Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Lithium-coated walls on plasma performance in FTU and in TJ-II F.L. Tabarés, and G. Maddaluno + the FTU and TJ-II Teams Laboratorio Nacional de Fusión. CIEMAT.Av. Complutense Madrid ENEA, Frascati Italy

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Lithium in Tokamaks Why Li? - Very low Z - High impurity getter (O 2,N 2,CO, H 2 O,CO 2 …) - 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??

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Progress in the FTU Liquid Lithium Limiter (LLL) experiment

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Experimental lay-out Capillary structure made of wire meshes with pore radius 15 µm and wire diameter 30 µm ( Capillary pressure ~ 10 5 Pa.) Melting point °C Boiling point 1342 °C Total area of Li surface ~170 cm 2 Effective plasma interaction ~ cm 2 Initial temperature of Li limiter > 200 o C Inventory of lithium 80 g Liquid lithium surface Heater Li source S.S. box with a cylindrical support Ceramic break Thermocouples 100 mm34 mm Meshes w/ Li Meshes w/o Li Mo heater accumulator

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Liquid Lithium Limiter Langmuir probes Thermocouples Heater electrical cables

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Liquid Lithium Limiter Toroidal Limiter

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Main features of lithium operations: 1.Zeff in ohmic discharges is well below 2 ( < n e < m -3 ) 2.Radiation losses less than 30% 3.With lithium limiter much more gas has to be injected to get the same electron density with respect to boronized and fully metallic discharges > 10 times 4.Operations near or beyond the Greenwald limit are easily performed 5.Plasma operations are more reliable with good plasma reproducibility and easier recovery from plasma disruptions 6.The LLL is able to withstand heat load up to 5 MW/m 2

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Recent results See Experiments on FTU with a liquid lithium limiter, G. Mazzitelli, M.L. Apicella, V. Pericoli Ridolfini et al., presented at the 34 th EPS Conference

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Peaked electron density discharges Spontaneous peaking of the density profile for n e > m -3 w/o lithium w/ lithium The SOL densities do not follow the FTU scaling law The strong particle depletion in the outermost plasma region is due to the strong pumping capability of lithium

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct A possible theoretical explanation is proposed in which electrostatic waves excited by thermal background in the plasma core enhance the turbulence at the edge via non-linear mode coupling. Quasi-quiescent MHD activity R. Cesario et. al, presented at the 34° EPS Conference- Poster T e at the edge is geneally higher than in boronized discharges

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct ECRH + LH Discharges t(s) 0.54 s0.59 s Strong and wide ITB develops after LH injection, with very high central Te up to 8 KeV in spite of the lower value of additional power Zeff is reduced by at least a factor 2 in lithium discharges increase of the LH current drive efficiency #30620 With LLL #27923 Without LLL R (m) T e [keV]

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Plasma dilution The dilution is strictly correlated with the plasma start-up phase and the low value of electron density. At higher electron densities dilution is negligible LLL Inside lithized metallic boronized Ip [MA] ne [x1020 m-3]

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Future activities Short term same limiter lay-out but actively cooled and equipped with a system for lithium refilling. Long term New limiter panel type actively cooled and equipped with a system for lithium refilling, able to withstand heat loads up to 10 MW/m 2 for 3 s LLL Toroidal lmiter LLL Top viewSide view

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Wall Conditioning in TJ-II -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: wall saturation ( <20 discharges)+ EPC

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct HV: line of sight mbar: diffusion - Ne GD+ ovens How? - 4 ovens, symmetric,tangential LOS - 4 g deposited each time - Role of background pressure: As deposited (HV) After 2 days Re-distribution by plasma: Improving with operation time!! Lithium coating in TJ-II Deposition on top of B-coated walls

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Li Coating Technique 4 Lithium ovens: 2 fixed (side windows) and 2 in retractable manipulators (top windows) 1 gr of Li per oven, heated to ~600 ºC during Ne GD Emission of Li injected from a retractable oven Emission of Li injected from a retractable oven

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Particle Control Li vs B He GD Factor 2-4 more gas required for similar densities in Li walls Total wall inventory > 4times, no sign of saturation

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Dynamic particle balance dN/dt= f. Qin-N/( p/1-R) For ECRH plasmas: f ~1, p eff ~30 ms, R~ Always in the EPC mode?

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Impurity behavior Limiter insertion New Li depos.

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Impurity behavior Hotter edge, similar density Te in Boron Higher central Zeff?

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct Ion confinement i-e. T i-e.ne- (Ti/ i +K cx n 0 Ti).ni=0, for high i-e, CX losses neglected Similar i No discontinuity at critical collision frequency (EPC aborted?) - Similar Ti in ECRH plasmas, but higher Te(0) EPC Critical collis. (Guimaraes et al, P2-014), shear layer develop. ( Pedrosa O-03,P2-012…)

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct NBI Plasmas Density control: Still limited by density ramp up: NBI fuelling enhanced by PWI, but lower dN/dt obtained in Li Record density and W dia at collapse obtained in Li walls Strong change of edge/core radiation ratio: Thermal vs radiative collapse(?) (M. A Ochando et al Nucl.Fus.1997

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct System Upgrade Searching for homogeneity: -8 ovens, loaded for repetitive, in situ evaporation ( 6-8 cycles) - Diffusion can help: dif.= -5 mbar He ~80cm parallel. = 1/2 n.v He n (x)~1/x 2.exp(-x/ ) diff (x) n 0 exp(-x/ )/.2 a, ~1/P He Free Parameters: -Type of gas -Bkgnd pressure -Distance between ovens Li beam Long term: LLL in TJ-II(?) a x

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct 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 Density control highly improved, long lasting effect Strong change in particle recycling Good impurity control, but hotter edge problematic if C(Me) is exposed to the plasmas: homogeneity problem? No major changes in confinement, but transition to EPC hindered Better control of NBI plasmas, but still to improve Change in radiation profiles may prevent radiation instability-driven collapse Improvement of technique in progress