HYPERVELOCITY EXPERIMENTS EXPLORING IMPACT-GENERATEDELECTROMAGNETIC PULSE PRODUCTION

Andrew Nuttall, Yayu Monica Hew, Ivan Linscott, Dave Lauben, and Sigrid Close

Stanford University

ABSTRACT

The space environment can pose many unique hazards tospacecraft. One of these possible hazards stems from themeteoroid and debris populations that spacecraft can en-counter. Traditionally mechanical damage has been theprimary concern, but it has been shown that impactorswith masses too small to cause significant mechanicaldamage have the potential to generate damaging electri-cal effects. These electrical effects can be further exacer-bated by different spacecraft charging conditions. Me-teoroids and orbital debris, collectively referred to ashypervelocity impactors, travel between 7 and 72 km/sin free space, and upon impact with a spacecraft con-vert their kinetic energy to ionization/vaporization energyover a very brief time-scale. The result of this impact isthe creation of a small, dense, and expanding plasma witha strong optical flash and possible radio-frequency (RF)emission. The impact generated plasma can also inducestrong RF emissions and arc discharges depending uponthe charging potential of the spacecraft. To better un-derstand the relationship between the impact parametersand the resulting emission of optical and RF radiation,ground-based tests were conducted under a variety of im-pact conditions. The ground-based experiments were per-formed using the light gas gun facility at NASA AmesVertical Gun Range (AVGR) for low velocity impacts(< 8 km/s), and using the Van de Graaff dust accelera-tor at Colorado Center for Lunar Dust and AtmosphericStudies (CCLDAS) for high velocity impacts(>15 km/s).This paper presents the successful deployment of a sen-sor suite to detect simultaneous RF, optical, and plasmameasurements across a range of impact conditions at theCCLDAS and AVGR facilities. The electrical effects dueto the impact were found to be related to the spacecraftcharging condition, and is connected to the time evolu-tion of the impact generated plasma.

Key words: Meteoroids; Hypervelocity; Impacts.

1. INTRODUCTION

Naturally occurring meteoroid populations can pose asignificant hazard to the spacecraft that they come into

contact with. This hazard traditionally stems from thepossible for catastrophic mechanical failure in the eventof a collision with a meteoroid of sufficient mass. How-ever, impacts with meteoroids too small to cause any me-chanical damage have been shown to produce unwantedand possibly damaging electrical effects [1]. The electri-cal effects of these low mass impacts are further primedand exacerbated by different spacecraft charging statesthat satellites can experience. To characterize the elec-trical hazards from low mass meteoroid and space de-bris impacts ground-based impact experiments were per-formed at accelerator facilities.

Typical meteoroid and orbital debris populations thatsatellites can encounter have velocities from 7 to 72km/s, and can have masses up to micrograms whilecausing negligible mechanical damage. When these hy-pervelocity impactors strike the surface of a spacecraft,the impact kinetic energy will be converted into ioniza-tion/vaporization energy within a very brief timescale.Thus, a small and dense expanding plasma will be pro-duced around the point of impact in addition to thevapor cloud. The study of hypervelocity impacts canyield invaluable information about meteoroid composi-tion, spacecraft protection and anomaly effects due totheir impacts, and other planetary science related topics.Researchers have detected strong radio-frequency andoptical emissions from the impact [2, 3, 4, 6, 10, 15, 16],and have applied various different optical characteriza-tion methods to estimate the temperature and density ofthe plasma and impact vapor cloud [3, 6, 7].

Characterizing and linking the optical flash to impactorparameters can provide a vital and economic diagnostictool for understanding the types of impacts a spacecraftcan encounter and also the types of impact induced elec-trical effects it is exposed to. In characterizing opticalemissions, both [3, 6] use Planck’s law to model the opti-cal emission as a blackbody for the entire time span afterimpact. However, the emission spectrum has been ob-served to evolve from optically thick emission body toan optically thin emission body in the experimental datapresented in this paper. Thus, this paper is motivated toinvestigate the domain in time where blackbody radiationis adequate for the impact flash.

Hypervelocity impact events can be studied via in-situmeasurements in space [5] or at ground-based experimen-

tal facilities[3, 6, 10]. Due to the resource availabilityand cost, the ground-based facilities offers an economi-cal controlled experimental environment for hyperveloc-ity impact studies. To characterize the effects associatedwith hypervelocity impacts using ground-based experi-ment, observations from diverse sets of impactor massand velocity configurations are required. While mete-oroids can achieve both large masses and velocities inorbit, ground-based facilities can only maximize one pa-rameter, either mass or velocity, as in Figure 1.

Figure 1. Mass and speed regimes for projectiles. [10]

Light gas gun facilities, such as the NASA Ames Re-search Gun Range (AVGR), can accelerate particles withmasses up to a gram at speeds up to 6 km/s, while electro-static accelerators, such as the Colorado Center for LunarDust and Atmospheric Studies(CCLDAS), can create im-pacts at speeds in excess of 100 km/s with picogram tofemtogram masses. Combining the results of experimentsconducted at both types of facilities can provide the mostcomprehensive picture available using ground-based test-ing. While these experiments cannot re-create the fullrange of impactor configurations that can be seen in inorbit, they provide a basis for initial characterization andscaling analysis for the electrical hazards associated withimpact. This paper demonstrates successful experimentcampaigns which capture both the RF and optical emis-sion signature from hypervelocity impact events, and theability to identify and measures plasma brightness tem-perature in the continuum dominant phase of the plasmaexpansion. Additionally, we will presents the result froman arc discharge event induced by an impactor onto a dif-ferentially charged target, which simulates the frequentspacecraft differential charging condition.

2. IMPACT CHARACTERIZATION

During a hypervelocity impact (HIP), the impactorrapidly vaporizes and ionizes itself and a portion of thetarget resulting in the creation of a small and dense ex-panding plasma. The state of the resulting plasma plumeis dependent upon the constituents of both the impactorand target materials in addition to impactor mass and ve-locity. The parameters of the impactor can then be probedthrough the optical emissions that the plasma produces.

After the plasma plume has been formed it rapidly ex-pands into the vacuum. This plasma acts as the conduitfor many of the unwanted and possibly hazardous electri-cal effects associated with the impact. The interplay be-tween the various spacecraft charging conditions and thisplasma can lead to wide-band electromagnetic spectrumemissions, which includes RF and visible optical emis-sion, and act as a catalyst for arc discharges to occur.

2.1. Optical Emissions

The strong optical flash generated by the hypervelocityimpact can not only damage the sensitive optical instru-ment on board the spacecraft, but also be used as a diag-nostic tools to relate the electrical damages, the hyper-velocity impactor properties, and the impact generatedplasma properties. We hypothesize an optical model forthe HIP as shown in Figure 2 in order to measure theplasma properties during an hypervelocty impact event.In this model, the optical emission in the early expan-sion phase is dominated by the continuum emission andcan be closely approximated as a radiating blackbody.The continuum brightness temperature of the plasma canbe estimated directly using the Plancks law as shownin Eq.1 ,where I is the spectral radiance of the body inWsr1m3, λ is the wavelength, k is the Boltzmann con-stant (1.38× 10−23m2kg s−2K−1), T is the equilibriumtemperature in Kelvin, c is the speed of light,and h is thePlanck’s constant (6.626× 10−34m2kg s−1).

I(λ, T ) =2hc2

λ51

ehcλkT − 1

(1)

Figure 2. Hypothesized HIP Optical Model

As the plasma expands, the continuum emissions be-gin to decay and the line emissions will stand out. Theplasma properties in this intermediate expansion regimecan be estimated using the ratio between the continuumand line emissions. These two expansion phases canalso be related to the conditions in the impact plasma.

Figure 3. Optical Evolution High Speed Images: The high speed images from AVGR experiment show the impact flashtransition from optically thick to thin.

Figure 4. The continuum plasma temperature measurement in early expansion phase.

In the early expansion plasma, it is non-ideal and col-lisional. Thus, it exhibits local thermal dynamic equi-librium (LTE) and gives an optically thick continuum-dominant emission presentation. The plasma in theintermediate expansion phase transitions into an idealand collision-less plasma. As the continuum emissionsweaken, the atomic/molecular line emissions dominate inthis optically thin plasma. This hypothesis is supportedby the impact flash images taken at AVGR as shown inFigure 3, and the Planck law fitting in Figure 4. In thispaper, we will focus on the early expansion phase of theimpact plasma emission spectrum via Planck’s law.

2.2. RF Emissions

High power RF emissions can induce damaging currentsin sensitive electronics and can create disturbances innominal sensor behavior and performance. In this paperwe explore the how the production of RF emissions fromimpact events evolves as a function of spacecraft poten-tial. One source of RF emission from impacts can occurfrom a bulk acceleration of electrons in negatively biasedspacecraft. After the impactor vaporizes and ionizes itselfand a portion of its target, all of the liberated electrons areaccelerated away from the impact crater due to the elec-trostatic fields created from the spacecraft’s negative bias.

The strength of this emission is proportional to the num-ber of liberated electrons and the spacecraft’s bias. Thefamiliar Larmor formula gives the radiated power of anaccelerated charge.

P =q4∣∣E2∣∣

6πε0c3mq(2)

where P is the radiated power in watts, q is the particlecharge, E is the electrostatic field induced by the targetsbias, c is the speed of light, mq is the mass of the chargedparticle, and ε0 is the permittivity of free space. ground-based tests have shown that the amount of charge liber-ated by a hypervelocity impact is given by approximately

Q = 0.1m( m

10−11

)0.02 (v5

)3.48(3)

[13] where Q is the total liberated charge, m is the massof the impactor in grams, and v is the velocity of the im-pactor in km/s. This RF emission mechanism is signifi-cantly weaker in positively biased targets as the acceler-ation of the ions is many orders of magnitude lower thanelectrons for the same electric field strength.

An additional possible source of RF emissions fromgrounded targets is through plasma instabilities withsmall-scale spatial correlations such as a coherent plasma

sheath oscillation [14], and the Weibel instability [17].After impact, as the isothermal plasma thermally expandsinto the vacuum of space a bulk separation occurs as theelectrons outpace the ions. This bulk separation inducesan internal electric field that starts an oscillation whichoccurs at the plasma frequency. Along the expandingedge of the plasma the produced electromagnetic fieldsfrom this oscillation are allowed to propagate outside ofthe plasma providing another RF emission mechanism.As the plasma expands and becomes less dense the fre-quency of this emission decays. The frequency of thisoscillation is given as

ωP =

√nee2

meε0(4)

where ne is the number density of electrons, e is the elec-tric charge of an electron, me is the effective mass of anelectron, and ε0 is the permativvity of free space.

2.3. Arc Discharge

Differential spacecraft charging also poses a risk forspacecraft health in the context of arc discharges inducedby hypervelocity impacts. Dielectrics can easily causedifferent portions of a spacecraft to reach different volt-age potentials. Spacecraft can be naturally inclined toachieving different voltage potentials due their orbit, ori-entation, surface topography, and on board components.One area where differential voltages are often found ison solar panels. Solar panels and all the other electri-cal components are designed to avoid a voltage break-down by providing a large enough spacing between areasof differential biases for the magnitudes of voltages thatare expected. However, if a hypervelocity impact wereto occur in the vicinity of a differential voltage the re-sulting plasma plume can act as a conductor between thetwo surfaces allow for a rapid and dangerous discharge ofcurrent across the dielectric.

3. EXPERIMENTS

3.1. Ames Vertical Gun Range

The NASA Ames Vertical Gun Range (AVGR) is a0.3 caliber light gas gun, which launches projectiles toaround 5-7 km/s. The AVGR was used to perform mil-ligram sized impact and arc discharge experiments. Theoptical and arc discharge data presented in this paper usesa fine grain sand projectile (0.0161 g) which will be accel-erated together with the sabot material, and the total massis 0.3938 g at a velocity of 4.65 km/s. The rest of the im-pactors are single aluminum impactors packed with nylonsabot material. The complete list of single particle alu-minum impactor mass-velocity configurations is shownin Figure 6. Due to the inherent limitations in using lightgas guns both the number of impacts and the variance in

their masses and velocities is low. The target is a copperplate which can be charged to various potentials to rep-resents different spacecraft charging surface conditions.For single particle shots the impact angle is at 30 degreesfrom the normal, and for the fine grain sand shot at 0 de-grees from the normal. The chamber pressure is set tobe at 0.71 torr, which presents a neutral-rich environmentfor the impact generated plasma when compared to thehigh vacuum provided by CCLDAS facility. The AVGRfacility and the inside chamber setup of the sensor suite isdepicted in Figure 5. The beam path is set to be perpen-dicular with the line of sight of the optical sensor suites,which are focusing onto the target from outside the cham-ber. (The optical sensors are placed outside the chamberto minimize the debris damage on the sensor, whereas theRF sensors are located inside the chamber.) The opticalsensor is shown in Figure 7.

Figure 5. NASA Ames Research Center Ames VerticalGun Range (AVGR) Test Facility Setup. The AVGR gunis in its vertical position shown in the left figure, and thesensor setup in the chamber is shown in the right figure.As seen from the diagram, the projectile beam path is per-pendicular to the line of sight of the optical sensors.

Figure 6. AVGR and CCLDAS Impactor Masses and Ve-locities

In the AVGR experiments, the goal is to characterizethe impact plasma that was produced, the electromag-

netic emissions in the visible spectrum (400-700 nm),and the electromagnetic emissions in the RF spectrum(100 to 1000 MHz). An inclined copper target wasplaced in the center of the chamber and was biased todifferent voltages to simulate different spacecraft charg-ing conditions. To perform the optical measurementsa suite of colored band-passed photomultiplier tubes(PMT) (Hamamatsu Inc., HC-124, 8MHz head-on photo-multiplier tubes) and photo-diodes (Pacific Silicon Sen-sor PS100-6-CER-2, 10 mm2 )were used in conjunc-tion with a few high speed cameras (1MHz; ShimadzuHPV-1, Phantom V10, and Phantom V12). The PMTcolor-filters are centered at 450nm, 550 nm, and 600 nmwith 40 nm bandwidth (CVI Laser Optics, F40-450.0-4-25.0M, F40-550.0-4-25.0M,F40-600.0-4-25.0M). Thecolored band-pass filter and PMT combination will en-able a high speed photometry unit to measure the visiblespectrum of the impact flash. The data from the photome-try sensors are collected by the high speed oscilloscope at1GHz (TeleDyne-Lecroy, WaveRunner 6Zi) after properamplification and conditioning through in-house circuit.These sensors were positioned outside of the chamberand made their observations though an optical view port.This configuration is shown in Figure 7. A anti-reflectiveshroud will be used a beam dump to avoid back scatter-ing of photons. To perform the RF measurements 3 patchantennas centered at 160, 315, 916 MHz were placed in-side the chamber one meter away from the target at a 90degree angle from the plasma expansion path. To char-acterize the plasma plume generated from the impacts aretarding potential analyzer (RPA) and a transient poten-tial analyzer (TPA) [18] were utilized.

Figure 7. AVGR Inside Chamber Sensor Setup. The op-tical sensor is located outside the chamber as shown,and pointed at the target inside the vacuum chamber. Aclose-in diagram of the optical sensor is shown on theright. The photomultiplier tube with red interference fil-ter (PMT-R), green filter (PMT-G), and blue filter (PMT-B) are mounted above three all-wavelength photo-diodes(PD) with small pin-hole apertures.

3.2. Colorado Center for Lunar Dust and Atmo-spheric Studies

The Colorado Center for Lunar Dust and AtmosphericStudies electrostatic accelerator facility is shown in Fig-

ure 8. The entire system is oil-free and can be pumpeddown to high vacuum of 10−6 torr. As shown in Fig-ure 8, the impact takes place in the large experimentalchamber, which is mounted at the end of the beam line.Picogram to femtogram sized iron particles where accel-erated at speeds from 15 km/s to 100 km/s and impactedinto tungsten targets in high vacuum. Impacts were per-formed at a variety of target potential conditions rangingfrom -1000 V to +1000 V to simulate different spacecraftcharging conditions. The particle sizes are many order ofmagnitude less massive than the ones in AVGR as seen inFigure 6.

Figure 8. The electrostatic accelerator facility atCCLDAS. Femto to picogram sized particles accumulatea surface charge and are then accelerated at speeds up to100 km/s by an electrostatic field in a vacuum.

During this experiment 3095 impacts were observed. Tocharacterize the impact event a multi-physics platformwas deployed to capture RF, optical, and plasma emis-sions at a distance of 10 cm from the impact site. Themulti-physics sensor suite is displayed in Figure 9. To

Figure 9. The sensor suite used during the CCLDAS ex-periments contained optical, RF, and plasma characteri-zation sensors.

perform RF measurements an array of 12 165 MHz patchantennas, two 315 MHz patch antennas, and two 916patch antennas were used to provide spatial and fre-quency bandwidth coverage. Three RGB bandpass fil-tered photomultipler tubes were used to make plasmatemperature measurements, and an all wavelength optical

photodiode tube was used to provide calibration and timeof impact data. Plasma measurements were collected on atransient plasma analyzer (TPA) and a retarding potentialanalyser (RPA).

4. RESULTS

Through sampling impact events from both extremes ofthe possible impactor mass/velocity combinations a morecomplete picture can be drawn about the electrical anddiagnosable effects associated with low mass meteoroidand space debris impacts. While the combination of elec-trostatic accelerator and light gas gun facilities cannotproduce the high mass, high velocity configuration thatimpactors can achieve in orbit, they provide a startingpoint for a scalability analysis.

4.1. RF Emissions

RF emission signatures were observed during impacts atboth CCLDAS and AVGR facilities. While the effects ofthe large neutral particle population in the AVGR facilityon RF production still needs to be understood, RF band-width emissions were observed in grounded, positivelybiased, and arc discharge test cases at the AVGR facility.The 315 MHz patch antenna response to a grounded tar-get impact is shown in Figure 10. A strong wide bandresponse is seen a few nanoseconds after impact. Addi-tional pulses are observed in the following microsecondspossibly corresponding to secondary impacts from ejecta.In the positively biased impact case there was no RF pulseobserved directly after the time of impact, but there wereseveral pulses in the following microseconds that exhib-ited the same distribution as was seen in the groundedcase. Unfortunately, the data acquisition system misfiredon the negatively biased target, so no information is avail-able on RF emissions from negatively biased targets atAVGR.

At the CCLDAS facility low power RF pulses were con-sistently observed 50 nanoseconds after impact in neg-atively biased targets. The strength of these low poweremissions scale with impactor mass and velocity sug-gesting that in the high mass/velocity impactor config-urations the emissions have the potential to become haz-ardous. Due to the high noise environment and low ex-pected power of emissions from grounded and positivelybiased targets no emissions were observed in the unpro-cessed data from CCLDAS.

4.2. Arc Discharge

Two plates were charged with a 400V differential poten-tial between them to simulate adjacent solar panels. Theplates were mounted on a dielectric slab and were sepa-rated by 1 mm. A fine grain sand was shot to ensure a

particle would impact the gap between the differentiallycharged plates. Figure 12 displays the all wavelengthPMT response in conjunction with the measured voltagedifferential between the plates and the calculated currentflow. Within 5-7 microseconds after impact as observedusing the PMT response, the expanding plasma beginsacting as a conductor between the differentially chargedplates allowing them to discharge. Over a 10 microsec-ond span an 190 V drop was observed in the differentialpotential as the plasma allowed current to flow betweenthe plates. During a period of the discharge the differen-tial bias goes negative, which could cause brownouts andother possible damages. A large time constant the voltagedivider setup masks the more transient nature of this dis-charge. A potential consequence of this is that the peakvoltage and current differentials could be much higher.Accompanying the arc discharge is a wideband RF pulse.

4.3. Optical Characterization

The proposed hypervelocity impact flash evolutionshown in Figure 2 is found to be consistent with the highspeed images captured during the experiment as shownin Figure 3. A continuum dominant section can be iden-tify in the early phase of the optical evolution shown bythe high speed images in Figure 3. The spectrum in thelater phases become optically thin, and thus deviates fromthe blackbody model as expected in the hypothesized op-tical model. For the continuum dominant regime, wehave been able to measure the plasma continuum tem-perature as shown in Figure 4. The spectral emission inthe first 40 µs after the impact follows the blackbody ra-diation model quite well, but the emission spectrum devi-ates from the Plancks law radiation model after this time.Thus, the first 40µs is roughly the continuum dominantregime for the AVGR experiment. The blackbody con-tinuum model can thus generate an estimate of thermalbrightness temperature as a function of time. Addition-

Figure 10. The figure displays the results from the 315MHz antenna of a ground target impact, and the PMTresponse as a time of impact fiducial. The dashed blackline represents the time of impact.

Figure 11. The figure displays the results from the 315MHz antenna of a 50V positively biased target impact,and the PMT response as a time of impact fiducial. Thedashed black line represents the time of impact.

Figure 12. The top figure is the PMT response which isused to determine time of impact. The second figure is themeasured voltage differential between the target plates,and the third figure is the computed current flow.

ally, we can leverage the high speed cameras, which haveisolated Red/Blue/Green pixels to acquire a spatially re-solved plasma temperature measurement as shown in Fig-ure 13. The isolated Red/Blue/Green pixels of the highspeed camera provide spatially resolved relative color ra-tio measurements. Since the initial impact flash is spa-tially confined to a small region, we can use the abso-lute color ratio and temperature measurements acquiredby the photometry PMTs to scale the relative color ra-tio from the high speed camera to gain a spatially re-solved temperature measurement of the impact flash. Asshown in Figure 13, the high temperature initial expan-sion front expands outward in the continuum temperaturemap, which is consistent with the expected radiation pat-tern predicted by the model [9].

We have also found positive correlation of the continuumplasma temperature and the scaled impactor energy, i.e.m0.64v2.74, as shown in Figure 14, which is consistent

with the previous charge production power law [11]. Inthe preliminary studies, the temperature is strongly corre-lated with the scaled energy in certain energy range, i.e.the solid trend-line presented in Figure 14. The contin-uum blackbody temperature can be calculated using ei-ther blue and green PMT (i.e. 450 and 550 nm) or blueand red PMT (i.e. 450 and 600 nm). If the spectrum isclose to an ideal blackbody, the temperature estimationfrom the two color ratios will be close. Thus, the devi-ation of the temperature estimation using two differentcolor ratio is an indicator of the deviation of the visiblespectrum from blackbody radiation. The RF emission hasalso found to have strong correlation with the impactorvelocity. Thus, the RF emission takes on strong corre-lation with the continuum plasma temperature. This isquite an exciting finding since this result enlightens theconnection between the RF emission, plasma generation,and impact flash directly.

Figure 13. Spatial Measurement of the continuum tem-perature at initial expansion phase for the AVGR data.The red-line presents a cross-section view of the temper-ature profile as shown in the bottom figure.

Figure 14. Continuum temperature and scaled energyfrom CCLDAS result. The light blue dash line representsthe lower bound of scaled energy where temperatures ex-hibit a strong correlation with the scaled energy.

5. CONCLUSIONS

This paper demonstrates the successful deployment ofa sensor suite to make simultaneous RF, optical, andplasma measurements across a diverse set of impactconditions. Measurements were taken at the light gasgun facility at AVGR and the electrostatic acceleratorat CCLDAS to provide the most comprehensive pictureavailable in ground based facilities. The simultaneousmulti-sensor approach is crucial to mapping out a timehistory of the plasma. It was observed that the electri-cal effects induced by impact are a function of the space-craft’s current charging condition. Experiments at AVGRdemonstrated how the impact plasma can induce arc dis-charges in differentially charged systems. RF emissionswere observed under multiple spacecraft charging con-ditions and at both electrostatic accelerator and lightgas gun facilities. The scaling of RF emission powerwith impactor mass and velocity that was observed atCCLDAS suggests that high mass, high velocity impactsthat can be seen in orbit have the potential to becomehazardous. The observed optical emissions generate atime-resolved continuum plasma temperature measure-ment using high speed photometry sensors, and a spa-tially resolved continuum plasma temperature measure-ment when high speed camera is used in conjunction. Theelectromagnetic signals, including RF and optical emis-sion, are also captured during the arc discharge eventinduced by differentially charged target surface, whichsimulates that the hazardous differential charging condi-tions on many spacecrafts. Strong correlations betweenthe optical emission and the RF emission have been de-tected, which can be a promising detection method forfuture characterization of spacecraft electrical damagesfrom meteoroids using optical and RF measurements.

ACKNOWLEDGMENTS

The authors of this paper would like to thank Ashish Goeland Paul Tarantino for their contributions at CCLDASand AVGR. Portions of this material are based upon worksupported by the National Science Foundation Gradu-ate Research Fellowship under Grant No. DGE-114747,the Stanford Graduate Fellowship (Gabilan Fellowship,2013-2016) and the generous support of the Bosack-Kruger Foundation.

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