wall conditioning techniques: status and perspectives

RFX-mod programme workshop 2011 February 7 - 9, 2011

Wall conditioning techniques: status and

perspectives

S.Dal Bello, A.Canton, L.Grando, P.Innocente, S.Munaretto

RFX-mod programme workshop 2011 - February 7- 9

Outline

Why wall conditioning is needed/desired

First Wall conditioning in RFX-mod

Effects of present wall conditioning

techinques on plasma properties in RFX-

mod and in other Experiments

Open points and activities aimed at

improving the present wall conditionig

effectiveness

Conclusions


FW cleaning (after vessel venting)

graphite is a good absorber of humidity(after the production, tiles were degassed at 2000°C)

even the vessel is continuously vented by dry N2 (overpressure of hundreds of mbar), during the shutdown some air retro diffuses through the open porthole

Why FW conditioning


Why FW conditioning (2)

Impurities control (performance)

the graphite wall prevents the presence of high-Z impurities

main impurities in RFX-mod: C, O2, B or Li (when conditioning procedures applied)

Zeff in the range 2-4


Wall desaturation


at a certain time, reached the “saturation level” (that is not a parameter, yet) the removal of Hydrogen from the wall is necessary to operate the machine at “allowed” density values


First Wall reset


Between to (very) different experimetal campaigns the status of the wall is considered needy of a reset.

The resetting wall treatment should remove the memory of different PW interactions


Possible control of recycling – density (performance)


We would try to modify the behaviour of the wall (recycling factor 1) increasing the operational range we have to modify/obtain the “desired” density, hopefully at different plasma currents.


FW conditioning in RFX-mod

BakingGlow Discharge Cleaning

Plasma discharges (i.e. He)

Boronization

Lithization

What are the possibilities we have:

RT Pellet injector (solid Li)

LLL used as limiter or evaporator (liquid Li)

(performed by Glow Discharge @170°C)


170 °C

Tav

time (hours)

10 20 30 40 50 60 70 80 90

H2GDC

He

GDC

25 °C

100

- Temperature rised to 170°C and kept for the whole sequence- Baking (4h) for outgassing pressure stabilization- H2 GDC (30-40h) to extract O2, N2 (CH4 also formed and

removed)- He GDC (6-10h) to remove H2

- Baking to favorite the removal of He (15h)

RF-assisted Glow Discharges in continuous flow

Executed at Room Temperature (during experimental sessions as desaturation technique) or at high temperature


170 °C

Tav

time (hours)

10 20 30 40 50 60 70 80 90

H2GDC

He

GDC

25 °C

100

B2H6

+

HeGDC

precursor gas: B2H6 (Diborane) 10% - He 90%

RF-assisted Glow Discharges in continuous flow

In 2010 (from Jan to Oct) we performed 4+1 boronizations: 30’, 120’, 240’, 240’ + 360’ (to possibly reduce the Li2CO3

formation/effects)

Boronization @170°C by


driver gas

sabot loader

pumping gate

optical detectors

bumper and recovery box

sabot

pellet

Pellet feature:

V = 8.8 mm3

4x1020 atoms

∆ne~1020 m-3

In both the cases, we deposited in or on (?) the wall approximately 2g of Lithium

Lithization: during pulses (pellets and LLL) or between pulses (LLL used as evaporator)









effectiveness

Conclusions


... on impurities ...

Impurity reduction

• The main sources of impurity at RFX-mod are C and O

• For both after Lithization the influx is lower than with clean graphite

• The main sources of impurity at RFX-mod are C and O

• For both after Lithization the influx is lower than with clean graphite

0

20

40

60

80

100C wall

Li wallB wall

0 1 2 3 4 5 6 7

<ne>*1019 (m-3)

0

1

2

3

4

5

0 1 2 3 4 5 6 7

C wall

Li wall

B wall

<ne>*1019 (m-3)


Graphite WallLi WallB Wall

0

1

2

3

4

5

6

7

8

0.0 10.0 20.0 30.0 40.0 50.0

Filled Particles (*1020)

0.0

1.0

2.0

3.0

4.0

-2.0 0.0 2.0 4.0 6.0

P (

*10

-3 m

Bar

)

t (s)

#27710

H discharge post boronization

Filled+Puffed atoms = 8.2*1020

Desorbed atoms = 6.9*1019

Flat top electron density versus total filled particles between two He-GDC performed to remove H from the wall

After Lithization a greater quantity of filled particles does not lead losing density control

Flat top electron density versus total filled particles between two He-GDC performed to remove H from the wall

After Lithization a greater quantity of filled particles does not lead losing density control

Increased wall gas adsorbing capacity

Increased wall gas adsorbing capacity

... on H2 recycling...


• Different density radial profile

• Statistically it can be verified by the Peaking Factor:

PF = n(0)/<n>

• On RFX-mod PF is a function of plasma current and average density

• After the Lithization it is clearly higher than with a clean graphite first wall

0.6

0.8

1.0

1.2

1.4

0.5 1.0 1.5 2.0

CLi

Fit_Pn=1.5I

p

0.29n-0.36

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.2 0.4 0.6 0.8 1.0

C wallLi wall

r/a

... on density radial profile modification...


•Measured cut-off layer position is averaged over similar density discharges

•In Li discharges cut-off layer position moves inward by about 1-2 cm

•Edge density with Thermal Helium Beam diagnostic

•After Lithization edge density is lower than on clean graphite

•It has been confirmed by the cut off layer position measurements of a fixed frequency reflectometer

reference

post Li

... on density at the edge ...


Gas Puffing Imaging diagnostic raw signals: measurement of edge density fluctuations

After Li

Reference

• larger edge blobs

• less plasma volume occupied

• different turbulent particles transport at the edge

• larger edge blobs

• less plasma volume occupied

• different turbulent particles transport at the edge

to quantify the differencesto quantify the differencesAutocorrelation time Packing

fractionC

Li

C

Li

C

Li

C

Li

... on edge transport ...


No difference on the energy confinement time with respect to the Graphite wall

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 1.0 2.0 3.0 4.0 5.0

C wallLi wallB wall

<ne>*1019 (m-3)

Lower Hα emission at the edge at the same density with a Li

wall

Improvement of the particles confinement

time

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 1.0 2.0 3.0 4.0 5.0

C wallLi wallB wall

<ne>*1019 (m-3)

... on global properties ...









effectiveness

Conclusions


Suppression of Oxygen contamiantion of plasma

this is the main effect on all experiments (TEXTOR, FTU, TORE SUPRA, DIII-D...)

Boron is a getter for Oxygen !

Suppression of contamination by wall material

In the case of high-Z wall materials, the advantage is the reduction of Zeff (Alcator C-Mod, Asdex-U with W divertor...)

In the case of carbon wall, boronization was seen to reduce the chemical sputtering of carbon due to Oxygen impurities (via

formation of carbon-dioxide)[Hino et al. Thin Solid Films 253 (1994) 518]

Effect of BORONIZATION


Reduction in Hydrogen recycling

DIII-D: They observe that particle fueling from plasma facing surfaces is reduced after boronization. They speculate that B binds H more effectively than C

[Jackson et al., J. Nucl. Mat. 196-198 (1992), 236]

TEXTOR: yes[Winter et al., J. Nucl. Mat. 162-164 (1989),

713]

TFTR: no significant change in Hydrogenic recycling properties[Dylla, J. Nucl. Mat. 176-177 (1990), 337]

Effect of BORONIZATION


TFTR Enhanced energy confinement time Reduction of the H recycling Peaking of plasma parameters Suppression of impurities

[Mansfield et al. 1996 Phys. Plasmas 3 1892]

NSTX Enhanced energy confinement time Increase in the total power radiated from the

plasma Reduction in the edge electron density

[Bell et al. 2009 PPCF 51 124054]

Effect of LITHIZATION


FTU Similar to Boronization but with better results Peaking of electron density radial profile Strong pumping effect of Lithium modificates

the SOL Te

[Apicella et al. 2007 Journal of Nuclear Materials 363-365 1346]

TJ-II Improved density control by external puffing Suppression of impurities

[Tabarès et al. 2008 PPCF 50 124051]

Effect of LITHIZATION









effectiveness

Conclusions


DISUNIFORMITY

(non uniform deposition/interaction with the FW during treatment)

and

QUANTITY ?

(thickness of Boron and Lithium layer

deposited on the wall)

Are the (small) effects of the wall conditioning we see strongly affected by


2D

9D

Glow Discharge disuniformity in toroidal direction

from ISIS measurements

Confirmed also by analyses of samples (SIMS at IENI institute)


By adding GDC anodes we can improve the uniformity, or better, the number of peaks but a good uniform density can only be reached with an number of GDC anodes that cannot be proposed !

Tests with a toroidal field (in the order of some mT) applied during the Glow Discharge process will be done in the near future (next P17 campaign)

We could (hopefully) discover that only one anode is necessary to have a very good and uniform glow discharge ... In this case “Thanks toroidal filed ! ”


If the GDC plant will be able to sustain the discharge with the toroidal field necessary to obtain a good uniformity the test will be done also during the boronizazion


If possible,we could try to modify the position of our LLL (as evaporator or limiter in equatorial ports) or install new evaporator(s)


CDU Proto-Liter evaporator

•100-200 mg/min evaporated lithium

•Cos Lithium distribution

•Horizontal set-up

... just for example ...


An idea could be the centrifugal Lithium multi-pellet injector ...


Just questions ...

Can we obtain the same plasma performances we obtained so far “simply” performing 5’He GDC shot by shot (or every two shots)?

What about to try with daily i.e. 20’H2 + 40’He to maintain

impurities at low values and a good wall absorbing capability far from boronization?


if, at the end, to change the wall conditioning technique will not lead to a good improvement of the First Wall behaviour …

… try to change the First Wall !!

wall conditioning techniques: status and perspectives

Documents

outlinewhy wall conditioning

wall recycling factor

resetting wall treatment

c temperature

glow discharges

c byin

plasma properties

electron density