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Time resolved measurements of deposition and dust in NSTX
C.H. Skinner, H. Kugel, L. Roquemore, E. Biewer, W. Davis, R. Maingi, N. Nishino,
C. Parker, and C. Voinier
47th APS – DPP MeetingOct 24-28, 2005
Denver, Co.
Supported byOffice ofScience
Culham Sci CtrU St. Andrews
York UChubu UFukui U
Hiroshima UHyogo UKyoto U
Kyushu UKyushu Tokai U
NIFSNiigata UU Tokyo
JAERIHebrew UIoffe Inst
RRC Kurchatov InstTRINITI
KBSIKAIST
ENEA, FrascatiCEA, Cadarache
IPP, JülichIPP, Garching
ASCR, Czech RepU Quebec
College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin
• ITPA DSOL priority topic: “Improve understanding of SOL plasma interaction with the main chamber”
Concerns on ITER Be wall:– heat load, erosion lifetime, repair difficulty– tritium migration,– coating diagnostic mirrors…
• ITPA Diagnostics priority topic:“…assessment of techniques for measurement of dust and erosion.”
Concerns: high dust levels are expected in ITER from long plasma duration and more intense plasma surface interactions.
– How to assure dust levels are below safety limit ?
– Will dust transport impurities to plasma core reducing fusion reactivity ? •Limits for C-and Be-dust are related to
an explosion (e.g., H produced by Be reactivity with steam).
•The limit for W-dust is related to the containment function of the ITER
building (is more flexible).
Dust Safety Issue Limits (kg) Beryllium Reactivity with steam
Toxic 10-20 on hot surfaces
Carbon Tritium retention Explosion with air
~100
Tungsten Activation 100-400
ITER
Motivation:
Open ST geometry facilitates diagnostic access and cost effective progress on these topics.
Dust particle trajectories in NSTX plasma:
Fast Camera divertor view, 800 kA, 3 MW NBI discharge, 13,500 frames/sec.Particles seen in minority of discharges
Note:Particle breakup @160 ms curved trajectory @ 340 ms
Apparent velocities up to 200 m/s.
Multiple camera views planned for 3D comparison to dust transport theory (S. Pigarov).
Dust particle trajectories in NSTX plasma:Surface Dust in NSTXSurface Dust in NSTX
Microscope image of Bay B lower viewport exposed 11 March - 5 August 2004
1,659 plasmas793 s total duration10,372 particles / mm2
(excl. those < 1 micron)Dust est. surface area 1.2 mm2 / mm2
-exceeds area of viewportCount Median Diameter = 2.07 µmGraphite (sp2 elemental carbon) identified by Raman analysis
Size distribution of dust on Bay B lower viewport
0.01%
0.10%
1.00%
10.00%
100.00%
1.00 10.00 100.00
projected area diameter (microns)
perc
enta
ge
100 µm (dia. human hair)
• A fine grid of interlocking traces (spacing down to 25 µm) is biased with 30-50 v DC.
• Impinging dust produces a short circuit and resulting current pulse vaporises the dust.
• Extensive lab data in air and vacuum ->>
• Higher sensitivity needed for NSTX dust levels (~ 5 ng/cm2/discharge).- should not be a problem in ITER
Dust detector installed in NSTX:
Bay B Bay C
Dust collection slide for calibration
Circuitgrid
Circuitgrid Grid
Spacing:25 µm
50 µm
75 µm100 µm125 µm
Lab results:
0
50,000
100,000
0 200 400 600 800
areal density (µg/cm2)
cou
nts
See poster GP1.00025 “Controlling surface dust in a tokamak” Colin Parker.
Deposition in NSTX
Quartz crystal microbalances located at Bay H top & bottom, 7 cm ‘behind’ 7 cm wide gap in tiles + Bay I midplane 10 cm ‘behind’ limiter.
• Quartz crystal oscillates at ~ 5.9 MHz, exact frequency depends on mass and on temperature.
• Temperature effect largely subtracted using thermocouple data. Density 1.6 g/cm3 assumed.
• Deposition inferred from change in frequency (measured to ~0.1 Å, ~0.1 Hz)
• Data accumulated continuously 24/7.
QuartzCrystal
Thermocouple
Boronization
0
500
1000
1500
5 6 7 8 9 10 11 12 1320050612
de
po
sitio
n (
Å)
rela
tive
Time of Day
Boronization not spatially uniform.
• Deposition of boron highest near to glow discharge electrodes 40 cm below midplane
• Recently installed movable glow probe should improve boronization of divertor.
Bay H top
Bay I mid
Bay H bottom 0.01
0.1
1
10
40 42 44 46 48
de
po
sitio
n r
ate
(Å
/min
ute
)
Boronization #
midplane
bottom
top
mini mini mini mini
Boronization Rate at midplane and divertor
Initial conclusions:• Discharge time marked by
transients from pickup and temperature differences between thermocouple and crystal.
• First shot of day always experiences deposition.
• Subsequent discharges can show deposition or erosion (unlike 2004 location 80 cm from LCFS which mostly accumulated deposition).
• Interaction at upper divertor increases in DN discharges.
• Interaction increases with plasma stored energy and pulse duration.
qmb
Deposition/erosion depends on plasma shape
• Incandescent particles with complex trajectories observed with fast camera in some NSTX plasmas.
• Carbon dust identified on surfaces after campaign.
• Electrostatic dust detector developed for time resolved measurements, however more sensitivity needed for NSTX dust levels (should not be a problem in ITER).
• Quartz microbalance (qmb) show boronization is non uniform, movable glow probe installed to address this.
• Quartz microbalance shows erosion and deposition on wall from plasmas.
– Deposition dominates on first discharge of day
– Erosion depends on plasma shape.
– Analysis continuing…
Conclusions:
More in poster GP1.00025 “Controlling surface dust in a tokamak” Colin Parker.
Extras….
• words
Deposition thickness at midnight during campaign
Midnight layer thickness Bay H
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
2005
0423
2005
0503
2005
0513
2005
0523
2005
0608
2005
0618
2005
0628
2005
0708
2005
0718
2005
0728
2005
0807
2005
0817
2005
0827
2005
0906
date
depo
sitio
n Å
(re
lativ
e, te
mp.
cor
rect
ed)
BayH bottom (FA1-DA1)*1.1
Bay H top (FA2-DA2)*1.1boron boronmini
boronboronboron
Modelling: John Hogan ORNL
BBQ details:Collision model Similar to LIM, WBC, ERO codes, detailed magnetic geometry (EFIT)
Background parameters assumed (ne, ,ln, Te, lT)[ local D+ flux amplification, sheath electrostatic (es) field, ESOL ] MZ dVZ/dt = -Ffriction -Fes + random // and ^ diffusionFZ = -MZ (VSOL-VZ) / ts ; Fes = Ze ESOL F// = Random // diffusion, D//=(8EZ/3pMi) t //
F^ = Random ^ anomalous diffusion ( D ^ )Particle energy (WZ)dWZ/dt = (kTi -WZ)/tT +Rfriction +Res
Molecular processes: Erhardt-Langer,Janev-Reiter hydrocarbon break-up ratesAtomic processes (ionization, recombination, D0 charge exchange) for carbonBirth gyro-collisions with surface
-1.60
-1.50
-1.40
-1.30
0.30 0.40 0.50 0.60
-1.633.441.810.185
R (m)
1.63
0
0
0.333
0.666
1Z (m) Localization of neutralC density near strike points
Configuration from EFITNSTX pulse 1108251 @ 235ms
CI density(rel.)
Poloidal localization around strike points
• BBQ calculations of quiescent cross-field transport of impurities generated at the divertor strike points find little mid-plane deposition, using conventional models.
• ‘Bursty’ low-field side, far-SOL transport and/or ELMs should give higher deposition rates
0
5
10
15
20
25
0 50 100 150 200 250 300 350
C neutral
C+
C2+
C3+
C4+
C5+
C6+
C totalC d
en
sity
(re
l.)
poloidal angle
outerstrike point
innerstrike point
Deposition/erosion depends on stored energy