Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct. 2007 Lithium-coated walls on plasma performance in FTU and in TJ-II F.L. Tabarés,

FL Tabarés et al. EU PWI TF..CIEMAT Oct. 2007 Laboratorio Nacional de Fusión

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 22. 28040 MadridENEA, Frascati Italy


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??


Progress in the FTU Liquid Lithium Limiter (LLL) experiment


Experimental lay-out

Capillary structure made of wire meshes with pore radius 15 µm and wire diameter 30 µm (Capillary pressure ~ 105 Pa.)

• Melting point 180.6 °C• Boiling point 1342 °C• Total area of Li surface

~170 cm2

• Effective plasma interaction ~50 - 85 cm2

• Initial temperature of Li limiter > 200oC

• Inventory of lithium 80 g

Liquid lithium surface

Heater

Li source

S.S. box with a cylindrical support

Ceramic break

Thermocouples

100 mm 34 mm

Meshes w/ Li Meshes w/o Li

Mo heater accumulator


Liquid Lithium Limiter

Langmuir probes

Thermocouples

Heater electrical cables


Liquid Lithium Limiter

Toroidal Limiter


Main features of lithium operations:

1. Zeff in ohmic discharges is well below 2 (0.15 1020 < ne< 3.1020m-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/m2


Recent results

See “Experiments on FTU with a liquid lithium limiter”, G. Mazzitelli, M.L. Apicella, V. Pericoli Ridolfini et al., presented at

the 34th EPS Conference


Peaked electron density discharges

• Spontaneous peaking of the density profile for ne > 1.0 1020 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

46.1eeSOL nn


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 2.019

Te at the edge is geneally higher than in boronized discharges


Te [KeV]264

Ip [MA]

.5

ne[1019 m-3]3

5

7

Te [KeV]

2

64

LH

ECH#27923P [MW]1

2

P [MW]

#30620LH

ECH0.40.3 0.5 0.6

1

2

ECRH + LH Discharges

t(s)

0.54 s 0.59 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-0,2 -0,1 0 0,10

2

4

6

8

B

0.54 s

0

1

2

3

4

5

6

-0,2 -0,1 0 0,1

0.59 s

B

R (m) R (m)

Te[keV]


Plasma dilution

0

1

2

3

4

5

0 0,2 0,4 0,6

neutrons/s [x1011]

t(s)

#27923

#30620

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

30584 LLL Inside29919 lithized28847 metallic28833 boronized

1.0

0.5

3.02.01.02.0

1.0

4.0

2.0

0.

Ip [MA]

ne [x1020 m-3]


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/m2 for 3 s

LLL

Toroidal lmiterLLL

Top viewSide view


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

0

10

20

1000 1050 1100 1150 1200 1250 1300

16455F

CV (a.u.)

ne (x1018

cm-3)

SXR (a.u.)

ECE5

ECE9

HD4 (a.u.)

H A5_TOP

BARATRON

BOLO

Time(ms)

ECRH (GR1+GR2)

cut-off

ne-crit. B walls: saturation

shot # 16455


- HV: line of sight

- 10-3-10-5mbar: 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


Li Coating Technique

4 Lithium ovens: 2 fixed (side windows) and 2 in retractable manipulators (top windows)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

1 gr of Li per oven, heated to ~600 ºC during Ne GD

Emission of Li injected froma retractable oven

Emission of Li injected froma retractable oven


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


Dynamic particle balance

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3lin

e d

en

sit

y (

101

2 cm

-3)

H wall (a.u)

EPC Trans.

0

2

4

1000 1050 1100 1150 1200 1250 1300

H D4H A5_TOP

ne

VALBARATRON

0

2

4

6

8

10

Time(ms)

dN/dt= f. Qin-N/(p/1-R)For ECRH plasmas:

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


Impurity behavior

0

1

2

3

4

5

6

7

8

0

0.5

1

1.5

2

16440 16460 16480 16500 16520 16540 16560 16580

Li/n

e,

Prad

/ne C

V/n

e

Series

Limiter insertion

New Li depos.


Impurity behavior

0

50

100

150

0

2 1012

4 1012

6 1012

8 1012

1 1013

0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1

Te(e

V)

ne

(cm-3)

r/a

1

1.5

2

2.5

3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Zeff

16370# B

16503# Li

r/a

Hotter edge, similar density

Te in Boron

Higher central Zeff?


Ion confinement

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25 30 35 40

Ti/(T

e-T

i)

nuei s

-1

Metal

Boron

Lithium

i-e.Ti-e.ne- (Ti/i+Kcxn0Ti).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…)


NBI Plasmas

0

1

2

3

4

5

6

0

50

100

150

200

250

300

1000 1050 1100 1150 1200

Li walls

edge emissivity (Wcm -3 )

Central emissivity (W cm -3 )

Line density (10 13cm-3 )IACCEL1 NBI

<Te> (keV)

BO

21

3-1

70

21

time(ms)

0

1

2

3

4

5

6

0

50

100

150

200

250

300

1000 1050 1100 1150 1200

B walls

<Te

>

time(ms)

•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


System Upgrade

Searching 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

•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

Laboratorio Nacional de Fusión FL Tabarés et al. EU PWI TF..CIEMAT Oct. 2007 Lithium-coated walls on plasma performance in FTU and in TJ-II F.L. Tabarés,

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