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