IEPC-2015-53／ISTS-2015-b-53
Analysis of Ignition of the Micro Cathode Arc
Thruster
IEPC-2015-53／ISTS-2015-b-53
IEPC-2015-53/ISTS-2015-b-53
Presented at Joint Conference of 30th International Symposium on Space Technology and Science,
34th International Electric Propulsion Conference and 6th Nano-satellite Symposium
Hyogo-Kobe, Japan
July 4–10, 2015
G. Teel ∗ , J. Lukas, A. Shashurin, M. Keidar
The George Washington University, Washington, D.C., 20052, U.S.A.
Abstract: This paper is an analysis of experimentation of a vacuum arc thruster designed and developed at the Micro Propulsion and Nano Technology Lab at the George
Washington University. The purpose of this paper is to provide a beginning analysis of
the triggering method of the Micro Cathode Arc Thruster in hopes to gain a better understanding of the triggering mechanism. Two series of experiments were preformed to
understand the triggering mechanism. The first experiment was a series of lifetime tests
in hopes to provide a quantitative means of a working thruster head. A series of resistance measurements were taken as this was the driving method of quality assurance, but
as this paper discusses, this method should be taken as general guidelines. The second
experiment was to take an even closer look at the triggering process. Since a re-coating is
the driving mechanism for long term applications, the physical process was studied using a
Scanning Electron Microscope. A view of the vaporization process and a physical look at
the re-coating has been recorded, and studied in conjecture with resistance measurements.
Nomenclature
anode
BRICSat − P
cathode
EDX
hz
ISP
μCAT
PPU
SEM
V AT
∗ PhD
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Positive electrode
Ballistic Reinforced Satellite - Propulsion
Negative electrode
Energy-Dispersive X-ray spectroscopy
Hertz
Specific Impulse
Micro Cathode Arc Thruster
Power Processing Unit
Scanning Electron Microscope
Vacuum Arc Thruster
Student, Department of Mechanical and Aerospace Engineering, and georgelteel@gmail.com.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
I.
Introduction
evelopment of micro and nano-satellites has become a major interest of government, businesses and
D
universities alike in the aerospace industry. A fully customizable spacecraft to test the individual needs
of an independent buyer is something many have dreamed of with hopes also of being a cheap means into
space. There is a catch however, and in order for these small satellites to be of any longterm use one needs a
means of propulsion. This desire and need has lead to many concepts for small satellite propulsion systems,
but few have actually been produced and flown. There are quite a few reasons for this such as size, safety, and
cost. The major factor of many systems is the footprint required to house the propulsion units. Few systems
utilize gas or liquid propellant which requires space. The rocket nozzles or propulsion devices themselves are
published as small, but the footprint for fuel tanks and hose systems may take up a large amount of space.
Another major factor is cost. Many high level thrust devices provide the propulsion that many mission
designers like, but they can simply be too expensive. For some such as universities this makes them
undesirable or unachievable. A last factor which is important is power
consumption. For small cubesatellites, these power costs must be a minimum which severely limits the propulsion options.
An answer to these small spacecraft propulsion needs has been developed in the Micropropulsion and Nano Technology Laboratory (MpNL)
at the George Washington University known as the μCAT 1,2,3 . Shown in
Fig. 1 the μCAT is a small, compact, power efficient electric propulsion
VAT, which utilizes the triggerless arc method proposed by Anders et al
4
. This thruster has applications targeted mainly at, but not specific to
nano-satellite applications such as cubesatellites. The μCAT recently has
been shrunk to roughly the size of a U.S. dime, and has matured enough
to become flight hardware. The μCAT has flown on its first flight with the
United States Naval Academy’s BRIC Sat-P. With space flight successfully underway, a desire for long term missions is on the rise, and a more
Figure 1: μCAT firing.
intricate understanding of the device is required. Therefore, this paper is
here to be a starting point of the triggering mechanism.
II.
μCAT
The μCAT concept is shown in Fig. 2, and utilizes two electrodes a cathode and anode. These two
electrodes are separated by an insulating layer which typically have some form of conductive layer to allow
current to flow through. This conductive layer allows small amounts of parasitic current to pass through but
is mainly used to hinder flow until threshold is reached. Once the circuit, PPU, can achieve enough voltage
to break down the cathode, cathode spots form which generate the plasma.
This cathode is the fuel and propellant of the μCAT. VATs are similar to
PPTs in that they both use two electrodes and PPUs to generate their
ignition. The difference between the two is that PPTs breakdown their
insulating layer to form their propellant, whereas the VATs do not desire
to do so. This insulator degeneration is an undesirable product however,
which is a problem to the VATs fundamental design. An understanding
of this mechanism is desired to use the μCAT for long term operation.
The μCAT uses the triggerless ignition method proposed by Anders et
al 4 . The triggerless arc method has become a common method of ignition
for a variety of vacuum arc applications and makes a perfect candidate for
micropropulsion as it utilizes a small foot print, low power consumption,
and non-combustible propellant.5 This ignition method has not undergone
extensive research beyond 105 and 106 arc pulses.6 Anders et al has disFigure 2: μCAT fundamentals. covered that each arc degrades both the insulator’s and cathode’s material
which ultimately leads to failure. For long term operation the insulating
boundary must be replenished during each arc. At MpNL however, thrusters have run for longer than what
Anders et al has published, and have been successful for very long term operations. A study in terms of
degradation has not been done until now.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
As the breakdown of the insulator and cathode happens, the re-coating supposedly replenishes this breakdown. With this idea, one could measure the resistance of the coatings to be a defining metric for thruster
effectiveness as this value would change. This study is to test that case, and our fundamental conception
has been found to be partially incorrect.
III.
Resistance
The concept to studying resistance is to find the correct characteristic to define the μCAT. By conceiving
the thruster head as a series of contacts between our cathode and insulator, each arc potentially breaks down
these connections. Cathode spots form, which erupt on the surface of the cathode and explode into plasma.
The longer the thruster runs, the more connections break. This is the resistance variation; a measure of
connections which allow current to travel through. The fewer connections there are, the more resistance
there will be, and as plasma is exhausted a re-coating must happen to rebuild these connections. This study
has found that resistance is not the main characteristic however, but can be taken as a major guideline to
show a working thruster.
A.
Experimental Setup
The experimental setup was running the μCAT in vacuum around 7.5∗10− 5 Torr. Measurements of resistance
were taken before and after a series of pulses. Each set was run on the same PPU with the same conditions
to provide a standard set of trials. Experiments were run at 100 pulses, 1000 pulses, and 10,000 pulese which
are presented below as Trials 1, 2, and 3 respectively.
These thrusters were minimally coated, less than 1/8 the insulator surface, in order to localize the
plasma arc location to provide a specific reason of breakdown. This forced degradation to happen quicker
in an attempt to gain quicker resistance variance.
B.
Results
Taking a look at these results one can clearly see some form of trend form, albeit haphazardly, through
each trial shown. This variation can be attributed to the degradation of the insulating layer. An initial
breakdown of the coating and insulator is the rise in resistance and the re-coating is then the decrease in
resistance. The first set of data can be seen in Fig. 3. Trial 1 shows the most variety in resistance readings.
Just before half way a large rise of resistance is seen with the peak value jumping from the average of 500
Ohms up to 75.3 kOhms, and then rapidly recedes back.
Figure 3: Trial 1 with range of 100 pulses.
The second experiment shown is in Fig. 4. Trial 2 shows a different curve with the larger pulse count.
Two gains in resistance with a final drop at the end is shown. This inconsistency in resistance readings has
provided difficult in forming any standard variation and predictability. During the experiment, most likely
was an initial degradation without much coating from 1000 to 2000 pulses, but then a sharp drop is shown.
Another degradation then happens but ultimately leads to some stability.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
Figure 4: Trial 2 with range of 1000 pulses.
The last experiment and trial is shown in Fig. 5. Run over a longer scale, a parabolic scheme is observed.
Over this scale one can see a larger increase over the first 10,000 pulses to the 20,000 but then another
decrease in resistance over the last 30,000 pulses.
Figure 5: Trial 3 with range of 10,000 pulses.
All three trials presented here are clearly showing signs of deposition, but a further look is required and
taken in the next section. Resistance measurement are not a conclusive characteristic, and this study has
shown that 1 MOhm is too a large a resistance in order for the μCAT to function properly. This was also
Anders conclusion.4
IV.
Plasma Re-Coating
A continuation of the resistance measurements leads to physical studies as it is quite apparent something
is happening to make these values change. A plasma jet emitted from the cathode spot contains ions,
electrons, a small fraction of neutrals (<1 %) and macroparticles.7 A macroparticle is a chunk of the cathode
that was not fully ionized which is a loss in energy, and formation of slow large particles. The ions, electrons,
and neutrals are most desired as the plasma is ionized to 99% 7 and then is used as high ISP exhaust.
By utilizing a SEM to analyze the thruster electrodes, one can have an intimate view of the two thruster
leads down to microns. Such resolution provides a physical insight as to what is being produced, what is
being coated, and how it is coated.
A.
Experimental Setup
This experimental setup is a modified form of the previous experiment in order for the SEM to be utilized.
Two thin metallic foils are used for electrodes; tungsten for the cathode and copper for the anode. These
thin foils allow for an unobstructed view to find the exact breakdown point, and also reduce the overall
area that the thruster could potentially ignite. The insulator was coated with the conductive paint, and the
4
Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
separations varied from one half a millimeter to one millimeter. The experiments were run in vacuum at
pressures of 7.5 ∗ 10− 5 Torr. With the PPU that we have used for our satellite subsystem, the experiments
were run at 5 hz for 6 minutes which is an average pulse count of 1800 pulses.
B.
Results
Shown in Fig. 6 and 7 are the before and after SEM images of two trials run for these experiments. It is quite
apparent the differences that the arcing makes. The ready can truly see the difference in surface texture of
the two electrodes. The electrodes become rough, and the cathodes show clear signs of degradation from the
smooth surfaces from earlier.
(a) Trial 1 before arcs.
(b) Trial 1 after arcs.
Figure 6: Trial 1 flat plate analysis
(a) Trial 2 before arcs.
(b) Trial 2 after arcs.
Figure 7: Trial 2 flat plate analysis
Shown in Fig. 8 and 9 are before and after photos and X-Rays taken by the SEM and EDX. The images
all follow the same format. In clockwise direction and starting with the top left the images are as follows:
SEM scan, Carbon, Tungsten, and Copper X-Ray maps. The X-Ray maps can be read as material being
present when white dots are present which give clear indication of material type.
One can note in the before scans in both trials are clear lines of both electrodes and carbon paint
separation. Each one with a distinct divide with no mixing. Looking at the after scans, both figures
display a cloud of tungsten coating, emanating from the cathode. The reader could imagine the explosions
continually happening near the center and the plasma re-coating the carbon surface.
In Fig. 10 a magnified view of the surface is shown with the X-Ray scans adjacent. Taking a specific
look at the tungsten element map it is quite noticeable that the coating appears only on the surface, with
a shadow deep down where the plasma could not reach. The ignition creates connection ”islands” to form
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
(a) Trial 1 before arcs.
(b) Trial 1 after arcs.
Figure 8: X-Ray maps of Trial 1, before and after.
(a) Trial 2 before arcs.
(b) Trial 2 after arcs.
Figure 9: X-Ray maps of Trial 2, before and after.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
that allow the current to pass. The distance therefore changes each pulse, altering the surface between the
two electrodes due to the arc. The plasma is then left to coat the altered surface as best it can, which can
only coat the very top surfaces leaving partial distances uncovered.
All trials showed a decrease in resistance of roughly 1 kOhm except Trial 1. Trial 1 had a macro particle
forming at the cathode element, and after a minute it was expelled. This created a huge alteration in surface
morphology. It is believed this led to the large resistance increase, which then relates to the distance between
the two electrodes as a major factor in resistance readings. Perhaps after a more prolonged operation time
the plasma would re-coat the surface back to a typical reading. A longer study will be looked into.
Figure 10: Magnified view of Trial 1.
V.
Conclusion
These results have provided a new way to characterize the μCAT. The resistance as a measurement to
provide primary proof to a working thruster head is now only a piece of the puzzle. This method should be
used as general guidelines, as we learned the μCAT cannot work higher than 1 MOhm.
On top of the measurements, we have found that the plasma forms ”islands” of connections which
sparsely re-fix the coating as best a surface coating can. This ultimately is effected by the surface topology
and morphology, so a heavy degradation of the insulating surface will cause rough changes in arc effectiveness.
A closer step has been found to the ultimate findings and understanding of the μCAT. Further studies to
discover how to increase effectiveness and prolong the thruster lifetime are needed to lead to better designs
in the future.
References
1
T. Zhuang, A. Shashurin, S. Haque, and M. Keidar, ‘ ‘Performance characterization of the micro-Cathode
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2
M. Keidar, T. Zuang, A. Shashurin, G. Teel, D. Chiu, J. Lucas, S. Haque, L. Brieda, ‘ ‘Electric Propulsion
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3
T. Zhuang,A. Shashurin,T. Denz,P. Vail,A. Pancotti, and M. Keidar, ‘ ‘Performance characteristics of
micro-cathode arc thruster,” J. Propulsion Power, 30 2934, 2014
4
A. Anders, I. Brown, R. MacGill, and M. Dickinson, “ ‘Triggerless’ triggering of vacuum arcs,”
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5
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6
A. Anders, J. Schein, and N. Qi, “ Pulsed vacuum-arc ion source operated with a “triggerless” arc
initiation method,” Rev. Sci. Instrum. , 584-587, 1998.
7
R. Boxman, D. Sanders, and P. Martins, Handbook of vacuum arc science & technology: fundamentals
and applications, Noyes Publishing, 1996.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015