Electrostatic Dust Detector with Improved Sensitivity

D. P. Boylea, C. H. Skinnerb, A. L. Roquemoreb

a Columbia University, New York, NY 10027 b Princeton Plasma Physics Laboratory, Princeton, NJ 0854318th International Conference on Plasma Surface Interactions, May 26-30, Toledo, Spain

Abstract

Methods to measure the inventory of dust particles and to remove dust if it approaches safety limits will be required in next-step tokamaks such as ITER. An electrostatic dust detector, based on a fine grid of interlocking circuit traces, biased to 30 or 50 V, has been developed for the detection of dust on remote surfaces in air and vacuum environments[1]. Gaining operational experience of dust detection on surfaces in tokamaks is important, however the level of dust generated in contemporary short-pulse tokamaks is comparatively low and high sensitivity is necessary to measure dust on a shot-by-shot basis. We report on modifications in the detection electronics that have increased the sensitivity of the electrostatic dust detector by a factor of up to 120, - a level suitable for measurements on contemporary tokamaks.

[1] C. H. Skinner, R Hensley A.L. Roquemore, J. Nucl. Mater., 376 (2008) 29.

Dust is a safety issue in future tokamaks

Electrostatic grid developed for dust detection

• Dust produced by ELMs, disruptions, chemical synthesis, crumbling co-deposits, and mechanical activity• Dust detection and removal essential for safe operation

• Ultrafine, interlocking circuit traces are biased 30-50V• Impinging dust particles cause temporary short circuits • Current ejects or vaporizes dust particles, restoring open circuit • Current pulses counted by electronics and related to mass of incident dust Dust particles on grid

with 25 µm spacing and trace width

13 mm and 51 mm copper grids, 25 µm spacing and trace width2006 grids:• PEEK substrate• Laser etched• Standoff ~30V DC

2007 grids:• Ultralam® substrate• Photolithography• Standoff >60 V DC

Dust Safety Issue

Beryllium • Reactive with steam, H2

• Toxic

Carbon • Tritium retention• Explosion with air

Tungsten • Activation

Circuit diagram of counting electronics

Dust versus pulses

0

200

400

600

800

1000

0 5000 10000 15000 20000 25000

Number of pulses

Dust (Kg)

Safety Limit 1000kG

Administrative Limit 670 kG

Projections of dust production and removal from C. Alejaldre, S. Ciattaglia, et al., CCB1 Meeting 25 March 08

Micrograms of dust delivered by fine mesh

Tray mesh - 22 mm

Dust footprint - 25 mmLarge grid - 51 mm

Size comparison (actual size)

Extreme care needed to measure micrograms of dustTweezers

Brush

Weighing Chamber

Dust Tray on Foil

Nitrile Glove

Compressed Gas for cleaning

Balance Readout

(mg)

Small grid - 13 mm

• Dust scraped from unused CFC tile• Sifted through 104 µm mesh• Placed in tray with 2 or 3 layer 104 µm mesh bottom• Dust tray put in receptacle on flange, placed at top of vacuum chamber• Chamber can be pumped to ~100 mtorr• Flange knocked with wrench, shaking micrograms of dust through mesh• Dust spreads out slightly as it falls on grid directly below

• Sartorius® microbalance has 1 µg readability & accuracy• NSTX relevance requires minimal quantities of dust• Many challenges must be overcome to achieve this level of accuracyChallenges Solutions

• Balance sensitive to air currents, vibrations, static charges, temperature, and humidity

• Experiment performed in windowless lab, balance vibration filter set to high, metal tray dissipates charge, balance calibrated at least twice per day

• Mesh bottom allows dust to fall when tray is moved

• Dust tray weighed and moved atop aluminum foil disk to catch any dropped dust

• Dust tray must be moved and put in chamber without spilling dust or picking up ambient dust

• Ambient dust blown away with compressed gas, foil and tray regularly blown and brushed clean between trials, foil kept in weighing chamber during trials

• Tray cannot be touched with bare hands - fingerprints hygroscopic, weigh 40 µg !

• Tweezers and gloves used to handle tray and foil disk

• Balance takes 5-10 minutes to reach stable reading

• Consistent criteria for final measurement: must remain stable for 60 seconds

• Early measurements demonstrated less than ideal repeatability

• Auto zero function disabled and empty reading subtracted from final reading, multiple measurements recorded and averaged

• Measurements without dust delivery show only 1.6 ± 1.2 µg lost in each trial

51mm grid, 30V bias, 50mV SCA, Linear fit

10

100

1,000

10,000

100,000

0.1 1 10Incident Areal Mass Density

(µg/cm2)

Counts

13mm grid, 30V bias, 50mV SCA, Linear fit

10

100

1,000

10,000

100,000

0.1 1 10Incident Areal Mass Density

(µg/cm2)

Counts

13 mm grid, Weighted avgs, Exponential fit

0.1

1

10

0 200 400SCA Threshold (mV)

(ng/cm

2 /count

13 mm grid, 30 V bias, Air

-0.4

0

0.4

0.8

0 10 20 30Time (µs)

Amplitude (V)

Detection threshold reduced 4-9x at lower SCA setting

• Reducing SCA setting to 50 mV reduced detection threshold 4-9x• Vacuum increases detection threshold 7-15x at 50V, 2x at 30V

Oscilloscope waveforms show most activity near 50 mV

Previous detector performance

C. V. Parker et al., J. Nucl. Mater., 1461 (2007) 363-365.

C. Voiner et al., J. Nucl. Mater., 346 (2005) 266-271.

• Typical current pulse amplitude ~100 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• Several current pulses occur within single SCA pulse - shows pulse pileup• Oscilloscope showed high frequency oscillations when low pass RC filter used• Since current pulses occur on fast time scale and oscillations produced counts, low pass RC filter disconnected

~90% of dust vaporized or ejectedOpen trace shows amount of incident dust

Finer spaced grids improve sensitivity ~30x

Overall detection threshold reduced 50-120x without RC filter

2006 data from C. H. Skinner, et al., J. Nucl. Mater., 376 (2008) 29.

50 V Vacuum

30 V Vacuum

7x

10x

2x

3x

3x

50x60x

120x

3x

‘07 Air

‘07 Vacuum

‘06 Air

‘06 Vacuum

‘07 Air

‘07 Vacuum

‘06 Air

‘06 Vacuum

Previous SCA Setting

Waveform from dust on grid

Previous counts

‘07 counts

‘07 SCA Setting

30 V Air50 V Air

µm: (grid spacing)

Grid Spacing

0

20

40

60

80

100

120

0 200 400 600 800Incident areal mass density (µg/cm2)

Counts (Thousands)

25 µm

51 µm

76 µm

127 µm

102 µm

• 2006 detection threshold (~2-5 ng/cm2/count) sensitive enough for ITER (~mg/cm2) but needs to be improved to detect lower dust levels on NSTX.

• 51 mm grid shows 6x higher sensitivity than 13 mm grid• No strong change of sensitivity with bias voltage (30v and 50v)

• Dust detection threshold reduced ~100x due to:- Lower single channel analyzer setting- No low pass RC filter- Larger grid

• 2006: ~2-5 ng/cm2/count• 2007: ~0.02 ng/cm2/count• Sensitivity now matches dust levels found in NSTX.• Optimal settings: 50 mV SCA, 30 V bias, 51 mm grid.• Preliminary large dust signals from NSTX correlate with limiter

contact.

First NSTX Results (preliminary)

Summary & Conclusions

AcknowledgementsThe authors would like to thank T. Provost, T. Holoman, R. Marsala, and D. LaBrie for technical assistance and A. Bader, C. V. Parker, C. Voinier, and R. Hensley for previous work. D. Boyle acknowledges support from the 2007 National Undergraduate Fellowship in Plasma Physics and Fusion Energy Sciences. Supported by US DOE Grant No DE-AC02-76CH03073.

Location: in lower portbehind 2 gate valves

Double layer of 51 mm grids covered with 102 µm mesh

Large signal on lower grid when plasma contacts RF limiter

Ch1 shorted

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

Ch1 upper gridCh2 lower grid

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