The Li-wall Stellarator Experiment in TJ-II Laboratorio Nacional de Fusión. CIEMAT. Madrid. Spain
Outlook Introduction Why Lithium? Li coating technique in TJ-II 2008 Results Particle recycling and confinement Plasma and Radiation profiles Electron energy confinement ELMs and L-H Transition Conclusions
The Stellarator Reactor Reactor issues: Stellarator characteristics Steady State operation OK Power loads No disruptions, no Type I ELMs High E confinement H modes High ne No Greenwald density limit Particle exhaust Intrinsic divertor configurations Low central Zeff(<1.6) Impurity accumulation (?) Stellarators are better suited for Fusion Reactor but low recycling (wall pumping), low Zeff still required
Why Li? Lithium in Tokamaks - Very low Z - Strong H retention (LiH) - Low melting point: Liquid PFC - High impurity getter (O 2,N 2,CO, H 2 O,CO 2 ) Very good results achieved 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?? TJ-II: first stellarator operated under Li walls
Heliac Stellarator 4 periods R=1.5 m <a>= 15-25 cm B T =1 T ECH : 2x300kW,53.2 GHz NBI:2x400 kw, >30 KeV Plasmas in TJ-II Vol Plasma ~ 1m 3 P 0 =5.10-8 mbar Low Z scenarios : - 2 Graphite Limiters - First Wall Boronization Scientific goals: Scan in magnetic configuration,high β operation Challenging for PWI control
P-W Interaction in TJ-II 0.12 Z (m) -0.12 0 Central Coil 2 Mobile Limiters @180 º But: no limiter effect for <2.5 cm insertion in ECRH plasmas -0.24-0.36 PWI mainly on Toroidal limiter (VV) -0.2 0 0.2 R-R 0 (m)
Density control under wall saturation (ECRH)
- HV: line of sight - 10-3 -10-5 mbar: diffusion Lithium coating in TJ-II Deposition on - 4 ovens, symmetric,tangential LOS - 4 g deposited each time (600ºC) - Role of background pressure: top of B-coated walls As deposited (HV) Re-distribution by plasma: Improving with operation time!! After 2 days
Li-wall experimental campaigns May-June 2007: 4 g fully evaporated under vacuum. Li-wall plasmas: ECRH/NBI H plasmas: Presented at the ISHW, Toki Oct.07 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 Operational Window: B vs Li wall / plasma x B / H Li / H Li / He Bivariate Fit of W_t_SXR_max By den_t_sxr_max Diamagnetic energy (kj) 5 4 3 2 1 * Li/He (NBI 1+2) 0 1 2 3 4 5 6 7 line density (e19m-3)
Density control evolution B wall Li wall 2007(full evap.)
Density control evolution (2008) 6 n e (x10 19 m -3 ) 4 2 0 17821 17822 17823 P NBI approaching nominal beam fuelling rate! (~10 20 e - /s) 3 17821 17822 17823 W p (kj) 2 m 2 =dne/dt 1 0 200 17821 17822 17823 P rad (kw) 100 0 3 17821 17822 17823 H! (a.u.) 2 1 puff NBI GAS PUFF 0 1000 1050 1100 1150 1200 time(ms)
Particle Control Li vs B Total wall inventory > 3 times, no sign of saturation H/Li~4.10 17 cm -2 ~ 80 nm, if H:Li=1!! Lab. Experiments: 4.2.10 17 cm -2 @ 1.7 KeV (Sugai et al)
dn/dt= f. Qin-N/(τp/1-R) For ECRH plasmas: f ~1, t p eff ~8 ms, R<0.2!! Dynamic particle balance He plasmas: R<1!!, enhanced contamination n e n e gas gas
Impurity composition/generation From spectroscopy, Φ Li / Φ H ~0.44% for R=20%, Φ Li / Φ H ~9.10-4 Li/H= S H /1-S ss, S ss = S ss -(1-Rn) Li/H sputt ~3.10-2 Impurity (Li) generation But expected?: Reduction>30x!! Efect of underlying coating?
Profile shape: impurity & n e behavior same global parameters but... non collapsing Prone to collapse BELL DOME central impurity peaking 4 n e DOME BELL 0,6 edge thermal instability 0.13 Z eff 3 Zeff n e 0,4 ne (10 20 m -3 ) 0.25 2 Zeff 0,2 1 0 0 0,2 0,4 0,6 0,8 1 r eff
Edge profile evolution BELL DOME
Plasma profile control by puffing 40 35 30 25 shot#19934 Lithium I ne 5 4 3 20 2 15 1 10 5 0 0-1 1100 1110 1120 1130 1140 1150 1160 1170 1180 bell dome r/a time
Energy Confinement Boron Li (dome) Li (bell) Energy Confinement τ E =W d /(P abs -P rad ) <Ne>(10 19 m -3 ) @Wmax
Energy Confinement Boron Li (dome) Li (bell) Energy Confinement τ E =W d /(P abs -P rad ) B < Li Dome ~ Li Bell <Ne>(10 19 m -3 ) @Wmax
ELM ELM activity and transitions
Density Fluctuations Ion saturation current (A) 0.8 0.6 0.4 0.2 0 #18940 1115 1125 1135 Time (ms)
L-H Transition
Sawtooth activity sawtooth-like events center line integrals time (ms) unfiltered photodiodes to visualize the whole plasma cross-section 1100 1120 1140 time (ms) edge H a +Toroidal rotation as a rigid body
Radial electric field in ECRH and NBI regimes j, V 600 500 400 300 200 100 0-100 -200-300 -400 #18973 ECRH t1046 t1057 t1067 t1077 t1087 t1103 t1113 t1123 t1133 t1144 t1154 t1164 t1174 NBI on -500-600 NBI -700-0.8-0.6-0.4-0.2 0.0 0.2 0.4 0.6 0.8 1.0 r There is a transition in the structure of plasma potential from pure ECRH to NBI plasmas. Negative edge radial electric fields can reach values in the order of 100 V/cm in the NBI phase.
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: very low R obtained! Good impurity control, but still C dominated (?) Strong confinement improvements in NBI plasmas. Sawtooth and ELM-like activity observed during transitions to enhanced confinement modes (L-H Transition) Change in plasma profiles controlled by fuelling strategy Improvement of technique still possible: Full Li wall (CPS?) + SMB fuelling in preparation