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

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

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

– Particle recycling and confinement– Edge parameters and impurity generation– Plasma profiles– Radiation and energy confinement– ELMs and Enhanced Confinement

• Conclusions


Vertical field coils

Toroidal field coils

Vacuum vessel

Central coil system

Plasma

4.6 m TJ-II stellarator


Plasmas in TJ-IIHeliac Stellarator 4 periodsR=1.5 m<a>= 15-25 cmBT=1 TECH : 2x300kW,53.6 GHzNBI: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 configurationhigh operation


Lithium in TokamaksWhy 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??

TJ-II: first stellarator operated under Li walls


-0.2 0 0.2

-0.36

-0.24

-0.12

0

0.12

R-R0 (m)

Z (m

)

Central Coil

P-W Interaction in TJ-II


-0.2 0 0.2

-0.36

-0.24

-0.12

0

0.12

R-R0 (m)

Z (m

)

gass-puff hole

H

2 Instrumented Limiters, 180 º

P-W Interaction in TJ-II

But: no limiter effect for <2.5 cm insertion in ECRH plasmas

Central Coil


PW Interaction and Edge Diagnostics

Edge diagnostics: -Supersonic He beam (ne, Te, Ti)1,2 - Recip. Langmuir Probes - Reflectometry - Fast Cameras3

- LIF….

0 1m

HeTurbopump

Observationoptics

He beam

pulsed valve & skimmer

=85,3°Manometer

PLASMA

VACUUMVESSEL

PWI mainly on Toroidal limiter (VV)

1. See P 1-18 Tabarés et al2. See P 2-43 Guzmán et al3. See P 3-39 Carralero et al


Density control under wall saturation (ECRH)

-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

Wall Conditioning:

Particle balance in B

Saturation @ 4.1020

Interaction area~ 1-2 m2

Shot (#)

smota

12280 12285 12290 12295 1230

GD He

10-15 dis.

.

Uncontrolled density

H

4.5x1020


Density control under wall saturation (ECRH)


- HV: line of sight- 10-3-10-5mbar: diffusion- Ne GD+ ovens

How?- 4 ovens, symmetric,tangential LOS- 4 g deposited each time (600ºC)- 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


Li Coating Technique4 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 froma retractable oven


Li-wall experimental campaigns in TJ-II

0,5

1

1,5

2

2,5

3

3,5

Diam

agne

tic e

nerg

y (k

J)

0 1 2 3 4 5line density (1019 m-3)

wall / plasma

x B / H

∆ Li / H

Li / He

Diamagnetic energy vs. plasma density

Operational Window: B vs Li

• 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-wall experimental campaigns in TJ-II

wall / plasma

x B / H

∆ Li / H

Li / He

Operational Window: B vs Li

• 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

1

2

3

4

5

Diam

agne

tic e

nerg

y (k

J)

0 1 2 3 4 5 6 7line density (e19m-3)

Bivariate Fit of W_t_SXR_max By den_t_SXR_max

* Li/He (NBI 1+2)


Impact on plasmas ECRH

B wall

Li wall


Particle Control Li vs B

Total wall inventory> 3 times, no sign of

saturation


0

2

4

6

8

10

0

0.2

0.4

0.6

0.8

1

1000 1050 1100 1150 1200 1250

H plasma(#17861)

gas puffCVH

, wall

Li emissionline density (1019 m-3)

time (ms)

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

For ECRH plasmas:f ~1, peff ~8 ms, R<0.2

He plasmas: R<1!!, enhanced contamination

0

2

4

6

8

10

0

0.2

0.4

0.6

0.8

1

1000 1050 1100 1150 1200 1250

He plasma(#18122)

gas puffCV

Li emissionline density (1019 m-3)

time (ms)

ne ne

gas gas


Density control improvingwith plasma operation: m2=dne/dt approachingnominal beamfuelling rate!(~1020 e-/s)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0

1

2

3

4

5

6

7

1000 1050 1100 1150 1200 1250

default-17825

ne HaGasCVNBIRX

H C

A

0

0.5

1

1.5

2

2.5

3

3.5

4

0

1

2

3

4

5

6

7

1000 1050 1100 1150 1200 1250

default-17813

ne HaGasCVNBIRX509

ne

Ha

t ms

m2

m2

Shot 17813

t ms

Shot 17825

12 shots later


Edge Parameters-1

0

2

4

6

8

10

0

0.5

1

1.5

2

2.5

3

3.5

4

1000 1050 1100 1150 1200 1250

B WallBaratronHWallHTopCVNBIBolo

ne (10 13 cm-3)

time (ms)

0

5

10

15

0

0.5

1

1.5

2

2.5

3

1000 1050 1100 1150 1200 1250

Li Wall

B

time(ms)

Li ______

ne (1013 cm

-3)

He beam at three different times: evolution of edge profiles


Edge Parameters-2

0

5 1012

1 1013

1.5 1013

2 1013

2.5 1013

0.8 0.85 0.9 0.95 1 1.05 1.1

#17628Density profiles

ne-1057ne-1107ne-1157

ne (c

m-3

)

rho20

40

60

80

100

120

140

0.8 0.85 0.9 0.95 1 1.05 1.1

17628Temperature profiles

Te-1057Te-1107Te-1157

Te (

eV)

rho

0

5 1012

1 1013

1.5 1013

2 1013

0.8 0.85 0.9 0.95 1 1.05 1.1

17844 Density profiles

ne-1057msne-1107msne-1157ms

ne (c

m-3

)

rho20

40

60

80

100

120

140

0.8 0.85 0.9 0.95 1 1.05 1.1

17844 Temperature profiles

Te-1057msTe-1107msTe-1157ms

Te (e

V)

rho

B wall

Li wall Te:Similar edge valuesbut steeper profiles. Transient cooling in Li wall Ne:Transient rise of density gradient, but recovers in density-controlled discharges

Te

Te

ne

ne

t1

t2

t3

t2

t1 t3


Impurity composition/generation

8 µm Be

33 µm Be

- No contribution of O radiation

SXR profilesImpurity (Li) generation

•From spectroscopy, Li/ H ~0.44%

•for R=20%, Li/ H ~9.10-4

•Li/H= SH/1-S’ss, S’ss= Sss-(1-Rn)

• Li/Hsputt~3.10-2

Reduction>30x!!

But…expected?:


0

1

2

3

4

5

0

50

100

150

200

250

1050 1100 1150 1200

ne (1

0-19 m

-3),

Wdi

a (k

J), S

RX (a

.u.)

Prad (kW)

time(ms)

SXR

ne

Wdia

Prad

Pnbi

Shot # 17931

LITHIUM

Profile shape: impurity & ne behavior

non collapsing collapsing

0

0,2

0,4

0,6

0,8

1

-1 -0,5 0 0,5 1

17941 10801130115011611180

emis

sivi

ty (W

cm-3

)

reff

0

0,1

0,2

0,3

0,4

0,5

-1 -0,5 0 0,5 1

17931 1080

1115

1125

1135

1140

emis

sivi

ty (W

cm-3

)

reff

0.25 Wcm-3

BELL DOME

same global parameters but...

edge thermal

instability

central impurity peaking

0.14 Wcm-3

0

1

2

3

4

5

0

50

100

150

200

250

1050 1100 1150 1200

ne (1

0-19 m

-3),

Wdi

a (k

J), S

RX

(a.u

.)

Prad (kW)

time(ms)

SXR

ne Wdia

Prad

Pnbi

Shot # 17941

LITHIUM


0

1

2

3

4

5

0

50

100

150

200

250

1050 1100 1150 1200

ne (1

0-19 m

-3),

Wdi

a (k

J), S

RX (a

.u.)

Prad (kW)

time(ms)

SXR

ne

Wdia

Prad

Pnbi

Shot # 17931

LITHIUM

0

1

2

3

4

5

0

50

100

150

200

250

1050 1100 1150 1200

ne (1

0-19 m

-3),

Wdi

a (k

J), S

RX (a

.u.)

Prad (kW)

time(ms)

SXR

ne Wdia

Prad

Pnbi

Shot # 17941

LITHIUM

non collapsing collapsing

0

0,2

0,4

0,6

0,8

1

-1 -0,5 0 0,5 1

17941 10801130115011611180

emis

sivi

ty (W

cm-3

)

reff

0

0,1

0,2

0,3

0,4

0,5

-1 -0,5 0 0,5 1

17931 1080

1115

1125

1135

1140

emis

sivi

ty (W

cm-3

)

reff

BELL DOME

same global parameters but...

DOME BELL

ne

ne

Zeff

Zeff

Profile shape: impurity & ne behavior

1

2

3

4

0

0,2

0,4

0,6

0 0,2 0,4 0,6 0,8 1

Z eff

ne (1020 m

-3)

reff


0

0.005

0.01

0.015

0.02

1 1.5 2 2.5 3 3.5 4 4.5 5

BoronLi (bell)Li (hat)

tau

E (s

)

n (t max W)

Energy Confinement

BoronLi (dome)Li (bell)


ELM activity and transitionsELM


Coating System UpgradeSearching for homogeneity:8 ovens, loaded for repetitive, in situ evaporation ( 6-8 cycles)

- Diffusion can help: dif.= 1/3 .v.n/a @10-5 mbar He ~80cm

parallel. = 1/2 n.v

He

n (x)~1/x2.exp(-x/)

diff (x) ＝ n0exp(-x/)/.2a, ~1/PHe

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

Li beam

Long term: LLL in TJ-II(?)

a x


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


Comments on Boronization

•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?)


H plasma

He plasma

•Similar Tivalues for H and He targetplasmas!

•Ti~Te at high ne (up to 140 eV in H targets)

H/He NBI

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

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