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

F.L. Tabarés, I. García-Cortés, D. Tafalla, J.A. Ferreira, M.A Ochando, R.
Balbín, F. Medina, and the TJ-II Team

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FL Tabarés et al. ITC/ISHW Oct. 2007 Toki

Oulook• Introduction• Lithiumization of TJ-II• Particle control• Impurity behavior• NBI plasmas• Conclusions

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FL Tabarés et al. ITC/ISHW Oct. 2007 TokiIntroduction
Plasma Wall Interaction in Stellarators:+ No disruptions, no Type I ELMS, no density limit(Greenwald..), intrinsic divertor…
- Larger aspect ratio...smaller <a> for given volume:-Higher S/V ratio-Higher λiz /a ratio+ Impurity accumulation

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Lithium in Tokamaks
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??
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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

Cut-off
B walls: saturation

Ne crit.

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Lithium coating in TJ-IIHow?
- 4 ovens, symmetric,tangential LOS- 4 g deposited each time
- Role of background pressure:- HV: line of sight- 10-3-10-5mbar: diffusion- Ne GD+ ovens

As deposited (HV)

Deposition on
top of B-coatedwalls
Re-distribution by plasma:
Improving with operation time!!

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After 2 days

FL Tabarés et al. ITC/ISHW Oct. 2007 TokiLi Coating Technique
44Lithium
Lithiumovens:ovens:22fixedfixed(side(sidewindows)windows)andand22in
inretractable
retractablemanipulators
manipulators(top(topwindows)windows)

Emission
Emissionof
ofLi
Liinjected
injectedfrom
fromaaretractable
retractableovenoven
11grgrofofLi
Liperperoven,oven,heatedheatedtoto~600~600ºCºC
during
NeGD
during Ne GD

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Impact on plasmas

B coated

Li coated

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Particle Control Li vs B
Factor 2-4 moregas required forsimilar densities
in Li walls

He GD

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Total wall inventory> 4times, no sign ofsaturation

FL Tabarés et al. ITC/ISHW Oct. 2007 Toki

Dynamic particle balancedN/dt= f. Qin-N/(τp/1-R)

For ECRH plasmas:

Always in the EPC mode?
f ~1, τ peff ~30 ms, R~0.6-0.7
1.2

neVAL
BARATRON

Hα D4
Hα A5_TOP

1-3line density (10 cm )1086240.8

1240.6
EPC Trans.
0.40.22001000

01050

11001150

1200

1250

00.5

11.5
Hα wall (a.u)

22.5

31300

Time(ms)

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Impurity behavior
28Limiter insertion

71.5

Li/ne,

5
14

CV/ne

Prad/ne6

New Lidepos.

30.5

2
100
16440

16460

16480

16500

16520

16540

16560

16580

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Impurity behavior
1 10 13150

38 101216370# B
16503# Li100

2.550

Te in Boron
00.7

2 10 1200.75

0.8

0.85

0.9
r/a

0.95

1

1.05

1.1

Zeff
2-34 10 12ne (cm )Te(eV)

6 10 12

1.5

100.1

0.2

0.3

0.4

0.5

0.6

0.7

r/a

Hotter edge, similar density

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Higher central Zeff?

FL Tabarés et al. ITC/ISHW Oct. 2007 Toki

Ion confinement- Similar Ti in ECRH plasmas, but higher Te(0)νi-e.ΔTi-e.ne- (Ti/τi+Kcxn0Ti).ni=0 , for high νi-e , CX losses neglected
EPC Critical collis. (Guimaraes et al, P2-014), shear layer develop.
( Pedrosa O-03,P2-012…)
0.25

MetalTi/(Te-Ti)

0.2

Boron•Similar τi

Lithium

0.15•No discontinuity at
critical collision frequency(EPC aborted?)

0.1

0.05

0051015nu20eis-1

25

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303540FL Tabarés et al. ITC/ISHW Oct. 2007 TokiNBI plasmas
BBECE

I accelP radNeHaNBI path
SXR
Ha wallLi

No density roll-overHigh SXR in NBI phase

LiLower dN/dt
Full duration

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FL Tabarés et al. ITC/ISHW Oct. 2007 TokiNBI Plasmasedge emissivity (Wcm-3

Central emissivity (W cm)<Te> (keV)-3)300

Li walls300

6B walls

5250

5250

4200

4200

3150

3150

2100

2100

150

150

001000

01000

1050

1100

1150

1200

<Te>

BO213-17021

13-3
Line density
6 (10 cm )IACCEL1 NBI

01050

1100

1150

1200

time(ms)time(ms)•Density control: Still limited by density ramp up: NBI fuelling enhanced byPWI, 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
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System Upgrade
Searching for homogeneity:-8 ovens, loaded for repetitive, in situ evaporation ( 6-8 cycles)- Diffusion can help:Φ dif.= 1/3 λ.v.n/a, λ @10-5 mbar He ~80cmax

He

Li beamΦ parallel. = 1/2 n.vn (x)~1/x2.exp(-x/λ)Φdiff (x)＝ n0exp(-x/λ)/λ.2πa, λ~1/PHe

Free Parameters:-Type of gas-Bkgnd pressure-Distance between ovens

Long term: LLL in TJ-II(?)
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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 theplasmas: 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

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FL Tabarés et al. ITC/ISHW Oct. 2007 Toki

Laboratorio
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FL Tabarés et al. ITC/ISHW Oct. 2007 Toki