Electrostatic dust detection and removal.

1/37IAEA RCM on Dust, Vienna, 10-12 December 2008

Electrostatic dust detection and removal.

Outline:

• TFTR & NSTX experience

• Dust Detection– Electrostatic detector

– Capacitive detector

• Dust Removal

• Path forward

Charles Skinner, Lane Roquemore, Henry Kugel, Charles Gentile, Steve Langish, PPPLwith National Undergraduate Fellows:

Aaron Bader, now at MIT; Chris. Voinier, College of New Jersey;Colin Parker, now at Princeton U.; Robert Hensley, Embry-Riddle Aeronautical Univ;

Dennis Boyle, Columbia University now at PPPL, Alejandro Campos, Rochester Univ.

Supported by US DoE DE-AC02-76CH03073


TFTR: flaking of thick codeposits

April ‘97 Codeposited layers weretenaciously attached during wholeperiod of TFTR plasma operationsending April ‘97

Oct. ‘97 No flaking was observed onvacuum opening, post operations.

Aug. ‘98 Unexpected surface featureswere observed on limiter surface onopening.

Nov.‘98 Flaking of co-deposited layer onlower part of limiter was photographed.

Jan. ‘99 Flaking observed on upper limiterAffected approximately 15% of theobservable limiter tiles.

[Nucl. Fus. 39 (1999) 1083]

! Thick codeposits expected on ITER(up to 400 !m a-C:H/ 10 day run period)Thermal and mechanical stabilityuncertain.


“Micro-seismology” in thick Li films on NSTX

• Occasionally see variation in quartz micro-

balance frequency after heavy Li deposition.

• NSTX Vessel still under vacuum.

• Flaking or detachment of layer possible

• ITER codeposition up to ~ 6-12 !m/day of BeT.

• Loose flakes could contaminate plasma and

complicate control of tritium inventory.

SEM image of deposit on Bay I crystal

Erratic behavior of !mass" of ~ 1 µm Li layer

103

104

105

106

24

25

26

27

17

18

19

20

21

22

PERSEC07.20070504

µg/c

m2

Time of Day (h)

<- Lower QMB

Upper QMB ->

µg/c

m2

115

120

125

130

135

140

25

30

35

40

45

50

23.5

23.6

23.7

23.8

23.9 24

PERSEC07.20070522

µg/c

m2

Time of Day (h)

µg/c

m2

<- Lower QMB

Upper QMB ->

End of Li deposition


TFTR: Tritiated dust levitationby beta induced static charge

Fus. Sci. Technol., 45 (2004) 11

• Radioactive decay of tritium via betaemission leaves a positive charge on adust particle.

• Tritiated particles could be uniquely

more mobile than other dust.

• Movie of tritiated dust from TFTR-->

Good News:

• D/C in TFTR dust only 0.007

T/C in TFTR dust only 0.0003

(TFTR D/T fueling ratio 3%)

Low D/C indicates high temperature

hydrogen isotope outgassing in dust

formation.

• cf. JET flakes D/C = 0.75

higher value similar to codeposits.


Duration of Dissolution (day)0 20 40 60 80 100 120 140

Rete

ntio

n o

f Tritiu

m

0.90

0.92

0.94

0.96

0.98

1.00

Sample #1Sample #2Fitted curve for retention

TFTR: Tritiated dust more hazardous than HTO

• Tritiated dust obtained from TFTR

• Size analysis showed it is respirable

– CMD = 1.23 !m, GSD = 1. 72 !m

• In-vitro dissolution rate measuredin simulated lung fluid.

Result:

• Only 8% of carbon tritide wasdissolved after 110 days.

• Low solubility means tritium willremain for long time increasingradiation dose to lung.

• Data needed on a:BeT dust todetermine allowable exposure !

Cheng et al., Fus. Technol., 41 (2002) 867

Tritium

activity


Electrostatic Detection

of dust settling on surfaces.

• A 30-50v bias is applied across agrid of interlocking traces on acircuit board.

• Impinging conductive dust creates ashort circuit and current pulse.

• Current pulse is input to nuclearcounting electronics and convertedto counts.

• Number of counts is proportional tomass of dust.

• Current also vaporizes or ejectsdust from the circuit boardrestoring an open circuit.

• Device works in air or vacuum.

500 µm

Schematic

Grid with 25 micron spacing

51"Single

ChannelAnalyser

100 µm

dia. of human

hair

Rev.Sci.Instrum. 75 (2004) 370


Waveform contains information on dust size

• Larger dust particles take longer tovaporize and create signals withhigher voltage and longer duration.

Electrostatic Dust Detectorin action

SCA pulses

filtered signal

unfiltered signal

~ 5-20 µm particle size

unfiltered signal

filtered signal

SCA pulses

~ 125-250 µm particle size


Dust signals:

25 µm gridspacing

50 µm

76 µm

127 µm

101 µm

Sensitivity increased30x with finer gridsDust signal recorded by 911 latching

scalar. (1.2 cm, 25 !m grid, air, 30 v)

0 20 40 60

time (seconds)

c

ounts

0 2

8

6

8

1

0

total 109 counts


Is response due to a few large particles ormany small particles ?

Unsifted

Dust

Viewing Area: 2.5 x3.4 mm

Sift carbon test particles in the“Sonic Sifter”…

Size distribution asmeasured inmicroscope aftersifting throughvarious sieves.(50 - 125 !m meansdust passed through125!m sieve but wasstopped by 50 !msieve).

‘Size’ is forequivalent sphericalparticles of sameprojected area.Filtering isincomplete.

Size Distribution


Counts depend on number of particles / cm2

size category

20-30 µm

5-20 µm

unsifted

53-125µm

125-250 µm

Response of 25 !m grid in air to different particle size categories.

Size category 20 - 30 !m means particles transmitted

by 30 !m sieve but retained by 20 !m sieve.

Much stronger response to small particles-favourable for tokamak dust as most is 1-micron scale.

Data on left replotted vs. estimated numberof particlesAnalysed microscope images give list of areas ofindividual particles.Assume volume = area(3/2) (cubic particles) andunit density to derive mass of imaged particles# particles / cm2 = mg/cm2 X # particles / mg

linear slope

sizecategory

20-30 µm

5-20 µmunsifted

53-125µm

125-250 µm

J. Nucl. Mater., 346 (2005) 266


Large area detectors:

• 5x5 cm grid has 16 times larger area

• 25 !m trace spacing

• 100 times more sensitive balance– 5 gram capacity with

0.000 001 gram readability

• Extreme precautions needed– (fingerprint weighs 40 !g)

• PEEK substrate to avoid fluorine– 30 volts max for PEEK– (spontaneous breakdown at 50 volts)

previous

1.2 x 1.2 cm grid

5 x 5 cm grid

Dust tray with double

104 micron mesh

Vacuum

Electrical

Feedthrough

Dust released

by vibration


Results with large area detector:

Results:

• Response linear down to "1 !g/cm2

• But dust levels in NSTX still lower.


Improvements in detection electronics

• Apply digital oscilloscope to analyse

the current waveform

• Typical pulse amplitude ~100s of mV

• Single channel analyzer produces

pulse as falling edge of waveform

crosses threshold

• Many more counts recorded at 50 mV

SCA setting than at 400 mV used

previously

• Some pulse pileup at lower threshold.

Present short-pulse tokamaks produceless dust than expected in ITER.Dust levels in NSTX were below 2006threshold of electrostatic detector.Improve sensitivity….

Previous SCA Setting

Waveform from dust on grid

Previous counts

‘07 counts

‘07 SCA Setting


Detection threshold reduced 4-9x at lower SCA setting

50 V Vac.

30 V Vac.

7x

10x

2x30 V Air50 V Air

• Detection threshold reduced 4-9xat lower SCA threshold (50 mvcompared to 400 mV).

• Detection threshold 7-15x higherin vacuum at 50V, 2x at 30V.

13 mm grid, weighted avgs, Exponential fit


Overall detection threshold reduced 50-120x without low pass RC filter

3x

3x

50x60x

A‘07 Air

C‘07 Vacuum

I‘06 Air

E‘06 Vacuum

120x

3x

A‘07 Air

C‘07 Vacuum

I‘06 Air

E‘06 Vacuum

Small grid Large grid


51 mm grid shows 6x highersensitivity than 13 mm grid A 51 mm grid C 13 mm grid

I 51 mm grid w/13 mm aperture

6x

51 mm grid

13 mm grid

Little change in sensitivity withbias voltages 25-50v

• Potential for multiple shorts fromsame particle

• Too low voltage may incompletelyvaporize dust result in continuousshort

• 51 mm grid gives 6x increase in counts

• smaller than 16x increase in area

• When covered by 13 mm aperture, 51 mm

grid has response similar to 13 mm grid

• Discrepancy could be explained by

increased pulse pileup in larger grid


Application to NSTX

• High sensitivity large area

5 cm x 5 cm grid mounted on

6” conflat behind 2 gate valves.

• Intention was to use lower grid

as a control to distinguish

electrical pickup.Location: in lower port

Double layer of 51 mm grids covered with 102 µm mesh

Ch1 upper grid

Ch2 lower grid


First preliminary results:

• Typically get a few counts.

• Surprisingly see counts also on Channel 2 lower

grid.• Dust also observed on lower grid after retrieval

from NSTX.

• Huge signal when plasma contacts RF limiter !

Typical signals from latching scalar (30 v bias, SCA 100 mV)

Ch1 shorted


• In the tokamak environment, largeparticles may fall on the grid,causing a permanent short.

• We have tested a helium gas puffsystem to clear off those particles.

• He @ 7 bar.• 160 V, 0.5s pulse drives piezo valve.• Plenum volume: 31 cm3.• Piezo valve - nozzle: 9.8 cm3.• Nozzle diameter: 0.34 mm @ 45°.

Before | After Puff into vacuum

7.6 cm 7.6 cm 1 cm

Carbon particles cleared from 4x4 cm area by He puffer:


Present status:

Electrostatic Dust Detector:• Dust detection threshold reduced ~100x due to:

- Lower single channel analyzer setting- No low pass RC filter- Larger grid

• 2006 sensitivity: ~2-5 ng/cm2/count• 2007 sensitivity: ~0.02 ng/cm2/count• Sensitivity now matches dust levels found in NSTX.• Maximal sensitivity @ 50 mV SCA, 30 V bias, 51 mm grid.• Large signals from NSTX correlate with dust production from

plasma-limiter contact but pickup contribution uncertain.

For 2009:• Cover lower grid with insulating coating to provide better isolation from dust

and unambiguous measure of electrical pickup.

• Add He puffer to clear particles from grid when needed.

• Operate at 50v voltage and 450mA current limit to ‘burn up’ large particles.

• Collaborate with Tore Supra on electrostatic dust detection.


Capacitive dust detection (Glenn Counsell):

•Capacitive Diaphragm Manometer (CDM)

Ceramic capacitive diaphragm

manometer adapted to be a

microbalance for dust detection

Tests of prototype show

- sensitivity of <0.5 mg/cm2

- dynamic range of >103.

- Remote electronics (can be controlled from 30m)

GF Counsell et al, Rev. Sci. Instrum. 77 (2006) 093501

Alternative/complementary method – using gravimetric principle

Prototype design courtesyof Inficon, Finland


CDM prototype tests:

Internal reference electrodes reducethermal expansion effects, but thermalshield still required

Early tests showed sensitivity to charge ondust – now eliminated by electrostatic shielding

Excellent linearity over wide dynamic range

Some hysteresis observed withthermal shield - optimisation stillrequired (thermal isolation from CDM)


Comparison:

Electrostatic Dust Detector:• Dust detection threshold recently

reduced ~100x– 2006: ~2-5 ng/cm2/count– 2007: ~0.02 ng/cm2/count

• Continuous detection.• Remote electronics

Tokamak demo ?• Sensitivity now matches dust

levels in NSTX– Tests planned for 2009 in

NSTX, Tore Supra.

• Other R&D needed:– adaptation to W and Be dust.– Fabrication in rad resistant

materials e.g. Silica.

• Extension to dust removal underdevelopment.

Capacitive Dust Detector:• Sensitivity 0.5 mg/cm2 at center

of diaphragm• Dynamic Range > 103

• Measures weight– insensitive to material species.

• Reference cell used.• Remote electronics

Tokamak demo?• Appears difficult at present dust

levels - sensitivity aimed at ITER

• Other R&D needed:– Residual thermal effects ?– Remote maintenance when full

of dust ?


Introduction of pre-characterized dust for dust transport studies in thedivertor and SOL

• Dust penetration of the core plasma in ITER can cause unacceptablyhigh impurity concentration and degrade performance.

• Introduction of pre-characterized dust from a known location offersa way to benchmark modeling of dust dynamics and transport.

ITPA DSOL-21 focus is on:• Characterization of core penetration efficiency and impact of dust of

varying size and chemical composition on the core plasma performance indifferent conditions and geometries

• Benchmarking of DustT modeling of dust transport and dynamics (Pigarov).• Joint Experiment with DIII-D, MAST, TEXTOR, LHD

NSTX Part:1. Penetration of particles from the scrape-off layer to the core plasma will

be studied by dropping Li particles into the scrape-off layer at variousangles and tracking their trajectory with fast cameras.

2. The new sample probe will be used to study the mobilization of surfacedust from the outer divertor region

New ITPA Div/SOL Joint Experiment Proposed:


DSOL-21 (1) drop dust into SOL to benchmark DustT

• 40 micron Li particles successfullydropped into NSTX plasma

• Views of individual dust particleshave been obtained with twospatially displaced fast cameras.

• 3-D trajectory code has recentlyproduced first trajectories. Codehas been written in freewarelanguage of Python

• Li dust in not ideal in that it is notpresently considered relevant asan intrinsic dust due to erosion.

• Plan to drop 1-2 milligrams ofpredetermined size and quantitiesof Li, C, B, and possibly, W and Al,into NSTX.

• Dust trajectory studies supportDUSTT code verification forextrapolation to ITER.


Propose to use sample probe to introducedust in SOL and see if it is mobilizedby ELMS, disruptions…

• Dust can be in mockup of a tile gap

• Measure influx spectroscopically andby fast cameras. (NSTX opengeometry is ideal to derive 3Dtrajectory).

• Potential materials: Al (as proxy forBe), B, Li, ….

• Intrepret results with DustT code.

DSOL-21 (2) Dust Studies with Sample Probe

48” Therm-ionics probe

Sample!briefcase"


Electrostatic Dust Removalfrom ITER ?

• Gaps between blanket modules

• Gaps between tile castellations

• Under divertor dome

• Under divertor cassette

140m2 of gaps

Dust typically accumulates at the bottomof a tokamak (TFTR diagnostic pipes,JET subdivertor…).

ITER has three constraints on dust:1. PUBLIC SAFETY: Cold dust: proposed 670 kg administrative limit. Projections suggest

dust removal when divertor cassettes exchanged could be sufficient.2. VACUUM VESSEL INTEGRITY: Explosion risk from potential production of H in an

accident with hot dust present. Relevant quantities of dust are 6 kg each of C, W andBe on hot surfaces. Diagnosis difficult.

3. CORE PLASMA CONTAMINATION: Migration of tungsten from dust produced byerosion or melt layer aerosols to the core plasma sufficient to cause the tungsten coredensity to be > 1e-5. Associated surface dust limit unknown.– Related issue: will dust in tile gaps be mobilizable ?

• Dust removal could have impact on ITER availability and major impact on DEMO.


Dust removal by electrostatic grid

• Dust removal required once inventorylimit approached.

• Long delays & millions EU if this neces-sitates removal of divertor modules.

– Overnight dust removal needed !

• The short circuit on electrostatic grid istemporary suggesting grid may be usefulfor the removal of dust from specificareas.

Dust in

mesh

container

Vibration

Grid

Funnel

Dust

Funnel

Grid

Dust in

mesh container

FunnelGrid

Vacuum

Chamber

The fate of the carbon dustparticles has been tracked bymeasurements of mass gain / loss.


Tests showed 91% of dust removed from board in vacuum

• We tracked the dust weight in the mesh container, funnel, and board.

• ~91% of dust is removed in vacuum.

– ~21% eliminated

– ~70% relocated onto funnel

• Some of the remaining dust was in a furrow due to a single open trace

Dust left on one board from anopen trace, trace widths are 50 !m

Dust removed from

energized area

Open Trace With

Dust Furrow

0.0%

20%

40%

60%

80%

100%

gri

d

fun

ne

l

mis

sin

g

Unenergized

Energized

fractional m

ass

(a)

J. Nucl. Mater., 363-365 (2007) 1461


Directed dust motion possible in 3-electrode grid

Triple Rotary Switch: Sequentially switches the three

voltages between the three sets of electrodes.

Tripolar grid

schematic

Isolated

PowerSupply

Traveling electrostatic wave: potential

gradient is created by three differentvoltages on the three electrodes.

• Potential to remove dust without machine venting and loss of plasma operational time.

• Critical need for higher duty factor of DEMO.

• A tripolar grid of fine interdigitated electrodes generates an electrostatic traveling

wave that transports particles to a “drain”.

• The electrodes are insulated with 300 nm thick silicon nitride (SiNx).

• Grids are produced by standard photolithography techniques.

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0

20

0

40

0

60

0

80

0

10

00

ele

ctr

ic f

ield

distance

V/2 V 0 V/2 V 0 V/2 V 0 V/2 V 0


Initial results show particle motion

• 50, 0, -50 V applied.

• So far, tests were performed with two working electrodes,since grids with three working electrodes were unavailable,

Carbon dust ontripolar grid with25 micron traces

Before voltage switch


Initial results show particle motion

Carbon dust ontripolar grid with25 micron traces

After voltage switch

• 50, 0, -50 V applied.

• So far, tests were performed with two working electrodes,since grids with three working electrodes were unavailable,


Help from NASA ?

• New collaboration with Dr. Carlos Calle ofNASA and Prof. Sigurd Wagner PrincetonMacroelectronics Group to apply NASAelectrostatic dust conveyers to fusion.

From “Dust Particle Removal by Electrostatic and Dielectrophoretic

Forces with Applications to NASA Exploration Missions”

C.I. Calle et al ESA Proc. Annual Meeting on Electrostatics 2008.

Before and after images of a dust shield prototype on a metal

plate to provide proof of concept for a sample handling system.

Mars Rover Spirit after a dust storm.


5 g/min (air) 1 g/min (vac) dust transported

(even vertically) by JAERI/Mitusubishi during EDA.

Overnight dust removal for ITER ?

Prof. S. Wagner, Princeton University Macroelectronics GroupY Oda et al., “Development of dust removal system for fusion reactor” J. Fus. Energy 16 (1997) 231

• Overnight dust removal without interrupting plasma operations would be ideal for ITER.

• Nanotechnology and large area displays are a rapidly evolving area.

• Propose to apply advances in macroelectronics to develop electrostatic dust transporter

• Mosaic of these devices could cover VV floor

• Use low activation substrate e.g. SiO2


Dust control achieved in SLR cameras

“Self Cleaning Sensor UnitA key element of minimizing dust is preventing itfrom clinging to the front surface of the imagingsensor. To combat against this, the EOS 40Dfeatures a Canon-designed Self CleaningSensor Unit. The low-pass filter at the front ofthe sensor shakes off dust automatically withultrasonic vibrations, removing dust from thesensor assembly.”http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcate

goryid=139&modelid=15653#ModelFeaturesAct

Need environment that can foster (and fund)similar creativity for ITER.


• Local measurement of dust on remotesurfaces in ITER required.

• Electrostatic and gravimetric detectiondemonstrated in laboratory.

• Adaptation to tokamak environment inprogress.

• Dust removal necessary once inventory limitis approached.

• Removal speed and efficiency needed forDEMO scales up ~ > x100 with duty cycle

• Prospect for continuous dust removal bylarge area flexible electrostatic grid.

Summary

500

µm


Thank you

Electrostatic dust detection and removal.

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