AIAA 2011-3147

3rd AIAA Atmospheric Space Environments Conference 27 - 30 June 2011, Honolulu, Hawaii

Study of Hypervelocity Impact Plasma Expansion Nicolas Lee∗, Sigrid Close†, David Lauben‡, Ivan Linscott§, Ashish Goel¶, and Theresa Johnsonk Stanford University, Stanford, CA 94305, USA

Ralf Srama∗∗ , Sebastian Bugiel†† , and Anna Mocker‡‡ Institut f¨ur Raumfahrsysteme, Universit¨at Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany Hypervelocity impact testing at the Max Planck Institute for Nuclear Physics was undertaken with a suite of RF, optical, and plasma sensors. Iron particles between 10−10 g and 10−15 g were accelerated to speeds of 1 km/s to 50 km/s using a Van de Graaff dust accelerator and were impacted on metallic targets. The retarding potential analyzers (RPAs) designed and constructed to measure impact-generated plasma are described in detail. Several optical signals from a photomultiplier tube were detected and found to be temporally coincident with impact events that also generated measurable plasma. The plasma measurements from the RPAs indicate a dependence on target electrical bias and material. The expansion speed of plasma generated from impacts on unbiased tungsten was found to be approximately 20 km/s.

I.

Introduction

Spacecraft are routinely impacted by meteoroids and orbital debris. Impact events that result in mechanical damage have been well-characterized, but the electrical effects of hypervelocity impacts are still relatively unexplained. Testing at ground-based accelerators have confirmed that hypervelocity impacts generate plasma. This expanding plasma can cause RF emission which is able to penetrate into the Faraday cage of a spacecraft chassis even when the particle has caused no mechanical damage. The emitted RF energy, in some cases, can couple into sensitive electronic circuits. Damage from this effect can range from a spurious signal to total loss of the spacecraft. In this paper we describe experiments studying impact plasma expansion, undertaken using the 2 MV Van de Graaff dust accelerator at the Max Planck Institute for Nuclear Physics. Simulation of impact events from meteoroids, which routinely travel at speeds between 11 km/s and 72 km/s, and orbital debris, which travel at 7 km/s, in a groundbased facility is the first step in a research program to fully characterize the electrical effects of hypervelocity impacts in space. Anecdotal evidence of electrical anomalies correlated with hypervelocity impact have been reported with a wide range of effects. Though we are able to link the timing of certain anomalies to meteoroid shower peaks, few incidences have direct measurement of an impact event. ESA’s Olympus satellite experienced a loss of gyroscope stability during the peak of the 1993 Perseid shower.1 Spacecraft operators were able to regain use of the malfunctioning gyroscope, indicating that the anomaly was electrical in nature. Olympus was declared a total loss due to the amount of fuel required to regain attitude control. In 2009, meteoroid impact from the Perseid shower may also have been responsible for a similar problem on Landsat 5.2 Electrical problems were also reported on two Japanese satellites that failed during meteoroid showers: ADEOS II during the 2002 Orionids and ALOS during the 2011 Lyrids; both experienced a sudden drop in power generation resulting in loss of mission.3, 4 In 2002, the Jason-1 satellite experienced a measurable momentum transfer followed immediately by increased solar array current over three subsequent orbits. The impact was significant enough to change the spacecraft’s orbital ∗ Ph.D.

Candidate, Department of Aeronautics and Astronautics, Stanford University, Student Member AIAA. Professor, Department of Aeronautics and Astronautics, Stanford University, Member AIAA. ‡ Senior Research Scientist, Department of Electrical Engineering, Stanford University. § Senior Research Scientist, Department of Electrical Engineering, Stanford University. ¶ Ph.D. Candidate, Department of Aeronautics and Astronautics, Stanford University. k Ph.D. Candidate, Department of Aeronautics and Astronautics, Stanford University, Student Member AIAA. ∗∗ Associate Professor, Institut f¨ ur Raumfahrsysteme, Universit¨at Stuttgart. †† Accelerator Engineer, Cosmic Dust Group, Max-Planck-Institut f¨ ur Kernphysik. ‡‡ Laboratory Director, Cosmic Dust Group, Max-Planck-Institut f¨ ur Kernphysik. † Assistant

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Impact

Plasma formation

Initial electron motion

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Figure 1. Depiction of the plasma generation and expansion process due to hypervelocity impact.

semi-major axis by 30 cm, but the mass and velocity of the impactor cannot be decoupled. As a result, we still cannot confirm whether the spacecraft was hit by a tiny meteoroid traveling quickly or a larger piece of orbital debris traveling more slowly. Additionally, many electrical anomalies with an “unknown catalyst” on-orbit may be attributed to meteoroid impacts, and many events likely remain unreported due to the classified or proprietary nature of the spacecraft. An impact-generated plasma can cause electrical anomalies through several mechanisms. Charge separation can generate a current pulse, plasma oscillations can cause electromagnetic radiation, and the presence of the plasma can trigger an electrostatic discharge of dielectric materials on the spacecraft.5 By studying the expansion behavior of impact plasmas at a ground-based hypervelocity impact facility, we expect to gain a better understanding of these mechanisms that can lead to spacecraft anomalies. In particular, we hope to determine whether the mechanism of electromagnetic radiation due to plasma oscillation is viable. This mechanism is especially concerning since it does not require a direct electrically-conductive path through the spacecraft chassis in order to cause damage to an internal component. In section II, we summarize the plasma expansion model that predicts a broadband RF emission, and discuss previous studies that shed light on the expansion process through in situ and ground-based measurements. We then describe our experimental configuration in section III and preliminary results from currently ongoing tests in section IV. Finally, we outline future studies we plan to undertake, both at hypervelocity impact facilities and in space, in section V.

II. A.

Previous Work

Plasma Expansion Model

The plasma expansion model describing a mechanism for EMP production is outlined in figure 1, and is described in detail by Close et al.6 When a meteoroid or debris particle hits a spacecraft surface, the kinetic energy of the particle is partially converted to vaporization and ionization energy.7, 8 The resultant small dense plasma is composed of material from both the impactor and the target surface. Electrons in this plasma expand outward faster than ions because of their higher mobility, creating an ambipolar electric field which pulls the electrons back toward the ions. As the ions expand out at their isothermal sound speed, the electrons oscillate coherently about the ion front, radiating at the plasma frequency. This frequency decreases with density as the plasma expands. The resulting RF signal is a chirp with the initial frequency dependent on the initial ion density and the rate dependent on density falloff. B.

In Situ Measurements

Several spacecraft have detected evidence of RF emission correlated with hypervelocity impact events on their science instruments. In particular, the Cassini spacecraft detected RF signals associated with impact events from nanometer-sized particles which had been accelerated to 450 km/s,9 and from micrometer-sized particles from Saturn’s rings moving at roughly 10 km/s relative to the spacecraft.10, 11 Waveforms were recorded from the 10 m dipole antennas on the Radio and Plasma Wave Science (RPWS) instrument.12, 13, 14 The STEREO pair of spacecraft have also detected electrical effects of impact events using the STEREO wave instrument (S/WAVES), which are coincident with optical measurements made by the two spacecrafts’ SECCHI instrument suite.15, 16 The Ulysses spacecraft detected “nonnominal signals” from its Unified Radio and Plasma wave (URAP) sensor (sensitive to RF signals from DC to 1 MHz) that were temporally correlated to dust stream impacts detected by the DUST impact-ionization detector sensor.17

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Figure 2. Projectile mass and velocity regimes for various accelerator technologies, as well as the meteoroids of interest.

C.

Ground-Based Testing

Many studies have been performed at hypervelocity impact facilities including Van de Graaff dust accelerators, light gas guns, and plasma drag accelerators. However, as seen in figure 2, there is no ground-based technology that fully replicates the projectile masses and speeds associated with meteoroid impact. Additionally, the ambient pressure achievable at most light gas gun facilities is on the order of 0.1 mbar. This corresponds to a mean free path of millimeters, resulting in a collisional expansion of the impact plasma into the ambient atmosphere. The effect of ambient atmosphere has been studied for expanding plasma plumes from laser ablation studies18 and is significant for impact-generated plasmas as well. Even with Van de Graaff dust accelerators, where the vacuum levels are typically on the order of 10−6 mbar, the size of the test chamber itself becomes the limitation, with reflected signals manifesting within nanoseconds of the impact event. As a result of these many challenges, there has been no complete analysis of the phenomenon of electromagnetic emission from hypervelocity impact. Instead, there remains much disagreement in the field about the mechanisms behind impact-induced radiation. Starks et al.19 attribute impact light flashes to rapid recombination of a fully-ionized plasma, while Burchell et al.20 conclude that light flashes are not due to recombination since they are not affected by his direct plasma measurements, which inhibit recombination. Takano’s research group21, 22, 23, 24 associates their microwave signals to microcracking while Starks et al. searched for microwave signals they attribute to plasma oscillation. Crawford and Schultz25, 26 posit a macroscopic charge separation, which may be due to their use of a powdered dolomite target rather than the more commonly studied solid metallic targets. Additionally, studies of hypervelocity impact plasma production typically use an accelerating grid at the target to separate the charge species and to direct them into a sensor. This is done in order to measure time-of-flight of different ion species and to measure the composition of the plasma. However, the grid also significantly changes the behavior of the expanding plasma.

III.

Experimental Approach

A preliminary series of hypervelocity impact experiments was undertaken at the Max Planck Institute for Nuclear Physics (MPIK) in December, 2010. A suite of RF and plasma sensors were used to study the impact of iron projectiles on various metallic targets. We believe that this is the first experiment to study RF emission from a freely-expanding plasma at a Van de Graaff dust accelerator. The lack of an accelerating grid at the target and the vacuum level achieved in the test chamber contribute to an experimental setup that best replicates the impact of a meteoroid or orbital debris particle on a spacecraft surface.

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PSU 2 MV Terminal Detector QP Dust Source

Detector 1 & 2 Chamber

Particle Deflector

Belt Potential Rings

Figure 3. Schematic of the Van de Graaff dust accelerator facility. Particles travel from the source at the right to the test chamber on the left, corresponding to photographs of the MPIK facility.

Figure 4. Dust accelerator drift tube and vacuum chamber. The Van de Graaff generator is located behind the wall to the right.

A.

Facility

At the heart of the Van de Graaff dust accelerator facility is a positively-charged dust source feeding a 2 MV acceleration path. A simplified schematic of the facility is shown in figure 3. Particles are housed in a small chamber at the 2 MV terminal, and caused to swirl by pulsing the voltage of the source chamber. As the dust moves, particles occasionally contact the tip of a charged tungsten needle, which provides the final amount of charge to inject the particle into the accelerator. The speed each particle attains as it drops through the potential coils is dependent on the charge and mass of the particle. As they enter the drift tube at the end of the accelerator (shown in figure 4), particles pass through induction loops that measure their charge and velocity. The mass of particles is determined from the measured charge and speed by equating the kinetic energy of the particle to the potential energy across the Van de Graaff terminals: 1 2 mv = qU, (1) 2 where m, v, and q are the mass, speed, and charge of the particle and U is the accelerating voltage. These measurements drive capacitor plates that deflect particles not meeting programmable selection criteria. Particles meeting the selection criteria are allowed to enter the vacuum chamber. The vacuum chamber is 1.4 m in diameter and can be depressurized to 10−7 mbar. Our experiments were performed at pressures between 2.5 × 10−6 mbar and 8.1 × 10−6 mbar, corresponding to a mean free path longer than the chamber diameter. The chamber contains a horizontally-translating platform (laterally across the beamline) with a mechanical feedthrough to allow manipulation of the internal geometry without breaking vacuum. The position of the platform is determined by measuring the resistance of a linear potentiometer and can be controlled to sub-millimeter precision. Two large flanges allow for a variety of electrical feedthroughs.

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Figure 6. Cumulative distribution of particle masses and speeds for each day of the experiment.

B.

Projectiles

Spherical iron particles between 10−10 g and 10−15 g were used for these experiments, though the facility has the capability to accelerate many other conductive projectiles, including organic particles coated in a conductive layer. Figure 5 shows a scatter plot of the particle masses and speeds recorded during the experiments, and figure 6 shows the cumulative mass and speed distributions for each day. The depletion of small (and therefore high-speed) particles after the first day was particularly significant. C.

Targets

Five targets (shown in figure 7) were used to study the effect of impacts on different materials and thicknesses. Two of the targets were active, serving as stub antennas with an integrated amplifier to measure the electric field at the point of impact. One of these used an 8 mm diameter brass knob as the target, which was smaller than the beamline scatter area. Due to this concern, the target was modified to include a 40 mm diameter copper disc. The other three targets were passive but could be biased to ±1 kV to simulate spacecraft charging. This bias was applied using a high-voltage source through an RC circuit to decouple any discharge events from the supply. The RC circuit included a 435 nF capacitor from the target to chamber ground and a 225 kΩ resistor feeding the capacitor from the voltage source. The three passive targets include 50.8 µm thick tungsten, 254 µm thick aluminum, and 12.7 µm thick aluminum. All of the targets were mounted horizontally in-line so that each could be translated into the beam line using the chamber’s mechanical feedthrough. Initially the targets were inclined down 30◦ from the horizontal beam line to point at the RF sensors, and were later inclined up 30◦ to direct the surface normal toward the plasma sensors.

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Figure 7. Left: Brass target. Center: Copper target. Right: Tungsten and aluminum targets.

Plasma sensors

Target

Photomultiplier Tube

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Figure 8. Left: Side view of the target and sensor configuration. Right: View of the target structure with photomultiplier tube.

D.

Sensors

The test chamber was outfitted with a suite of plasma and RF sensors (as shown in figure 8) to characterize the expanding plasma and any associated RF emission. A Hamamatsu photomultiplier tube (PMT) was used to detect coincident optical response. This PMT was mounted above and offset to the side of the passive target, at a distance of 15 cm from the tungsten foil. It has a photocathode area of 78 mm2 and is sensitive to wavelengths between 300 nm and 650 nm. The anodeto-cathode voltage was -800 V and the output current was measured directly by the 5 Gs/s oscilloscope on a 1 MΩ DC coupled channel. The RF suite was composed of two log-periodic arrays (LPAs) sensitive to frequencies between 50 MHz and 4 GHz, three VLF loops designed to detect signals between 300 Hz and 50 kHz, and the E-field sensor embedded in the active targets. The two LPAs were arrayed to measure horizontally and vertically polarized signals and were mounted on the horizontally-translating platform about 1 m from the impact point. Each signal was passed through two low-noise amplifier (LNA) stages and sampled at 5 Gs/s. The VLF loops were also mounted on the platform. However, the 100 kHz data acquisition system used by the VLF system was too noisy to run simultaneously with the LPAs and the plasma sensors. The signal from the E-field sensor, after passing through its internal amplifier stage, also was fed into two low-noise amplifier stages before being sampled at 5 Gs/s. A retarding potential analyzer (RPA) was designed specifically for this experiment, and two units were constructed to measure the constituents of the expanding plasma. The design of the RPA is similar to that described by Marresse et al.27 and Heelis and Hanson28 and is outlined in figure 9. The sensor has an effective collecting area of 10 cm2 behind a series of four electrode grids. These are, from front to back, the floating, repeller, threshold, and suppressor grids. The floating grid is electrically connected to the RPA chassis to shield the internal electric fields of the RPA and is constructed of a stainless steel mesh (229 µm wire thickness) with 73.3% transmissibility. This mesh was chosen for mechanical strength to protect the other grids. The repeller grid is biased to allow only electrons or only ions through. The threshold grid prevents low-energy particles from passing through. Finally, the suppressor grid prevents secondary ionization from causing electrons to escape the collector plate. These three internal grids have a 6 of 11 American Institute of Aeronautics and Astronautics

Repeller grid Threshold grid Suppressor grid Vout+ VoutTransimpedance Differential amplifier driver

Housing ground

Figure 9. Simplified schematic of the retarding potential analyzer.

Figure 10. Retarding potential analyzer used in hypervelocity impact experiments.

wire thickness of 30 µm and a transmissibility of 88%. The grids are held apart by nylon spacer discs, and their bias voltages are transmitted through the screws holding the assembly together. The signal from the collector plate is passed through a transimpedance amplifier and a differential driver before being passed out of the vacuum chamber. The embedded amplifier provides direct pickup without any front-end cable loss, and the differential driver mitigates plasma-generated RF interference and accelerator-generated EMI. The two amplifier stages are AC coupled. The assembled RPA is shown in figure 10. Two RPAs were used to measure the plasma at different ranges from the impact point, as shown in figure 11 in order to compute a plasma expansion speed. These two RPAs were mounted at 75 mm and 150 mm from the impact point, at angles of 15◦ and 30◦ from the target normal, respectively. The RPAs were positioned using a network of tension lines to the walls of the chamber, minimizing the amount of mechanical and electrical interference to the expanding plasma. Due to the limited number of oscilloscope channels available, the RPA differential signals were sampled at both 1 Gs/s and 5 Gs/s. In order to maintain the same frequency content from both RPAs, each sensor had one half of its differential signal sampled at 5 Gs/s and the other half at 1 Gs/s. The lower-rate signal was then interpolated linearly to be combined with the higher-rate signal.

IV.

Results

Many impact events resulted in a plasma signal detected by the nearer RPA (denoted RPA-A). Representative waveforms are shown in figure 12. The number of plasma signals detected out of the total number of impact events recorded yields a signal detection rate for each target and for each bias configuration, as shown in figure 13. The impacts on biased aluminum clearly show a higher detection rate than on unbiased aluminum. The same trend occurs for positively-biased tungsten, but not for negatively-biased tungsten. This may be a result of tungsten’s higher electronegativity. From the temporal spacing in the peaks between the two RPAs, the plasma expansion speed was computed for

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RPA-B RPA-A

Particle beam line Figure 11. View looking upward at the targets and RPAs from below. 40 20 RPA−A [mV]

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Figure 12. Typical signals recorded from impacts on unbiased tungsten. Top: Fast rising and exponential decaying peak from a 4.7 km/s impact of a 1.86 × 10−12 g particle. Middle: Slow rising and falling peak from a 4.1 km/s impact of a 2.62 × 10−12 g particle. Bottom: Oscillatory rise and exponential fall from a 5.2 km/s impact of a 1.91 × 10−12 g particle.

approximately 100 impacts. A histogram of the computed speeds is shown in figure 14. The mean expansion speed is 20.8 km/s with a standard deviation of 3.5 km/s, which is consistent with an expansion at the isothermal sound speed. A detailed analysis of the results from the plasma sensor suite is presented by Lee et al.29 The data from the RF sensor suite is still under study. The PMT yielded 28 detectable signals corresponding to the particle masses and speeds indicated in the scatter plot in figure 15. A respresentative PMT waveform is also shown. Out of the 28 signals, 21 occured within the 3 µs preceding a detected RPA peak. From the timing of these signals, it does not appear that this response is purely due to impact flash, which would have occurred earlier. Instead, the PMT response may be associated with some optical phenomenon in the expanding plasma. The long transient of the waveform is potentially a result of line capacitance and the measurement configuration. However, more study is required to validate this hypothesis and determine the mechanism behind this optical response.

V.

Future Work

A second experiment campaign is planned at the Van de Graaff dust accelerator facility to address some unanswered questions. A deeper study of the plasma plume geometry is expected, as well as a wider scope of target materials. Additionally, we plan to use a similar sensor suite to characterize plasma expansion at a light gas gun facility in order to obtain data from the larger projectile mass regimes. The primary challenge with light gas gun facilities is that typical vacuum levels are much poorer than at Van de Graaff accelerators (10−1 mbar compared to 10−6 mbar), resulting in a much shorter atmospheric mean free path. The plasma-atmosphere interactions will have to be incorporated into the plasma expansion model in order to interpret results from these tests.

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Figure 13. Plasma signal detection rate on RPA-A for different targets and biases.

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Figure 14. Histogram of plasma expansion speeds computed from impacts on unbiased tungsten.

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Figure 15. Left: Scatter plot of detected PMT signals as a function of particle mass and speed. Right: Representative PMT response (from a 31 km/s impact of a 2.7 fg particle on positively-biased tungsten). The time axis is referenced to the start of the PMT transient response.

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Figure 16. Rendition of CubeSat with deployable MMOD impact screen on orbit.

In order to fully characterize the expasion of impact plasma in space, in situ study will be required. The best vacuums achieved at ground-based hypervelocity impact facilities cannot replicate conditions above 150 km altitude in terms of atmospheric mean free path and are ultimately constrained by the size of the test chamber. To provide an in situ platform for studying the expansion of hypervelocity impact plasma in space, we plan to construct a CubeSat with a deployable meteoroid impact screen to maximize exposed area, as shown in figure 16. The CubeSat will use miniaturized plasma particle detectors and an RF sensor suite to characterize the expanding plasma. Since a CubeSat will be impacted by meteoroids of unknown mass, density, and speed, we will use the data obtained from ground-based testing to solve the inverse parameter estimation problem. RF and plasma signals will be used to provide an estimate of the projectile properties, which can be compared against truth. This estimator will then be used to determine projectile properties from impact data on the spacecraft.

Acknowledgments N. Lee thanks the Canadian Natural Sciences and Engineering Research Council (NSERC) for providing a graduate research fellowship which helped make this work possible. The accelerator experiments were funded through Los Alamos National Laboratory. The authors gratefully acknowledge the contributions from Dr. Patrick Colestock and Stan Green.

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11 Srama, R., Kempf, S., Moragas-Klostermeyer, G., Helfert, S., Ahrens, T. J., Altobelli, N., Auer, S., Beckmann, U., Bradley, J. G., Burton, M., et al., “In situ dust measurements in the inner Saturnian system,” Planetary and Space Science, Vol. 54, No. 9-10, 2006, pp. 967–987. 12 Kurth, W., Averkamp, T., and Gurnett, D., “Cassini RPWS Observations of Dust Impacts in Saturn’s E-ring,” Dust in Planetary Systems: Workshop Program and Abstracts, Vol. 1280, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX, 77058-1113, USA,, 2005, pp. 102–103. 13 Wang, Z., Gurnett, D. A., Averkamp, T. F., Persoon, A. M., and Kurth, W. S., “Characteristics of dust particles detected near Saturn’s ring plane with the Cassini Radio and Plasma Wave instrument,” Planetary and Space Science, Vol. 54, No. 9-10, 2006, pp. 957–966. 14 Meyer-Vernet, N., Lecacheux, A., Kaiser, M. L., and Gurnett, D. A., “Detecting nanoparticles at radio frequencies: Jovian dust stream impacts on Cassini/RPWS,” Geophysical Research Letters, Vol. 36, No. 3, 2009, pp. L03103. 15 Meyer-Vernet, N., Maksimovic, M., Czechowski, A., Mann, I., Zouganelis, I., Goetz, K., Kaiser, M. L., St. Cyr, O. C., Bougeret, J. L., and Bale, S. D., “Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?” Solar Physics, Vol. 256, No. 1, 2009, pp. 463–474. 16 St. Cyr, O. C., Kaiser, M. L., Meyer-Vernet, N., Howard, R. A., Harrison, R. A., Bale, S. D., Thompson, W. T., Goetz, K., Maksimovic, M., Bougeret, J. L., et al., “STEREO SECCHI and S/WAVES Observations of Spacecraft Debris Caused by Micron-Size Interplanetary Dust Impacts,” Solar Physics, Vol. 256, No. 1, 2009, pp. 475–488. 17 Grun, E., Zook, H., Baguhl, M., Fechtig, H., Hanner, M., Kissel, J., Lindblad, B., Linkert, D., Linkert, G., and Mann, I., “Ulysses dust measurements near Jupiter,” Science, Vol. 257, No. 5076, 1992, pp. 1550. 18 Harilal, S. S., Bindhu, C. V., Tillack, M. S., Najmabadi, F., and Gaeris, A. C., “Internal structure and expansion dynamics of laser ablation plumes into ambient gases,” Journal of Applied Physics, Vol. 93, 2003, pp. 2380. 19 Starks, M. J., Cooke, D. L., Dichter, B. K., Chhabildas, L. C., Reinhart, W. D., and Thornhill III, T. F., “Seeking radio emissions from hypervelocity micrometeoroid impacts: Early experimental results from the ground,” International Journal of Impact Engineering, Vol. 33, No. 1-12, 2006, pp. 781–787. 20 Burchell, M. J., Kay, L., and Ratcliff, P. R., “Use of combined light flash and plasma measurements to study hypervelocity impact processes,” Advances in Space Research, Vol. 17, No. 12, 1996, pp. 141–145. 21 Takano, T., Murotani, Y., Maki, K., Toda, T., Fujiwara, A., Hasegawa, S., Yamori, A., and Yano, H., “Microwave emission due to hypervelocity impacts and its correlation with mechanical destruction,” Journal of Applied Physics, Vol. 92, 2002, pp. 5550. 22 Maki, K., Takano, T., Fujiwara, A., and Yamori, A., “Radio-wave emission due to hypervelocity impacts in relation to optical observation and projectile speed,” Advances in Space Research, Vol. 34, No. 5, 2004, pp. 1085–1089. 23 Ohnishi, H., Maki, K., Soma, E., Chiba, S., Takano, T., and Yamori, A., “Study on Microwave Emission Due to Hypervelocity Impact Destruction,” URSI-GA, Delhi, October 2005. 24 Ohnishi, H., Chiba, S., Soma, E., Ishii, K., Maki, K., Takano, T., and Hasegawa, S., “Study on microwave emission mechanisms on the basis of hypervelocity impact experiments on various target plates,” Journal of Applied Physics, Vol. 101, 2007, pp. 124901. 25 Crawford, D. A. and Schultz, P. H., “The production and evolution of impact-generated magnetic fields,” International Journal of Impact Engineering, Vol. 14, No. 1-4, 1993, pp. 205–216. 26 Crawford, D. and Schultz, P., “Electromagnetic properties of impact-generated plasma, vapor and debris,” International Journal of Impact Engineering, Vol. 23, No. 1, 1999, pp. 169–180. 27 Marrese, C. M., Majumdar, N., Haas, J., Williams, G., King, L. B., and Gallimore, A. D., “Development of a single-orifice retarding potential analyzer for Hall thruster plume characterization,” 25th International Electric Propulsion Conference, No. IEPC-97-066, Cleveland, OH, 1997, pp. 397–404. 28 Heelis, R. A. and Hanson, W. B., “Measurements of thermal ion drift velocity and temperature using planar sensors,” Measurement techniques in space plasmas: particles, 1998, pp. 61–71. 29 Lee, N., Close, S., Lauben, D., Linscott, I., Goel, A., Johnson, T., Yee, J., Fletcher, A., Srama, R., Bugiel, S., Mocker, A., Colestock, P., and Green, S., “Measurements of freely-expanding plasma from hypervelocity impacts,” International Journal of Impact Engineering, 2011 (In review).

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