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
2/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
3/37IAEA RCM on Dust, Vienna, 10-12 December 2008
“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
4/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
5/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
6/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
7/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
8/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
9/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
10/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
11/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
12/37IAEA RCM on Dust, Vienna, 10-12 December 2008
Results with large area detector:
Results:
• Response linear down to "1 !g/cm2
• But dust levels in NSTX still lower.
13/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
14/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
15/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
16/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
17/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
18/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
19/37IAEA RCM on Dust, Vienna, 10-12 December 2008
• 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:
20/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
21/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
22/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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)
23/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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 ?
24/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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:
25/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
26/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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"
27/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
28/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
29/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
30/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
31/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
32/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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,
33/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
34/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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
35/37IAEA RCM on Dust, Vienna, 10-12 December 2008
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.
36/37IAEA RCM on Dust, Vienna, 10-12 December 2008
• 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
37/37IAEA RCM on Dust, Vienna, 10-12 December 2008
Thank you
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