outboard edge from magnetics PSI-TRI code

23. Thomson T e vs normalized radius from equilibrium reconstruction (C. Hansen)  Edge data is noisy early and late in the discharge (low density)

24. Lithium safety  CDX/LTX experiments have run 15 years without incident  Extensive engineering controls for lithium systems –Secondary stand-by vacuum system (Roots blower) maintains reduced pressure in LTX, even if a vacuum window cracks –Tertiary turbopump system on 15 min. uninterruptible power –Heaters are interlocked to pressure sensors –ALL windows are mounted on gate valves  No direct water cooling of the vacuum boundary or internal sturctures  No argon gas pressurization to transfer liquid lithium  No use of demountable joints for lithium containment –Liquid lithium containment employs welded or formed stainless steel or tungsten structures  Vacuum boundary is NOT heated above the melting point of lithium –Lithium will freeze out on the wall. No possibility of egress into air