study of hypervelocity impact plasma expansionnot require a direct electrically-conductive path...

Study of Hypervelocity Impact Plasma Expansion

Nicolas Lee�, Sigrid Closey, David Laubenz,Ivan Linscottx, Ashish Goel{, and Theresa Johnsonk

Stanford University, Stanford, CA 94305, USA

Ralf Srama��, Sebastian Bugielyy, and Anna Mockerzz

Institut fur Raumfahrsysteme, Universitat Stuttgart, Pfa�enwaldring 31, 70569 Stuttgart, Germany

Hypervelocity impact testing at the Max Planck Institute for Nuclear Physics was undertaken with a suiteof RF, optical, and plasma sensors. Iron particles between 10�10 g and 10�15 g were accelerated to speeds of1 km/s to 50 km/s using a Van de Graa� dust accelerator and were impacted on metallic targets. The retardingpotential analyzers (RPAs) designed and constructed to measure impact-generated plasma are described indetail. Several optical signals from a photomultiplier tube were detected and found to be temporally coincidentwith impact events that also generated measurable plasma. The plasma measurements from the RPAs indicatea dependence on target electrical bias and material. The expansion speed of plasma generated from impactson 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 damagehave been well-characterized, but the electrical e�ects of hypervelocity impacts are still relatively unexplained. Testingat ground-based accelerators have confirmed that hypervelocity impacts generate plasma. This expanding plasma cancause RF emission which is able to penetrate into the Faraday cage of a spacecraft chassis even when the particle hascaused no mechanical damage. The emitted RF energy, in some cases, can couple into sensitive electronic circuits.Damage from this e�ect 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 deGraa� 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 ground-based facility is the first step in a research program to fully characterize the electrical e�ects of hypervelocity impactsin space.

Anecdotal evidence of electrical anomalies correlated with hypervelocity impact have been reported with a widerange of e�ects. Though we are able to link the timing of certain anomalies to meteoroid shower peaks, few incidenceshave direct measurement of an impact event. ESA’s Olympus satellite experienced a loss of gyroscope stability duringthe 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 fuelrequired to regain attitude control. In 2009, meteoroid impact from the Perseid shower may also have been responsiblefor a similar problem on Landsat 5.2 Electrical problems were also reported on two Japanese satellites that failedduring meteoroid showers: ADEOS II during the 2002 Orionids and ALOS during the 2011 Lyrids; both experienceda 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 increasedsolar 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.yAssistant Professor, Department of Aeronautics and Astronautics, Stanford University, Member AIAA.zSenior Research Scientist, Department of Electrical Engineering, Stanford University.xSenior Research Scientist, Department of Electrical Engineering, Stanford University.{Ph.D. Candidate, Department of Aeronautics and Astronautics, Stanford University.kPh.D. Candidate, Department of Aeronautics and Astronautics, Stanford University, Student Member AIAA.��Associate Professor, Institut fur Raumfahrsysteme, Universitat Stuttgart.yyAccelerator Engineer, Cosmic Dust Group, Max-Planck-Institut fur Kernphysik.zzLaboratory Director, Cosmic Dust Group, Max-Planck-Institut fur Kernphysik.

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American Institute of Aeronautics and Astronautics

3rd AIAA Atmospheric Space Environments Conference27 - 30 June 2011, Honolulu, Hawaii

AIAA 2011-3147

Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Impact Plasma expansion Initial electron motionPlasma formation

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 stillcannot confirm whether the spacecraft was hit by a tiny meteoroid traveling quickly or a larger piece of orbital debristraveling more slowly. Additionally, many electrical anomalies with an “unknown catalyst” on-orbit may be attributedto meteoroid impacts, and many events likely remain unreported due to the classified or proprietary nature of thespacecraft.

An impact-generated plasma can cause electrical anomalies through several mechanisms. Charge separation cangenerate a current pulse, plasma oscillations can cause electromagnetic radiation, and the presence of the plasma cantrigger an electrostatic discharge of dielectric materials on the spacecraft.5 By studying the expansion behavior ofimpact plasmas at a ground-based hypervelocity impact facility, we expect to gain a better understanding of thesemechanisms that can lead to spacecraft anomalies. In particular, we hope to determine whether the mechanism ofelectromagnetic radiation due to plasma oscillation is viable. This mechanism is especially concerning since it doesnot require a direct electrically-conductive path through the spacecraft chassis in order to cause damage to an internalcomponent.

In section II, we summarize the plasma expansion model that predicts a broadband RF emission, and discuss previ-ous studies that shed light on the expansion process through in situ and ground-based measurements. We then describeour 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. Previous Work

A. Plasma Expansion Model

The plasma expansion model describing a mechanism for EMP production is outlined in figure 1, and is describedin detail by Close et al.6 When a meteoroid or debris particle hits a spacecraft surface, the kinetic energy of theparticle is partially converted to vaporization and ionization energy.7, 8 The resultant small dense plasma is composedof material from both the impactor and the target surface. Electrons in this plasma expand outward faster than ionsbecause 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, radiatingat the plasma frequency. This frequency decreases with density as the plasma expands. The resulting RF signal is achirp with the initial frequency dependent on the initial ion density and the rate dependent on density fallo�.

B. In Situ Measurements

Several spacecraft have detected evidence of RF emission correlated with hypervelocity impact events on theirscience instruments. In particular, the Cassini spacecraft detected RF signals associated with impact events fromnanometer-sized particles which had been accelerated to 450 km/s,9 and from micrometer-sized particles from Saturn’srings moving at roughly 10 km/s relative to the spacecraft.10, 11 Waveforms were recorded from the 10 m dipoleantennas on the Radio and Plasma Wave Science (RPWS) instrument.12, 13, 14 The STEREO pair of spacecraft have alsodetected electrical e�ects of impact events using the STEREO wave instrument (S/WAVES), which are coincident withoptical 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 to1 MHz) that were temporally correlated to dust stream impacts detected by the DUST impact-ionization detectorsensor.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 Graa� dust accelerators,light gas guns, and plasma drag accelerators. However, as seen in figure 2, there is no ground-based technologythat fully replicates the projectile masses and speeds associated with meteoroid impact. Additionally, the ambientpressure achievable at most light gas gun facilities is on the order of 0.1 mbar. This corresponds to a mean free pathof millimeters, resulting in a collisional expansion of the impact plasma into the ambient atmosphere. The e�ect ofambient atmosphere has been studied for expanding plasma plumes from laser ablation studies18 and is significant forimpact-generated plasmas as well. Even with Van de Graa� dust accelerators, where the vacuum levels are typicallyon the order of 10�6 mbar, the size of the test chamber itself becomes the limitation, with reflected signals manifestingwithin nanoseconds of the impact event. As a result of these many challenges, there has been no complete analysis ofthe 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 a�ected by his direct plasma measurements,which inhibit recombination. Takano’s research group21, 22, 23, 24 associates their microwave signals to microcrackingwhile Starks et al. searched for microwave signals they attribute to plasma oscillation. Crawford and Schultz25, 26 posita macroscopic charge separation, which may be due to their use of a powdered dolomite target rather than the morecommonly studied solid metallic targets.

Additionally, studies of hypervelocity impact plasma production typically use an accelerating grid at the target toseparate the charge species and to direct them into a sensor. This is done in order to measure time-of-flight of di�erention species and to measure the composition of the plasma. However, the grid also significantly changes the behaviorof the expanding plasma.

III. Experimental Approach

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

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PSU

ChamberParticle

Deflector

Detector 1 & 2

Detector QP

2 MV Terminal

BeltPotential Rings

DustSource

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

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

A. Facility

At the heart of the Van de Graa� dust accelerator facility is a positively-charged dust source feeding a 2 MVacceleration path. A simplified schematic of the facility is shown in figure 3. Particles are housed in a small chamberat the 2 MV terminal, and caused to swirl by pulsing the voltage of the source chamber. As the dust moves, particlesoccasionally contact the tip of a charged tungsten needle, which provides the final amount of charge to inject theparticle into the accelerator. The speed each particle attains as it drops through the potential coils is dependent on thecharge and mass of the particle. As they enter the drift tube at the end of the accelerator (shown in figure 4), particlespass through induction loops that measure their charge and velocity. The mass of particles is determined from themeasured charge and speed by equating the kinetic energy of the particle to the potential energy across the Van deGraa� terminals:

12

mv2 = qU; (1)

where m, v, and q are the mass, speed, and charge of the particle and U is the accelerating voltage. These measurementsdrive capacitor plates that deflect particles not meeting programmable selection criteria. Particles meeting the selectioncriteria 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 performedat pressures between 2:5� 10�6 mbar and 8:1� 10�6 mbar, corresponding to a mean free path longer than the chamberdiameter. The chamber contains a horizontally-translating platform (laterally across the beamline) with a mechanicalfeedthrough to allow manipulation of the internal geometry without breaking vacuum. The position of the platform isdetermined 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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2 4 6 8 10 20 30 40 50

10−15

10−14

10−13

10−12

10−11

10−10

10−9

Particle speed [km/s]

Par

ticle

mas

s [g

]

Figure 5. Distribution of particle masses and speeds. Lower-speed particles are achievable but were deflected by the particle selection unit.

10−15

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10−13

10−12

10−11

10−10

0

0.2

0.4

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1

Particle mass [g]

Fra

ctio

n lig

hter

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100

101

102

0

0.2

0.4

0.6

0.8

1


Fra

ctio

n sl

ower

12/0912/1012/1512/1612/1712/20

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 thecapability 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 showsthe cumulative mass and speed distributions for each day. The depletion of small (and therefore high-speed) particlesafter the first day was particularly significant.

C. Targets

Five targets (shown in figure 7) were used to study the e�ect of impacts on di�erent materials and thicknesses. Twoof the targets were active, serving as stub antennas with an integrated amplifier to measure the electric field at the pointof impact. One of these used an 8 mm diameter brass knob as the target, which was smaller than the beamline scatterarea. Due to this concern, the target was modified to include a 40 mm diameter copper disc. The other three targetswere passive but could be biased to �1 kV to simulate spacecraft charging. This bias was applied using a high-voltagesource through an RC circuit to decouple any discharge events from the supply. The RC circuit included a 435 nFcapacitor from the target to chamber ground and a 225 k resistor feeding the capacitor from the voltage source. Thethree passive targets include 50.8 �m thick tungsten, 254 �m thick aluminum, and 12.7 �m thick aluminum. All ofthe targets were mounted horizontally in-line so that each could be translated into the beam line using the chamber’smechanical feedthrough. Initially the targets were inclined down 30� from the horizontal beam line to point at the RFsensors, 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.

Particle beam line

RFsensors

Plasma sensors

Target

PhotomultiplierTube

Target

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 theexpanding plasma and any associated RF emission. A Hamamatsu photomultiplier tube (PMT) was used to detectcoincident optical response.

This PMT was mounted above and o�set to the side of the passive target, at a distance of 15 cm from the tungstenfoil. It has a photocathode area of 78 mm2 and is sensitive to wavelengths between 300 nm and 650 nm. The anode-to-cathode voltage was -800 V and the output current was measured directly by the 5 Gs/s oscilloscope on a 1 M DCcoupled channel.

The RF suite was composed of two log-periodic arrays (LPAs) sensitive to frequencies between 50 MHz and4 GHz, three VLF loops designed to detect signals between 300 Hz and 50 kHz, and the E-field sensor embeddedin the active targets. The two LPAs were arrayed to measure horizontally and vertically polarized signals and weremounted on the horizontally-translating platform about 1 m from the impact point. Each signal was passed throughtwo 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 theLPAs 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 constructedto measure the constituents of the expanding plasma. The design of the RPA is similar to that described by Marresseet al.27 and Heelis and Hanson28 and is outlined in figure 9. The sensor has an e�ective collecting area of 10 cm2

behind a series of four electrode grids. These are, from front to back, the floating, repeller, threshold, and suppressorgrids. The floating grid is electrically connected to the RPA chassis to shield the internal electric fields of the RPAand is constructed of a stainless steel mesh (229 �m wire thickness) with 73.3% transmissibility. This mesh waschosen for mechanical strength to protect the other grids. The repeller grid is biased to allow only electrons or onlyions through. The threshold grid prevents low-energy particles from passing through. Finally, the suppressor gridprevents secondary ionization from causing electrons to escape the collector plate. These three internal grids have a

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Transimpedanceamplifier

Differentialdriver

Repeller grid

Threshold grid

Suppressor grid

Vout+

Vout-

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 �m and a transmissibility of 88%. The grids are held apart by nylon spacer discs, and their biasvoltages are transmitted through the screws holding the assembly together.

The signal from the collector plate is passed through a transimpedance amplifier and a di�erential driver beforebeing passed out of the vacuum chamber. The embedded amplifier provides direct pickup without any front-end cableloss, and the di�erential driver mitigates plasma-generated RF interference and accelerator-generated EMI. The twoamplifier stages are AC coupled.

The assembled RPA is shown in figure 10. Two RPAs were used to measure the plasma at di�erent ranges fromthe impact point, as shown in figure 11 in order to compute a plasma expansion speed. These two RPAs were mountedat 75 mm and 150 mm from the impact point, at angles of 15� and 30� from the target normal, respectively. The RPAswere positioned using a network of tension lines to the walls of the chamber, minimizing the amount of mechanicaland electrical interference to the expanding plasma.

Due to the limited number of oscilloscope channels available, the RPA di�erential signals were sampled at both1 Gs/s and 5 Gs/s. In order to maintain the same frequency content from both RPAs, each sensor had one half of itsdi�erential signal sampled at 5 Gs/s and the other half at 1 Gs/s. The lower-rate signal was then interpolated linearlyto 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). Representativewaveforms are shown in figure 12. The number of plasma signals detected out of the total number of impact eventsrecorded yields a signal detection rate for each target and for each bias configuration, as shown in figure 13. The im-pacts on biased aluminum clearly show a higher detection rate than on unbiased aluminum. The same trend occurs forpositively-biased tungsten, but not for negatively-biased tungsten. This may be a result of tungsten’s higher electroneg-ativity. From the temporal spacing in the peaks between the two RPAs, the plasma expansion speed was computed for

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Particle beam line

RPA-A

RPA-B

Figure 11. View looking upward at the targets and RPAs from below.

0

20

40

0

20

40

RP

A−A

[mV

]

0 5 10 15 20 25 300

40

80

Time [us]

Figure 12. Typical signals recorded from impacts on unbiased tungsten. Top: Fast rising and exponential decaying peak from a 4.7 km/simpact 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 is20.8 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 sensorsuite is still under study.

The PMT yielded 28 detectable signals corresponding to the particle masses and speeds indicated in the scatterplot in figure 15. A respresentative PMT waveform is also shown. Out of the 28 signals, 21 occured within the 3 �spreceding a detected RPA peak. From the timing of these signals, it does not appear that this response is purely dueto impact flash, which would have occurred earlier. Instead, the PMT response may be associated with some opticalphenomenon in the expanding plasma. The long transient of the waveform is potentially a result of line capacitanceand the measurement configuration. However, more study is required to validate this hypothesis and determine themechanism behind this optical response.

V. Future Work

A second experiment campaign is planned at the Van de Graa� dust accelerator facility to address some unansweredquestions. 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 orderto obtain data from the larger projectile mass regimes. The primary challenge with light gas gun facilities is that typicalvacuum levels are much poorer than at Van de Graa� accelerators (10�1 mbar compared to 10�6 mbar), resulting in amuch shorter atmospheric mean free path. The plasma-atmosphere interactions will have to be incorporated into theplasma expansion model in order to interpret results from these tests.

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Tungsten Aluminum Al Foil0%

25%

50%

75%

100%

Target materialS

igna

l det

ectio

n ra

te

−1 kVFloat+1 kV

Figure 13. Plasma signal detection rate on RPA-A for di�erent targets and biases.

0 10 20 30 40 50 600

5

10

15

20

25

30

Plasma expansion speed [km/s]

Qu

anti

ty

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

2 4 6 8 10 20 30 40 50

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ticle

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]

0 10 20 30−0.64

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−0.58

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−0.54

PM

T [V

]

Time [us]

Figure 15. Left: Scatter plot of detected PMT signals as a function of particle mass and speed. Right: Representative PMT response (froma 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 bestvacuums achieved at ground-based hypervelocity impact facilities cannot replicate conditions above 150 km altitudein terms of atmospheric mean free path and are ultimately constrained by the size of the test chamber. To provide anin situ platform for studying the expansion of hypervelocity impact plasma in space, we plan to construct a CubeSatwith a deployable meteoroid impact screen to maximize exposed area, as shown in figure 16. The CubeSat will useminiaturized 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 dataobtained from ground-based testing to solve the inverse parameter estimation problem. RF and plasma signals will beused to provide an estimate of the projectile properties, which can be compared against truth. This estimator will thenbe 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 graduateresearch fellowship which helped make this work possible. The accelerator experiments were funded through LosAlamos National Laboratory. The authors gratefully acknowledge the contributions from Dr. Patrick Colestock andStan Green.

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study of hypervelocity impact plasma expansionnot require a direct electrically-conductive path...

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