Plasma Potential Measurements in the Plume of a Colloid Micro-Newton Thruster
Colleen Marrese-Reading§, John K. Ziemer**, Manuel Gamero-Castaño**, David Bame** California Institute of Technology, Jet Propulsion Laboratory, Pasadena, CA, 91109, U.S.
and
Nathaniel Demmons†† and Vlad Hruby‡‡ Busek Co, Nantick, MA, 01760, U.S.
The plume produced by a single-emitter colloid micro-newton thruster was interrogated for potential field characteristics using an emissive probe. Plasma potential distributions in the plumes were characterized for 2-4 kV beam energies with various thruster electrode potentials and beam target potentials. A two-dimensional translation stage allowed both radial and angular profiles to be obtained. The results show that beam targets can significantly affect the plasma potential measurements in these low charge density plumes. It is shown that secondary electrons from the beam target will partially neutralize the positive charge density in the plume unless they are suppressed. The measurements also showed that biasing the outer beam target grid to 100 V significantly broadened the beam potential profile. The presence of a low-energy ion population is suggested to explain the results from varying beam target screen potentials. The measured plasma potential profiles indicate potentials between 0-30 V within an unneutralized beam that maintains a constant expansion angle past 10 cm downstream of the thruster head.
Introduction Colloid micro-newton thrusters (CMNTs) developed by Busek and JPL will be flight demonstrated as part of the
Space Technology 7 (ST7) mission in late 2009 [1]. ST7 is part of the NASA New Millennium Program and will be a payload on the ESA LISA Pathfinder spacecraft. This mission serves to demonstrate technologies critical for the detection of gravity waves with Laser-Interferometry Space Antenna (LISA) mission. The colloid thrusters employ an ionic liquid for propellant to generate an electrospray of charged droplets that are accelerated through 2-8 kV for Isp levels of ~100-300 s [2,3]. The ST-7 colloid thrusters are capable of producing 5-30 microNewtons of thrust with 0.1 microNewton precision from an array of nine emitters in each of four thrusters in a cluster. Carbon nanotube field emission electron sources are used for charge neutralization of the electrospray beam and spacecraft.
Electrospray thruster plume characterizations are underway to further develop the tools required to properly integrate these thrusters with other spacecraft systems. In this study, the thruster plume potential profile was characterized. The beam potentials provide charge density distribution in the plume and can quantify plume neutralization. Since future missions with electrospray propulsion technology are expected to require more thrust, potentially higher currents per emitter or larger emitter arrays in each thruster, it is important to identify the current density limitations for the transition between charge neutralizers required for spacecraft charge and for spacecraft charge and beam neutralization. The latter scenario will require neutralizers to be co-located with the thrusters ;otherwise, multiple neutralizers could be used on the spacecraft in convenient locations.
This paper presents the potential distribution measurements in the plume of a single emitter of a colloid thruster at various beam energies and the impact of beam target potentials on the plume potentials.
§ Senior Engineer, Thermal and Propulsion Engineering Section, [email protected], Member AIAA. ** Senior Engineer, Thermal and Propulsion Engineering Section, Member AIAA †† Research Engineer, [email protected] ‡‡ President, [email protected], Senior Member AIAA
42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit9 - 12 July 2006, Sacramento, California
A. Thruster The Busek colloid thruster shown in Figure 1 was used in these plume
characterizations. It consists of one capillary emitter in the center position of a nine emitter manifold. The capillary emitter is positioned behind the extractor electrode. Charged droplet emission typically starts with 1.85 kV between the emitter needle and extractor electrode. The potential between the emitter needle and ground determines the beam energy. An accelerator electrode at -1 kV with respect to ground is downstream of the extractor electrode to prevent electron back-streaming to the emitters. The thruster unit shown has an integrated feed system consisting of a pressurized bellows propellant reservoir, which feeds a piezovalve capable of a maximum flow rate of ~50 ug/s. Hydraulic resistance in the capillary emitter provides stable flow.
B. Facility The thruster plume characterizations were conducted in a 2 m x 2 m
ultra high vacuum chamber located in a class 10 compatible cleanroom that is currently being operated as a class 100 cleamroom. This facility is shown in Figure 2. The 2 m diameter door has a double O-ring seal that is differentially pumped with a turbomolecular pump. The main chamber vacuum is maintained by four CTI CryoTorr 10 pumps. The facility has a load-lock chamber with its own pumping system that includes a turbomolecular pump and a track for guiding a thruster from the load lock port into the center of the main chamber. The facility was designed to achieve 10-9 Torr base pressure with all of the four cryopumps operating. With only two of the cryopumps operating, a base pressure of 8x10-8 Torr has been routinely demonstrated. A probe positioning system in the facility has both rotational and linear stages capable of 180° and 400 mm travel, respectively. A National Instruments LabvVIEW data acquisition system controls and monitors the thruster, the positioning system and diagnostics.
C. Diagnostics The thruster, probe, probe boom and beam target test configuration in
the UHV chamber are shown in Figure 3. The probe was positioned within +/-90° from the thruster axis and up to 270 mm from the thruster accelerator electrode. The probe boom was electrically isolated from the grounded positioning system and the probe body was electrically isolated from the boom to minimize the impact of the grounded positioning system on the plasma potential measurements. The stainless steel beam target consisted of a collector and two screens. The screen near the collector was biased to suppress secondary electron emission from the collector at V2. The second screen was biased to deflect low energy ions that originated downstream of the charged droplet emitter at V1. The beam target, beam and extractor electrode currents were measured with electrometers. Plasma Potentials were measured with respect to facility ground using emissive probes. Emissive probes are more sensitive for plasma potential measurements than Langmuir probes in the low density and high energy ion beams generated by colloid thrusters. This diagnostic creates its own population of low temperature electrons and then primarily characterizes their response to the changing potential of the probe with respect to the plasma[4,5]. These probes have been proven to be effective in vacuum environments of 10-6 Torr. The floating probe technique was applied for the measurements acceptable results. In this technique, the probe filament heating current is increased to increase the current emitted from the filament and drive the probe towards the plasma potential in balancing the collected ion and emitted electron current against the collected electron current. The emitted electron current will continue to increase until the probe potential is biased more positive than the plasma potential by several
Figure 1. Busek EM CMNT.
Figure 2. Facility, thruster and diagnostic experimental configuration.
Figure 3. The emissive probes, boom and target configuration.
3
Tw/e, where Tw is the emissive wire temperature since the electrons are emitted with a distribution of electron energies. At this potential, space-charge limits further emission and changes in the probe floating potential. Once the required current is identified that satisfies the requirement for both extremes of the plasma being interrogated, the thermionically emitting floating probe can be swept through the beam while measuring its floating potential to quickly map the plasma potential profile in the beam. A dual emissive probe configuration was used because of the delicate filaments employed to minimize the emitted current levels required for the measurements. The emissive probes consisted of hairpin loop tungsten filaments that were contacted to copper lead wires in ceramic sleeves. The length of the probe was 5 cm. The probe that was built for these experiments is shown in Figure 3. The filament was heated by a power supply that was electrically isolated from ground with an isolation transformer. Isolation amplifiers were used for ground referencing signals for the data acquisition system. The error in the measurements with the applied technique is expected to be within +2.4 V and -1.35 V.
Plasma Potential Measurements The plasma potential profile was characterized for various beam target potentials and beam energies on the thruster single emitter both with and without a beam. Potential distribution profiles were obtained at 100, 200 and 270 mm radial positions and through +/- 90° from the axis of the thruster. The operating conditions for all of the plasma potential profile measurements are shown in Table 1. Initial measurements focused on the vacuum potentials without a thruster beam for reference. With no beam and no applied potentials to the thruster or target the beam potential was 0V at 100, 200 and 300 mm and +/-90°. With 100 V on the outer target electrode and all of the thruster potentials applied without a beam, the measured potentials were 0V at 100 mm, ~4 V at 200 mm and 8 V at 270 mm. Measured beam profiles are shown in Figures 4-8 for various beam currents, beam energies and target potentials. The operating conditions are described in Table 1 with a data set number and number of the figure with the data. The current level of the beam was higher than desired from the single emitter. It was not controllable with valve voltage because of bubbles in the feed system; however the beam current was stable during each of the measurements. The current was sensitive to extraction voltage, which was maintained at a relatively high voltage, 2 kV, to effectively ionize the high flow of propellant. The beam profiles followed a single or double Gaussian distribution with the critical parameters identified in the following relationship and specified in Table 1 for the data taken at constant radial positions through +/- 90°.
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Table 1. The operating conditions for each of the data sets presented.
Thruster Target Gaussian Beam Profile Fit Parameters Data
Axial beam profiles were obtained with and without the -1 kV applied to the extraction electrode to characterize the impact of the voltage on that grid on the local space potentials. Figure 4 presents this data and shows that the space potential is not very sensitive to the extractor electrode potential. At 13 mm from the electrode, the difference in potentials is only 14 V. Beyond 40 mm from the accelerator electrode, the potentials are not affected by it. This graph also shows where most of the potential fall occurs downstream of the accelerator electrode along the axis of the thruster from 13 mm to 270 mm from the accelerator electrode. Two sets of data are shown to demonstrate the repeatability of the measurements. While the beam is not operating with a charge neutralizer, the value of potential was higher than anticipated and the structure of it was surprising and not yet understood. This structure was observed in all of the measurements.
The beam potential distributions were characterized for a ~0.7 uA beam at 2 kV with a -1 kV accelerator grid voltage for four different beam target electrode potential configurations. The measured distributions are shown in Figure 5 with the beam target potentials identified on the graph. The operating conditions of the thruster and beam target are identified in Table 1. The measurements show that the beam potential distribution followed a single Gaussian distribution if the beam target electrodes were not biased. The Gaussian distribution parameters are shown in Table 1. Biasing the collector electron suppression grid resulted in higher beam potentials, as expected, since those secondary electrons from the beam target were repelled back to the collector and therefore, could not contribute to neutralizing the positive charge in the beam. With 100 V on the outer beam target grid and no secondary electron emission suppression from the target collector, those secondary electrons were accelerated up to 100 eV before entering the beam and therefore had less of an impact on the beam potentials with their lower charge density. This result is illustrated in the comparison of the two curves in Figure 5 for the biased beam target. Those 100 eV electrons were also capable of ionizing ambient neutrals to generate a more broad positive charge density in the plume. Biasing the beam target outer electrode changed the potential profile in the beam from a single Gaussian distribution to a double Gaussian distribution as shown in Figure 5 with the curve fit parameters given in Table 1. The secondary electrons could generate a second population of ions in the beam as the double Gaussian profile suggests. Ion evaporation from the droplets also contributes to a second population of ions in the beam [6]. These ions get evaporated radially from the charged drops from the high surface electric fields with energies dictated by local potentials including the 100 V on the beam target that could deflect it towards the wall of the vacuum chamber and contribute to the charge density distribution broadening shown in Figure 5. The effect of beam potential on plasma potentials was characterized for 2, 3 and 4 kV beams at approximately 0.7 uA. Arcing between the electrodes and ground prevented testing at higher beam energies. These results are shown in Figures 6-8. As expected with increases in beam energy and decreases in charge density, decreases in beam potentials and beam divergence were observed. Equipotential contour plots are shown for the 2 and 4 kV beams in Figures 7 and 8.
60
56
52
48
44
40
36
32
28
24
20
16
12
8
4
0
Volt
age
(V)
26024022020018016014012010080604020
Radial Position on Axis-Relative to Accel. Electrode (mm)
Ibeam= 0.97 uA
Iext= 0.18 uA
Icoll= 0.65 uA
Vbeam= 2 kV
Vext= 2 kV
Vtarget= 0 V, -100 V
Vacel= 0 kV
Vacel= 0 kV
Vacel= -1 kV
Figure 4. Axial potential distribution for 0 and -1 kV accelerator electrode potential.
5
30
20
10
0
Pla
sma
Po
ten
tial
(V
)
-80 -60 -40 -20 0 20 40 60 80
Angular Position w.r.t. Thruster Axis (°)
30
20
10
0
Pla
sma
Po
ten
tial
(V
)
30
20
10
0
Pla
sma
Po
ten
tial
(V
)
2 kV
3 kV
4 kV
200 mm
100 mm
270 mm 2 kV
3 kV
4 kV
2 kV
3 kV
4 kV
Figure 6. A comparison of the plasma potential distributions for 2, 3 and 4
kV beam at 100, 200 and 270 mm.
22
20
18
16
14
12
10
8
6
4
2
0
Pla
sma
Po
ten
tial
w.r
.t.
gro
un
d (
V)
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
Angular Position from Thruster Axis (°)
Vtarget = 0, 0V
Vtarget= 0, -30V
Vtarget= 100, 0V
Vtarget= 100, -30V
Figure 5 Plasma potentials in the beam with and without beam target potentials.
Figure 7 (top), 8 (bottom). Contour plots of the plasma potential distribution in the 2 and 4 kV beams.
7
Conclusion The plasma potential profiles were characterized for various beam target conditions, beam energies and thruster
electrode biases during operation of a colloid thruster with a single emitter needle and without an electron source to neutralize the beam of charged droplets. The results showed that a biased beam target can significantly affect the beam potential distribution in the plume. If not suppressed, the secondary electrons from the beam target will partially neutralize the positive charge density in the plume. The measurements showed that biasing the outer target grid to 100 V significantly broadened the beam potential profile. It is believed that the ions evaporated from the charged droplets are causing this broadening of the potential distribution as they are repelled back into the beam by the beam target or towards the wall by the beam potentials. This population of ions should be further characterized as they represent an ion distribution that could be accelerated back to the thruster or other spacecraft surfaces. The results showed that the accelerator potential had little affect on the beam potentials at -1 kV. The plasma potential profiles measured indicate very low beam potentials for the low current level tested.
The data presented represents a small contribution to a larger effort to fully characterize the beam potential and current density distribution as a function of beam current and voltage, facility conditions and neutralizer electron energies. The data will be used for model validation. The models will be used to identify requirements on the beam neutralizer characteristics. Future measurements will focus on the effect of neutralizer electrons on beam potential and current density distribution in the plume for various electron energies. The measurement and modeling results will be applied to identify the conditions under which a thruster will require an electron source for beam charge neutralization, instead of only spacecraft charge neutralization.
Acknowledgments The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of
Technology, under a contract with the National Aeronautics and Space Administration. The authors would like to acknowledge Al Owens for fabrication of the probe components and test fixtures.
References 1 T.M. Randolph, J.K. Ziemer, V. Hruby, D. Spence, N. Demmons, T. Roy, and W. Connolly, E. Ehrbar, J. Zwahlen, R. Martin, and C. Gasdaska, “Microthrust Propulsion for the Space Technology 7 (ST7) Technology Demonstration Mission,” 42nd AIAA Joint Propulsion Conference, Sacramento, CA, AIAA-2006-4320. 2 Gamero-Castano, M., Hruby, V., “Characterization of a Colloid Thruster Performing in the Micro-Newton Thrust Range,” IEPC-01-282, 27th International Electric Propulsion Conference, Oct. 15-19, 2001, Pasadena, CA. 3 Gamero-Castano, M., Hruby, V., “Electrospray as a Source of Nanoparticles for Efficient Colloid Thrusters,” AIAA 2000-3265, 36th AIAA Joint Propulsion Conference, July 16-19, 2000, Huntsville, AL. 4 Kemp, R., and Sellen, J., “Plasma Potential Measurements by Electron Emissive Probes,” Rev. Scientific Instruments, Vol. 37, No. 4, April 1966, pp. 455-461. 5 Measurements of plasma potential using collecting and emitting probes,” N. Hershkowitz and N. M. Cho, J. Vac. Sci. Technol.A Vol. 6, No. 3, May/Jun 1988. 6 Gamero-Castano, M., Fernandez de la Mora, J, “Direct Measurement of Ion Evaporation Kinetics from Electrified Liquid Surfaces,” J. Chem.Phys. Vol. 113, No. 2, July 2000.
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