Locations Bay H top & bottom, 7 cm ‘behind’ 7 cm wide gap in outer divertor tiles
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Locations Bay H top & bottom, 7 cm ‘behind’ 7 cm wide gap in outer divertor tiles Bay I midplane 10 cm outboard of limiter, data accumulated continuously 24/7. Deposition changes crystal oscillating frequency. • Frequency measured by pulse accumulator controlled by 20 MHz reference oscillator. • Commercially available system (Infincon XTM/2) is relatively fast (1/4 s), precise (0.1Å), and has immunity from mode hopping C.H. Skinner a , H.W. Kugel a , A. L. Roquemore a , P Krstic b , and A. Beste b a Princeton Plasma Physics Laboratory, b Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA 18th International Conference on Plasma Surface Interactions, May 26-30, 2008, Toledo, Spain P2-59 Mass changes in NSTX surface layers with Li conditioning as measured with quartz microbalances Motivation: Abstract Dynamic retention, lithium deposition, and the stability of thick deposited layers were measured by four quartz crystal microbalances (QMB) deployed in plasma shadowed areas at the upper and lower divertor and outboard midplane in the National Spherical Torus Experiment (NSTX). Deposition of 185 µg/cm 2 over 3 months in 2007 was measured by a quartz crystal microbalance (QMB) at the lower divertor while a QMB on the upper divertor, that was shadowed from the evaporator, received an order of magnitude less deposition. During helium glow discharge conditioning both neutral gas collisions and the ionization and subsequent drift of Li + interupted the lithium deposition on the lower divertor. Some lithium was redirected by the electric field of the glow to the tiles in the vicinity of the evaporator. Occasionally strong variations in the QMB frequency were observed after lithium deposition. suggesting relaxation of mechanical stress and/or flaking or peeling of the deposited layers. QMB locations in NSTX Quart z Cryst al Thermo- couple • Track lithium deposition during LiTER experiments. D + Li -> LiD He + Li -> trapping • Measure hydrogenic retention on a shot- to-shot basis Quartz Microbalances (QMBs) offer both space and time information, in contrast to tile/coupon samples (only well defined spatially) and gas balance (only well defined temporally). Lithium Evaporator (LiTER) used to coat NSTX walls See: P 2-58 H. W. Kugel “Evaporated Lithium Surface Coatings in NSTX”. P 2-60 W. R. Wampler et al., “Measurement of Lithium and Deuterium on NSTX Carbon tiles” P 1-54 J. P. Allain et al., “Lithiated Graphite as a hydrogen -pumping surface for improved plasma performance” Day-by-day deposition thickness & Li evaporation 2007: Transient response to discharge similar with Li Li evaporated (g) Large step-up on 1st discharge of the day. Transient rise and decay at discharge time similar to behavior before LiTER. Rising slope on lower QMB due to Li deposition. Li deposition interrupted by neutral gas collisions and He-GDC Gas only pulse (no plasma) during Li evaporation. Vessel pressure rises to 1.3 mtorr Deposition ceases on lower QMB Deposition appears on upper QMB (not in direct line of sight to evaporator). But no upper QMB deposition during GDC. 0.5m Acknowledgements: The authors thank T. Holoman, D. Labrie, P. Roney and the NSTX team for technical assistance. Supported by DOE Contract No. DE-AC02-76-CH03073. Conclusions : Fluctuations observed in thick films (micro-seismology) • QMB’s offer time and space resolved data on surface layers. • Li deposition redirected by glow discharge and neutral gas collisions for pressures > ~ 1e-5 torr. Li+ drifts to anode tiles. • Behavior consistent with QM calculations of mean free path. • Instability observed in thick films may have implications for ITER. Erratic behavior after heavy deposition Occasionally see variation in QMB frequency after heavy Li deposition. Conclude layer is not always firmly attached to quartz crystal. Flaking of layer possible Similar behavior expected in thick deposited layers. Could be an issue for ITER. Peak codeposition ~ 6-12 µm/day of BeT. Loose flakes could contaminate plasma and complicate control of tritium inventory. Tritiated dust can self charge from Beta decay and levitate in E fields. SEM image of deposit on Bay I crystal film mass frequency change crystal mass bare crystal frequency = Li+ returns to anode (wall) Calculations of mean free path: Mean free path density velocit ies From: T.I. Ganbosi, Gaskinetic Theory, Cambridge University Press 1994 Momentum transfer cross section Fully quantum mechanical calculations of Li - D mean free path based on LiH ground singlet and triplet potenti Li - D 2 mean free path calculated by analogy with H - D and H - D 2 results. • Similar calculation of Li - He mean free path based on Hamel et al., J. Phys. B19 (1986) 4127 • Similar calculation of Li + - He mean free path based on Elford et al., J. Phys. B32 (1999) 1873. Results: • mean free path = 3.94 cm for Li at 627 C in 1 mtorr of atomic D at 27C • mean free path = 7.7 cm for Li at 627 C in 1 mtorr of molecular D 2 at 27C • mean free path = 2.7 cm for Li at 627 C in 2.5 mtorr of He at 27C • mean free path = 0.50 cm for Li + at 627 C in 2.5 mtorr of He at 27C Consistent with observed time history of Li deposition 103 104 105 106 24 25 26 27 PERSEC07.20070504 Time of Day (h) <- Lower QMB Upper QMB -> Discharge Turn off LiTER 115 120 125 130 135 140 25 30 35 40 45 50 PERSEC07.20070522 Time of Day (h) <- Lower QMB Upper QMB -> 10 mins 66 68 70 72 0 20 40 60 PERSEC07.20070423 Time of Day (h) Gas only pulse Upper QMB X10 (offset ) Lower QMB Upper QMB rate Lower QMB rate 30 35 40 45 50 0 10 20 30 40 50 PERSEC07.20070423 Time of Day (h) LiTER entrance port Upper QMB X10 (offset ) Lower QMB Upper QMB rate Lower QMB rate glow glow Discharge time He-Glow Discharge Cleaning 0 50 100 150 200 Year-month-da 5 April 2007 23 June 2007 Upper QMB (-20 µg/cm 2) (no direct line of sight) Lower QMB (direct line of sight to LiTER) 1 micron at density 1.6 g/cm3 Boron (10 g ea.) Boron (10 g ea.) 22 24 26 28 30 PERSEC07.20070423 Time of day (h) Lower QMB Upper QMB Begin Li evaporation First plasma plasma plasma plasma plasma Plan View
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0.5m. P2-59 Mass changes in NSTX surface layers with Li conditioning as measured with quartz microbalances. - PowerPoint PPT Presentation
Transcript of Locations Bay H top & bottom, 7 cm ‘behind’ 7 cm wide gap in outer divertor tiles
Locations Bay H top & bottom, 7 cm ‘behind’ 7 cm wide gap in outer divertor tiles
Bay I midplane 10 cm outboard of limiter, data accumulated continuously 24/7.
Deposition changes crystal oscillating frequency.
• Frequency measured by pulse accumulator controlled by 20 MHz reference oscillator.
• Commercially available system (Infincon XTM/2) is relatively fast (1/4 s), precise (0.1Å), and has immunity from mode hopping
C.H. Skinnera, H.W. Kugela, A. L. Roquemorea, P Krsticb, and A. Besteb aPrinceton Plasma Physics Laboratory, bOak Ridge National Laboratory, Oak Ridge, Tennessee, USA
18th International Conference on Plasma Surface Interactions, May 26-30, 2008, Toledo, Spain
P2-59 Mass changes in NSTX surface layers with Li conditioning as measured with quartz microbalances
Motivation: AbstractDynamic retention, lithium deposition, and the stability of thick deposited layers were measured by four quartz crystal microbalances (QMB) deployed in plasma shadowed areas at the upper and lower divertor and outboard midplane in the National Spherical Torus Experiment (NSTX). Deposition of 185 µg/cm2 over 3 months in 2007 was measured by a quartz crystal microbalance (QMB) at the lower divertor while a QMB on the upper divertor, that was shadowed from the evaporator, received an order of magnitude less deposition. During helium glow discharge conditioning both neutral gas collisions and the ionization and subsequent drift of Li+ interupted the lithium deposition on the lower divertor. Some lithium was redirected by the electric field of the glow to the tiles in the vicinity of the evaporator. Occasionally strong variations in the QMB frequency were observed after lithium deposition. suggesting relaxation of mechanical stress and/or flaking or peeling of the deposited layers.
QMB locations in NSTX
QuartzCrystal
Thermo-couple
• Track lithium deposition during LiTER experiments.
D + Li -> LiD He + Li -> trapping
• Measure hydrogenic retention on a shot-to-shot basis
Quartz Microbalances (QMBs) offer both space and time information, in contrast to tile/coupon samples (only well defined spatially) and gas balance (only well defined temporally).
Lithium Evaporator (LiTER) used to coat NSTX wallsSee: P 2-58 H. W. Kugel “Evaporated Lithium Surface Coatings in NSTX”. P 2-60 W. R. Wampler et al., “Measurement of Lithium and Deuterium on NSTX Carbon tiles” P 1-54 J. P. Allain et al., “Lithiated Graphite as a hydrogen -pumping surface for improved plasma performance”
Day-by-day deposition thickness & Li evaporation
2007: Transient response to discharge similar with Li
Li evaporated (g)
Large step-up on 1st discharge of the day. Transient rise and decay at discharge time similar to behavior before LiTER. Rising slope on lower QMB due to Li deposition.
Li deposition interrupted by neutral gas collisions and He-GDC
Gas only pulse (no plasma) during Li evaporation. Vessel pressure rises to 1.3 mtorrDeposition ceases on lower QMBDeposition appears on upper QMB (not in direct line of sight to evaporator).But no upper QMB deposition during GDC.
0.5m
Acknowledgements:The authors thank T. Holoman, D. Labrie, P. Roney and the NSTX team for technical assistance.Supported by DOE Contract No. DE-AC02-76-CH03073.
Conclusions:
Fluctuations observed in thick films (micro-seismology)
•QMB’s offer time and space resolved data on surface layers.
•Li deposition redirected by glow discharge and neutral gas collisions for pressures > ~ 1e-5 torr. Li+ drifts to anode tiles.
•Behavior consistent with QM calculations of mean free path.
• Instability observed in thick films may have implications for ITER.
Erratic behavior after heavy depositionOccasionally see variation in QMB frequency after heavy Li deposition.Conclude layer is not always firmly attached to quartz crystal.Flaking of layer possibleSimilar behavior expected in thick deposited layers. Could be an issue for ITER. Peak codeposition ~ 6-12 µm/day of BeT. Loose flakes could contaminateplasma and complicate controlof tritium inventory. Tritiated dust can self charge fromBeta decay and levitate in E fields.
SEM image of deposit on Bay I crystal
film mass frequency change
crystal mass bare crystal frequency=
Li+ returns to anode (wall)
Calculations of mean free path:
Mean free path
density
velocitiesFrom: T.I. Ganbosi, Gaskinetic Theory, Cambridge University Press 1994Momentum transfer
cross section
Fully quantum mechanical calculations of Li - D mean free path based on LiH ground singlet and triplet potentials. Li - D2 mean free path calculated by analogy with H - D and H - D2 results.
• Similar calculation of Li - He mean free path based on Hamel et al., J. Phys. B19 (1986) 4127• Similar calculation of Li+ - He mean free path based on Elford et al., J. Phys. B32 (1999) 1873.Results:• mean free path = 3.94 cm for Li at 627 C in 1 mtorr of atomic D at 27C• mean free path = 7.7 cm for Li at 627 C in 1 mtorr of molecular D2 at 27C
• mean free path = 2.7 cm for Li at 627 C in 2.5 mtorr of He at 27C• mean free path = 0.50 cm for Li + at 627 C in 2.5 mtorr of He at 27C Consistent with observed time history of Li deposition
103
104
105
106
24
25
26
27
17 18 19 20 21 22
PERSEC07.20070504
µg/cm
2
Time of Day (h)
<- Lower QMB
Upper QMB ->
µg/cm
2
DischargeTurn off LiTER
115
120
125
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135
140
25
30
35
40
45
50
23.5 23.6 23.7 23.8 23.924
PERSEC07.20070522
µg/cm
2
Time of Day (h)
µg/cm
2
<- Lower QMB
Upper QMB ->
10 mins
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0
20
40
60PERSEC07.20070423
Time of Day (h)
Gas only pulse
Upper QMBX10 (offset )
Lower QMB
Upper QMBrate
Lower QMBrate
30
35
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0
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20
30
40
50
1616.1 16.2 16.3 16.4 16.5 16.6
PERSEC07.20070423
µg/cm
2 (offset) µg/cm
2/h
Time of Day (h)
LiTER entrance port
Upper QMBX10 (offset )
Lower QMB
Upper QMBrate
Lower QMBrate
glow glow
Discharge time
He-Glow Discharge Cleaning
0
50
100
150
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20070405 20070415 20070425 20070505 20070515 20070525 20070613 20070623Year-month-day
µg/cm2
5 Apr
il 200
7
23 J
une
2007
Upper QMB (-20 µg/cm2) (no direct line of sight)
Lower QMB(direct line of sightto LiTER)
1 m
icro
n a
t densi
ty 1
.6 g
/cm
3
Bor
on (
10 g
ea.
)
Bor
on (
10 g
ea.
)
22
24
26
28
30PERSEC07.20070423
Time of day (h)
Lower QMB
Upper QMB
Begin Lievaporation
Firs
t p
lasm
a
pla
sma
pla
sma
pla
sma
pla
sma
PlanView
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