Electrostatic dust detector with improved sensitivity

D. P. Boyle^a^, **C. H. Skinner**^b\*^, and A. L. Roquemore^b^.

^a^ *Columbia University, New York, NY 10027*

*^b^ Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA.\
*

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. 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.

PSI keywords: Dust, Erosion & Deposition, ITER.

JNM keywords: P0500 Plasma-Materials Interaction, S0100 Safety of
Nuclear Reactors and Components, C0100 Carbon.

PACS codes: 52.40.Hf Plasma-material interactions; boundary layer
effects,\
52.90.+z Other topics in physics of plasmas and electric discharges.

Corresponding and presenting author address:

Charles H. Skinner, Princeton Plasma Physics Laboratory, POB 451,
Princeton NJ 08543 USA.

Email: <cskinner@pppl.gov>, Phone: 609 243 2214, Fax: 609 243 2665

1\. Introduction

The increase in duty cycle and erosion levels in next-step devices will
cause a large scale-up in the amount of dust particles produced
\[[^1],[^2],[^3],[^4],[^5]\]. This has important safety consequences as
the dust particles may be radioactive from tritium or activated metals,
toxic, and/or chemically reactive with steam or air. A value of 1000 kg
of dust inside the ITER vacuum vessel is used for safety assessments; in
addition very much smaller quantities on hot surfaces are important in
potential chemical reactions. Measuring the dust particle inventory is a
challenge in existing tokamaks, let alone one with the radiological
environment and scale of ITER. Diagnostics that could provide assurance
that ITER is in compliance with its dust inventory limits are in their
infancy. A separate challenge is demonstrating techniques that could
remove dust from the tokamak, once the limits are approached.

A novel device to detect the settling of dust particles on a remote
surface has recently been developed in the laboratory \[[^6],[^7]\]. A
grid of two closely interlocking conductive traces on a circuit board
was biased to 30 -- 50 V. Test particles, scraped from a carbon fiber
composite tile, were delivered to the grid by a stream of nitrogen.
Miniature sparks appeared when the particles landed on the energized
grid and created a transient short circuit. Typically, the particles
vaporized in a few seconds restoring the previous voltage standoff. The
transient current flowing through the short circuit created a voltage
pulse that was recorded by standard nuclear counting electronics and the
total number of counts was related to the mass of dust impinging on the
grid. The device worked well in both at atmosphere and in vacuum
environments. The sensitivity has been enhanced by more than an order of
magnitude by the use of ultrafine grids \[[^8]\]. The response to
particles of different size categories was compared and the sensitivity,
expressed in counts / areal density (mg/cm^2^) of particles, was maximal
for the finest particles. This is a favorable property for tokamak dust,
which is predominantly of micron scale \[Error: Reference source not
found\]. Larger particles produce a longer current pulse, providing
qualitative information on the particle size.

The short circuit is temporary suggesting the device may be useful for
the removal of dust from specific areas. The fate of the dust particles
has been tracked by measurements of mass gain / loss. Heating by the
current pulse caused up to 90% of the particles to be ejected from the
grid or vaporized, the removal efficiency depending on the experimental
geometry\[[^9]\], an encouraging result for the management of dust
inventories.

Real time measurements of dust on surfaces in contemporary tokamaks are
challenging because of the comparatively low dust levels. For example,
the average flux of dust in the National Spherical Torus Experiment
(NSTX) that accumulated on a glass slide was 5.6 ng/cm^2^/discharge, a
level below the estimated sensitivity of the 13 x 13 mm grid detector of
36 ng/cm^2^/count that was extrapolated from previous measurements at \~
mg/cm^2^ dust levels \[Error: Reference source not found\]. Experiments
with microgram quantities of dust and a large area (51 x 51 mm) grid
showed a doubling of sensitivity and linear response down to the lowest
measurable masses\[[^10]\]. The present work is aimed at optimizing the
detection electronics to further increase the sensitivity of the
detector and enable measurements on contemporary tokamaks. We note that
100 kg of dust evenly distributed on the lower part of ITER amounts to
\~ 60 mg/cm^2^ so sensitivity will not be an issue for ITER.

2\. Experimental setup.

The basic experimental setup has been previously described in ref.
\[Error: Reference source not found\]. Small (13 x 13 mm) and large (51
x 51 mm) area grids were fabricated by photolithography on Ultralam
substrates (in contrast ref. \[10\] used a laser write process on a PEEK
substrate). The traces and intertrace space were 25 µm wide as before
and the circuit could standoff at least 60 V. Simulated dust particles
were scraped from a CFC tile and pre-filtered through a vibrating mesh
with square apertures 104 µm a side. The particles were spread evenly
over a dust tray with a 22 mm square double or triple 104 µm mesh
bottom; the multiple layers helped retain the dust until needed. The
tray was weighed with a Sartorius ME5-F microbalance with 1 µg
resolution, 5 g capacity and 50 mm diameter pan that was recalibrated at
least twice / day. The dust tray was weighed and moved with an aluminum
foil disc underneath to catch any dust that fell through the mesh.
Extreme care was used to get high accuracy readings. Small temperature
differences between the larger objects and the balance could cause air
currents and stabilization times of 5 - 10 minutes. A reading was
considered final when it was stable for more than 60 seconds.

The mesh tray was carefully positioned in a holder suspended below a 6"
Conflat flange and the flange carefully placed on top of the test
chamber (Fig.1). The grid was mounted on the bottom flange and dust was
delivered to the grid by mechanically tapping on the chamber. The mass
loss of the tray was determined by measuring the mass of tray, foil disk
and dust before and after dust delivery. The area on which the dust fell
was measured by positioning white paper at the grid position. Most of
the dust fell within a 25 x 25 mm square area so that a 51 mm square
grid received all of the dust lost by the tray. The fraction of dust
falling on the 13 mm square grid was measured with a 13 mm square tray
at the grid position and was 28%. The accuracy of the mass measurements
was estimated by performing a weighing cycle without tapping the chamber
to deliver dust in both air and vacuum conditions. These showed a very
small average mass loss of 1.6 ± 1.2 µg that was subtracted from the
calculated mass difference delivered to the grid. Extraneous dust was
removed from the grid with compressed gas before every trial. For
experiments in vacuum, viton gaskets were used to seal the chamber and
avoid the mechanical vibration associated with tightening copper gaskets
and the chamber was evacuated to 50 - 100 mtorr. A baffle was installed
over the evacuation port in the chamber to deflect air currents from the
grid during evacuation or venting. To avoid disturbing the dust, pumping
was done slowly over 3 - 5 minutes and then the pumping valve closed for
dust measurements.

3\. Optimization of the Detection electronics.

The short circuit caused by the impinging dust particles caused current
to flow in a 51 ohm resistor in the detection circuit. In previous work
this signal was conditioned by high and low pass RC filters with a
bandpass of 1.3 -- 31 kHz (Fig.2). The low pass filter was intended to
intended to avoid pulse pile-up and saturation of the electronics for
the mg quantities of dust used in earlier experiments. For the present
experiment a switch was added that could disable the low pass RC filter
to increase the count rate. The signal was then input to an Ortec 550
single channel analyser (SCA) that produced an output pulse every time
the input signal decreased past a level of 400 mV and the pulses were
counted by an Ortec 775 counter. The input waveforms and output pulses
were also recorded by a Tektronix TDS50545B-NV 5 GS/s digital
oscilloscope and an example is presented in Fig 3.

With dust delivery to the grid, the voltage across the 51 ohm resistor
displayed irregularly shaped pulses of 50 mV to 2 V height with pulse
widths of about 1 µs. These typically occurred in bunches of 5 -- 10
that continued for several seconds. To assess the relation between SCA
threshold and number of output pulses the input waveform with the low
pass RC filter disconnected was input to 4 SCAs with settings of 50,
160, 200, 425 mV. Fig. 3 shows that there is a large increase in the
number of pulses from the SCA at a setting of 50 mV. There was some
pulse pileup at the lower threshold, possibly accentuated by the
impulsive nature of the dust delivery by tapping the chamber. Fig. 4
shows the dust detection threshold, expressed as the ratio of the flux
of dust falling on the grid to the number of counts generated for 30 and
50 V bias voltages in air and vacuum conditions. The points are an
average over 2 or 3 measurements weighted by the uncertainties of the
measurements, as there was some variation in sensitivity between trials.
However, the relative improvement from reducing the SCA setting from 400
mV to 50 mV was consistent between trials, and reduced the detection
threshold by a factor of 4-9x depending on conditions. Compared to the
value in air, the detection threshold under vacuum increased by 7-15x at
50 V and 2x at 30 V bias.

The largest number of counts was obtained by disconnecting the low pass
RC filter and using a 50 mV SCA threshold. Fig. 5 shows the results in
air and vacuum for 13 and 51 mm grids and for comparison previous
results from ref. \[Error: Reference source not found\] with the low
pass RC filter and 400 mV SCA threshold. It can be seen that the
sensitivity has increased by 50-120x compared to the earlier results. In
these comparisons, the flux of dust on the 13 mm grid is calculated as
28% of the mass loss by the tray (see sect. 2) divided by the area of
the 13 mm grid. For the 50 mm grid case the flux is the mass loss
divided by the area of the 50 mm grid. In ref. \[Error: Reference source
not found\] the relative sensitivity of the 51 mm grid on the PEEK
substrate was 2x higher than previous measurements with a 13 mm grid on
the Ultralam substrate, a factor much lower than the 16x increase in
area. In the present work both 13 and 51 mm grids were on Ultralam
substrates and a 6x higher number of counts was seen from the larger
grid \[conditions: 30 V bias voltage, air, 50 mV SCA\]. Covering the 51
mm grid with a 13 mm aperture resulted in the same number of counts as
the 13 mm grid. The scatter of the data points around the linear fit is
attributed to variations in the dust delivery to the grid and in the
subsequent pulse pile up. The sublinear increase in the response with
grid area could also be due to pulse pileup in the larger grid.

The dependence on number of counts with bias voltage was also
investigated. Lower voltages may result in multiple pulses from the same
particle if it is not immediately ejected from the grid. The sensitivity
of the 13 mm grid increased a factor of two when the bias voltage was
decreased from 50 to 25 V while the 51 mm grid did not show a clear
dependence of sensitivity on bias voltage \[conditions: air, 50 mV
SCA\]. On the other hand, bias voltages much lower than 30 V could
result in a continuous short when exposed to dust particles.

4\. Conclusions

The ability to measure ultralow levels of dust is important for
demonstrations of time resolved detection of dust on surfaces in
contemporary tokamaks. We have optimized the detection electronics for
the electrostatic dust detector by removing a low pass RC filter and
lowering the SCA setting from 400 mV to 50 mV. This resulted in an
increase in sensitivity, by a factor of up to a 120x and corresponding
reduction in the detection threshold from 2-5 ng/cm^2^/count to 0.02
ng/cm^2^/count for a 51mm square grid at 30 V bias voltage. There was
not a strong dependence of sensitivity on bias voltage and power supply
current limit. The sensitivity is now suitable for contemporary
tokamaks. Tests on NSTX are currently in progress and the results will
be reported elsewhere.

Acknowledgments

The authors thank T. Holoman, R Marsala and T. Provost for technical
assistance. D. Boyle acknowledges support from the 2007 National
Undergraduate Fellowships in Plasma Physics and Fusion Energy Sciences.
The grids were supplied by MicroConnex of Snoqualmie, WA. This work was
funded by U.S. DOE Contract Nos. DE-AC02-76CH0307.

Figure Captions:

Fig. 1 Dust tower used to deposit microgram quantities of dust on
electrostatic grid in air or vacuum environments. The dust tray was
weighed and loaded in the stand on the left. The flange + tray + catch
foil was then transferred to the test vessel on the right.

Fig. 2. Circuit used to convert dust signals to discrete counts.

Fig. 3 Waveform at input to SCAs (a) and corresponding 6V SCA output
pulses, offset for clarity, for the 13 mm grid, with bias voltage 30 V
in air. Label (b) denotes the 400 mV SCA setting and output pulses and
label (c) the 50 mV SCA setting and output pulses.

Fig. 4 Detection threshold as a function of SCA threshold. The points
represent a weighted average, the lines are to guide the eye. The labels
on the right denote the bias voltage and conditions. The dashed line
followed to the left shows a 7x decrease in detection threshold as the
SCA setting is decreased from 400 mV to 50 mV. The vertical arrows show
a 2x and 10x increase in detection threshold at 30V and 50V respectively
in changing from air to vacuum conditions.

Fig. 5 Overall detection threshold for (a) 13 mm grid and (b) 50 mm grid
with the low pass RC filter off under air and vacuum conditions. The
lines are a linear fit to the data.

![](media/image1.pct){width="3.0444444444444443in"
height="2.7555555555555555in"}

Fig. 1. Dust tower used to deposit microgram quantities of dust on
electrostatic grid in air or vacuum environments. The dust tray weighed
and loaded in the stand on the left. The flange + tray + catch foil was
then transferred to the test vessel on the right.

![](media/image2.pct){width="2.7555555555555555in"
height="1.711111111111111in"}

Fig. 2. Circuit used to convert dust signals to discrete counts.

![](media/image3.pct){width="2.8777777777777778in"
height="2.7777777777777777in"}

Fig. 3 Waveform at input to SCAs (a) and corresponding 6V SCA output
pulses, offset for clarity, for the 13 mm grid, with bias voltage 30 V
in air. Label (b) denotes the 400 mV SCA setting and output pulses and
label (c) the 50 mV SCA setting and output pulses.

![](media/image4.pct){width="2.8222222222222224in"
height="2.7666666666666666in"}

Fig. 4. Detection threshold as a function of SCA threshold. The points
represent a weighted average of 2-3 trials, the lines are to guide the
eye. The labels on the right denote the bias voltage and conditions. The
dashed line followed to the left shows a 7x decrease in detection
threshold as the SCA setting is decreased from 400 mV to 50 mV. The
vertical arrows show a 2x and 10x increase in detection threshold at 30V
and 50V respectively in changing from air to vacuum conditions.

![](media/image5.pct){width="2.7777777777777777in"
height="4.455555555555556in"}

Fig. 5 Overall detection threshold for (a) 13 mm grid and (b) 50 mm grid
with the low pass RC filter off under air and vacuum conditions. The
lines are a linear fit to the data.

[^1]: \[?\] G. Federici, et al., Nucl. Fus. 41 (2001) 1967.

[^2]: \[?\] J. P. Sharpe, D. A. Petti, H. --W Bartels, Fus. Eng & Des.,
    63-64 (2002) 153.

[^3]: \[?\] Z. Wang et al., in New Aspects of Plasma Physics,
    Proceedings of 2007 ICTP Summer College on Plasma Physics, Trieste
    30 July - 24 August 2007, ed. P. K. Shukla, L. Stenflo and B.
    Eliasson, World Scientific (Singapore) 2008.

[^4]: \[?\] J. Winter, Plasma Phys. Controlled Fusion 40 (1998) 1201.

[^5]: \[?\] M. Rubel *et al*., Nucl. Fusion 41 (2001) 1087.

[^6]: \[?\] A. Bader, C. H. Skinner, A.L. Roquemore, and S. Langish,
    Rev. Sci. Instrum. 75, (2004) 370.

[^7]: \[?\] C. H. Skinner, A. L. Roquemore, A. Bader and W. R. Wampler,
    Rev. Sci. Instrum., 75 (2004) 4213.

[^8]: \[?\] C. Voinier, C. H. Skinner and A. L. Roquemore, J. Nucl.
    Mater. 346 (2005) 266.

[^9]: \[?\] C. V. Parker, C. H. Skinner and A. L. Roquemore, J. Nucl.
    Mater., 363-365 (2007) 1461.

[^10]: \[?\] C. H. Skinner, R. Hensley and A. L. Roquemore, J. Nucl.
    Mater., 376 (2008) 29--32.