laboratory investigations of impact&generated plasma · moffett field, california. high frame...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. E3, PAGES 18,807-18,817, OCTOBER 25, 1991

Laboratory Investigations of Impact-Generated Plasma

DAVID A. CRAWlOre) AND PETER H. SCHULTZ

Department of Geological Sciences, Brown University, Providence, Rhode Island

The production of magnetic fields by hypervelocity meteoroid impact has been proposed to explain the presence of palcomagnetic fields in some lunar samples as well as on the lunar surface. Impact-generated magnetic fields also may be significant for the palcomagnetic record on a variety of cratered surfaces in the solar system, such as the Moon, Mercury, Phobos, and asteroids. Previous experiments with the two-stage hydrogen light gas gun at the NASA Ames Vertical Gun Range demonstrated that hypervelocity impacts can produce impact-generated magnetic fields by the expansion of an impact-derived ionized vapor cloud (impact-generated plasma). Here we further characterize impact-generated magnetic fields and plasma with hyperv•.locity impact experiments at the NASA Ames Vertical Gun. Spherical projectiles (0.32 cm Fe, Cu; 0.64 cm AI, nylon) with velocities from 5.2 to 6.0 lcm/s were impacted at impact angles of 15% 30 ø, 60% and 90 ø (measured from horizontal) into powdered dolomite, silica sand, and aluminum plate. Three sets of experiments using search coils as magnetic detectors characterized impact-generated magnetic fields as functions of ambient field strength and often- tation, projectile/target composition, and impact angle. Experiments using Langmuir probes indicated a charged particle density (between 109 and 10" ions/el cm '3) and an electron temperature (-4500 K) of the impact-generated plasma, the inferred source of impact-induced magnetic fields. These new results demonstrate that impact-generated magnetic fields at the laboratory scale exhibit spatial and temporal complexity dependent on impact angle, velocity, and projectile/target composition thereby suggesting that crater-related palcomagnetism associated with this mechanism should exhibit similar complexity with spatial wavelengths on the order of a fraction of the crater radius.

INTRODUCTION

The natural remanent magnetization of the lunar surface as displayed in returned lunar samples and the data returned by the Apollo subsatellite magnetometer has an unexpectedly high magnitude and exhibits spatial variation at all scales. Hypothe- ses for the origin of the lunar remanent fields fall into two general categories: crustal remanence of a core dynamo field occurring early in lunar history prior to extensive modification by impact [Runcorn, 1983]; and remanence of transient fields, par- ticularly associated with impacts, occurring on a local scale throughout lunar history [Gold a•nd Soter, 1976; Srnka, 1977; Hood and Huang, 1991]. Provided that a magnetic field is present during an impact event, thermal remanence (TRM) can be acquired by cooling ejecta and impact melt and shock remanence (SRM) can be acquired in a•zone surrounding the impact point where shock pressures exceed 20 kbar [Cisowski et al., 1975, 1976]. The main methods for p•clucing impact-induced magnetic fields include magnetic fieldl induced by the expansion of an impact-derived ionized vapor cloud (impact-generated plasma) [Srnka, 1977; Hood and Vickery, 1984; ttood and Huang, 1991], shock-induced piezomagnetic fields produced as the shock wave from the impact propagates into the target [lvanov et al., 1977; Nitsan, 1977] and compression of the solar wind or cometary field during collision of a cometary coma with the lunar surface [Gold and Soter, 1976; Schultz and Srnka, 1980]. This paper examines some of the characteristics of plasma produced by laboratory hypervelocity impacts with par- ticular emphasis on the production of plasma-related magnetic fields and addresses first-order extrapolations of these observations to broader scales.

Hypervelocity collision between objects in the solar system

Copyright 1991 by the American Geophysical Union.

Paper number 91JE02012. 0148- 0227/91/91JE- 02012505.00

will, in general, vaporize and ionize portions of the projectile and target materials. While ionization and formation of plasma by impact of fast dust particles have been studied extensively for investigations of cometary dust particles on space flight missions [qqe Kissel and Krueger, 1987], a quantitative model for the formation and evolution of impact-generated plasma due to macroscopic meteorold impact on planetary surfaces is needed in order to understand the generation of magnetic fields during these events. Significant theoreti.gal and ekperimental work has been done with macroscopic, v•ertical hypervelocity impacts in the presence of relatively strong ambient magnetic

fields to 't•ossibly explain the production of radiofrequency emissions d•rtfing earthquakes [Bianchi et al., 1984; Martelli and Cerroni, 1985] and to understand the nature of shock remanent magnetization [Cisowski et al., 1975, 1976; Martelli and New- ton, 1977; Srnka et al., 1979].

The high specific energy and nonequilibrium conditions within impact-generated plasma led Hide [ 1972] to suggest that hypervelocity impacts could produce magnetic fields through hydromagnetic interaction between the impact-generated, ele•- •ti'ically conductive plasma and the ambient magnetic field. Al- though Hide's mechanism may not produce significant ambient field enhancement from laboratory scale [Crawford and Schultz, 1988b] up to small crater scale (meters to kilometers), it may be significant at large scales (kilometers to hundreds of kilometers) or under certain impact geometries [Hood and Vickery, 1984; Hood and Huang, 1991].

Experiments performed at the NASA Ames Vertical Gun Range revealed that impacts at low angles (15ø-30 ø from h0ri- zontal) enhanced vaporization and induced a self-luminous ionized cloud, virtually nonexistent at high angles [Schultz, 1988]. .The production Of spontaneous magnetic fields by magnetohy- drodynamic processes independent of the ambient field intensity within the impact-produced plasma was first proposed by Srnka [1977], but experimental confu'mation through the analysis of the ionized Cloud observed at low angles did not come until later [Crawford and Schultz, 1988b].

18,807

18,808 •w-vo• n•rr) ScartJL•: L•O•TOR¾ INVESTIGATIONS OF IMPAC•-G•'mD PriMA

THEORY where thermal gradients are relatively small. Magnetic field generation due to plasma expanding in a relatively large ambient

Srnka [1977] proposed that thermally driven electrical cur- field environment will be dominated by field amplification at rents and their associated magnetic fields could be generated in plasma clouds produced during the early stages of hypervelocity virtually all scaleS. In addition, equation (3) shows that the pro-

duction of spontaneous impact-generated magnetic fields is not impacts. Such currents and fields have been observed during laser-target interactions where strong temperature and density gradients drive the plasma electron motion [Stamper et al, 1971; Pert, 1977]. He developed a simple theoretical model of impact-generated magnetic fields by combining the generalized Ohm's law for a low-temperature plasma with Faraday's law of induction. In this approach, an equation describing the time rate of change of magnetic induction (•B/•t) is derived from a generalized Ohm's law of the form

J=o E+-uxB+ (1) c en .!

where J is the electric current distribution, E and B are the elec-

tric and magnetic fields; u and (• are the plasma's fluid velocity and electrical conductivity, respectively; c is the speed of light; k is Boltzmann's constant; and n, T and e are the electron number density, temperature, and charge, respectively. Applying Maxwell's equations to (1) yields

•B c 2 V2 ck (VT x Vn) (2) •'• = V x u x B + 4"• B + e n

The first term on the right of (2) represents advection of B due to fluid motion; the second represents diffusion of B through the electrically conductive plasma. The third term is a source term which arises from drift currents due to pressure gradients arid is nonzero when the electron temperature and density gradients are not aligned [Srnka, 1977].

The maximum expected magnetic field can be estimated from equation (2) with a simple model where u and T are spherically symmetric, and n can vary throughout space. This is a reasonable approximation to the distribution of plasma produced by laboratory hypervelocity impact where u and T are dominated by the neutral component of the vapor cloud which is observed to expand in a regular (nearly hemispherical) fashion [Schultz, 1988]. The distribution of the ionized fraction (n) within the vapor cloud is unknown but may exhibit considerable complexity due to jetting, entrained ricochet fragments and internal shear heating of the projectile and target [Schultz and Gault, 1990b]. If the diffusion term in equation (2) can be neglected [Srnka, 1977; Hood and Vickery, 1984], an order-of-magnitude estimate of the maximum magnetic field (Bm•) arising from impact-generated plasma is found by setting •B/3t = 0 to find

Trr' (3)

where u is the magnitude of the plasma velocity (usually approximated by the gas expansion velocity) and •T/•r and Anln represent temperature and density gradients as functions of po- sition within the plasma cloud. Similar results were obtained by a more conservative approach of linearly extrapolating the source term of equation (2) from the time of first contact to terminal engulfment of the projectile [Srnka, 1977] and by a straightforward scaling analysis [Hood and Wickcry, 1984].

Equation (3) indicates that the generation of magnetic fields by hypervelocity impact is dominated at small (distance) scales by spontaneous field production where thermal gradients are relatively large and at large scale by ambient field amplification

strongly affected by the absolute level of electron density (n) but relies instead on fluctuations of electron density (An/n). It is important to note that the spatial and temporal distributions of impact-generated plasma (most notably the functions u, VT and Vn) depend in a complex fashion on the impact parameters such as impact angle, velocity and projectile/target composition. Consequently, laboratory experiments are necessary to characterize impact-generated plasma.

EXPEmMi•NTS

Several experimental techniques were implemented for the study of plasma produced during macroscopic hypervelocity impact experiments at the NASA Ames Vertical Gun Range, Moffett Field, California. High frame rate photography (20,000-35,000 frames per second) was used to measure expansion of the self luminescent impaCt-generated plasma cloud and the mass and energy density of the cloud for certain target materials [Schultz, 1988]. Previously, impact-generated magnetic fields were observed and an estimate of the electrical conductiv-

ity (or • 105 - 107 f2 '1 m 'l) was obtained from the resIxmse of a magnetic search coil to the expanding impact-generated plasma in a high magnetic field environment [Crawford and Schultz, 1988a]. In the present work, the magnetic field (actually •B/•t) is further characterized, and some preliminary measurements of the electron number density (n) and the electron temperature (T) are made with electrostatic probes.

The NASA Ames Vertical Gun is a two-stage hydrogen light gas gun capable of launching macroscopic projectiles at up to 7 km/s with the angle of impact varying from nearly horizontal to vertical in increments of 15 ø [see Gault and Wedekind, 1978]. The large impact chamber was evacuated to • 0.8 mm Hg for all of the experiments and the impact velocity was typically be-

..,

tween 4 and 6 km/s with spherical aluminum (0.05 g, 0.38 g), nylon (0.16 g), iron (0.13 g), and copper (0.15 g) projectiles impacting targets of powdered dolomite (Mg0aCa0aCO3) silica sand (SiO4) and aluminum plate. The ambient magnetic field was controlled by large field coils, some arranged cubically to reduce the ambient field and others embedded in the target to increase the ambient field. Normally, the ambient field was oriented horizontally with a magnitude of about 30 p.T, but with the field coils, the ambient field could be varied from +1 IxT in a small region immediately surrounding the impact point to approximately ñ200 •tT throughout the target region.

Magnetic probe experiments

Magnetic detectors, consisting of up to 15 search coils in various locations above and below the target surface and of varying orientation provided measurements of •B/•t. To posi- tion the search coils, we used a Cartesian coordinate system (x,y,z) with the origin at the point of impact, where +z measured the height (in centimeters) above the target surface, +y the distance uprange of the impact point, and +x the distance away from the projectile line of flight in a right-handed sense. The coils consisted of several hundred turns of 30 gauge copper magnet wire wound helically on a plastic form approximately 8 cm in diameter. Four of the coils (type A) were made with twice as many turns as the remaining 11 (type B coils) for ira-

CRAWFORD AND SCHULTZ: LABORATORY INVESTIOATIONS OF IMPACT-GENERATED PLASMA 18,809

proved sensitivity at low frequencies. All the coils and leads were electrostatically shielded by 1-nun-thick grotreded aluminum foil. In addition, each experiment was conducted with at least one pair of coils oriented back to back to distinguish true magnetic signals from any trowanted electrostatic signals which may have penetrated the electrostatic shielding. In the experiments reported here, the electrostatic component of the magnetic probe signals was negligible.

The signal from each probe was routed through an amplifier with software-selectable gain settings of 10, 100, 1000, and 10,000 and fed to one of the digitizer channels. The signals

In order to determine the relative contribution of the ambient

field interactions and the spontaneous field generation terms of equation (2), it is necessary to conduct impact experiments in a range of ambient field environments. Figure 1 shows the restfits of a series of hypervelocity impacts (0.64-cm Al projectiles, 30 ø, mean velocity 5.5 kin/s) conducted in various ambient field environments. The data consist of the magnetic signals recorded by two vertically oriented search coils embedded in a powdered dolomite target and located at the same radial distance uprange (location x=-5, y=13, z=-6 era) and downrange (12,-9,-2) of the impact point. Due to the "clipping" of the stronger signals by

from the 16 amplifier channels were simultaneously converted the data collection system as mentioned previously, the esti- into 8-bit digital form at a rate of 500 kHz (2 Its per conversion) mated maximum amplitude of the actual magnetic field strength for later computer readout. With the limited dynamic range of is 10-50% greater than the integrated data shown in Figure 1; the 8 bit A/D converter some of the stronger signals were however, the consistency of the data allows a first-order charac- "clipped" in order to retain information from the weaker signals. terization of impact-generated magnetic fields to be made. Both Each channel had 8192 time samples for a total time that the uprange and downrange data sets show the influence of spanned from about 0.5 ms before impact to about 15.8 ms after spontaneous field generation and ambient field interaction. The impact. The response of the search coils cut off above 120 kHz; uprange signal is a simple combination of a high-frequency hence aliasing was not a problem. spontaneous signal and a low-frequency ambient field interac-

A Fourier transform technique, described in Appendix A, was tion signal, whereas the downrange signal appears to be a more used to determine the magnetic field B(t) by integrating the complicated mixture. Figure la shows the signals observed in a magnetic probe signal V(t)with the relation low-field environment, and they are very close to what we

..-1VF[V(t)]• B(t)=r L A(k) / (4)

where A(k) is the spectral response of the search coil and F[V(O] represents the Fourier transform of V(t). The average search coil response between 0.1 and 100 kHz is about 0.25 mV nT '1

would expect to see in a null field environment. Figures lb-le show the signals observed in progressively stronger ambient field environments. Most notably, the signals recorded in the strongest field environment (Figures ld and le) show compression of the ambient field lasting about 1.5 ms due to expansion of the impact-generated plasma cloud.

Since every other data set of Figure 1 was collected in oppos- for type A coils and 0.5 mV nT '1 for type B coils. Plots of the ing polarity ambient field environments, their average should actual spectral responses of both search coil types can be found approximate the spontaneous magnetic field signal. A cross cor- M Appendix A. relation of the data in Figure 1 determined the offsets required

30 Uprange

30

,5O

1D

,.5O

30

• 15

Downrange 5o

30

30

--1

--30-

30

15

_ -

--0.50 0.00 0.50 1.00 1.50 2.00 2.5

Time After Impact (ms) Time After Impact (ms)

--0.50 0.00 0.50 1.00 1.50 2.00 2.5

a) 5.41 km/s _+1 gT

b) 5.60 km/s +31 gT

c) 5.56 km/s -28

d) 5.33 km/s +166 gT

e) 5.66 km/s -167

Fig. 1. Vertical component of the magnetic field observed uprange (left) and downrange (fight) of hypervelocity impacts (0.64-cm A1 projectiles, powdered dolomite target, 30 ø from horizontal) conducted in a variety of ambient field environments (Figures la- le). The impact velocity and strength of the ambient field are indicated to the fight of the figures. The data have been shifted so that impact occurs at time zero.

18,810 CRAWFORD • SCmU.U.TZ: LABORATORY •TIOATIONS OF IlViPACr-G•'r•D •SMA

0.8

0.5

0.4

o Uprange I

_

0.3 5.3 5.4 5.5 5.6 5.7

Impact Velocity (km/s)

Fig. 2. Arrival times of the uprange and downrange signals as determined by cross-correlation of the data in Figure 1. The systematic decrease with increasing impact velocity is consistent with expected increasing plasma expansion velocity.

to stack the data. The maximum correlation yielded the signal arrival times, while the width of the correlation peak at half its maximum value provided an estimate of the correlation error (see Figure 2). There are two factors contributing to the variation of the signal arrival times: (1) a systematic decrease of arrival time with increasing velocity that is consistent with increasing plasma cloud expansion velocity, and (2) a relatively small, random error in the arrival time most likely due to variations in the trigger signal from the Vertical Gun electronics. The solid line of Figure 3 is the average of the data sets of Fig- ure 1; the dashed lines denote the standard deviation. The relatively low deviation among these data sets shows that averaging the data sets in this way allows a first-order determination of the spontaneous field term.

Figure 4 shows the results of a series of hypervelocity impacts (0.64-cm AI projectiles, 30 ø, mean velocity 5.75 km/s) conducted with various projectile/target materials. Figures 4a-4d

show the results using different projectile materials impacting into a powdered dolomite target. Figure 4e shows the results obtained with an iron projectile impacting into a silica sand target. The differences between the data sets, though subtle, are reproducible as demonstrated in Figures 1-3. Ascertaining the relative contribution of spontaneous field generation and ambient field interactions requires many experiments in various ambient field environments which have not yet been performed. Although the relative contribution of the piezomagnefic effect [lvanov et al., 1977] for certain targets (most notably the silica sand targe0 also remains to be determined, the observation of magnetic fields above the target surface and during impacts into solid alturtimurt targets (discussed below) strongly suggests that the observed magnetic fields are plasma related.

Two sets of otherwise identical experiments at various impact angles, holding the impact velocity as constant as possible, doc- tunented the dependence of impact-generated magnetic fields on impact angle. Figures 5a-Sd show the results of a suite of impacts (0.32-cm Fe, mean velocity 5.7 km/s, powdered dolomite target) conducted at 15 ø , 30 ø , 60 ø , and 90 ø respectively, whereas Figures 5e and 5f show the results of two impacts (0.64-cm AI, 5.22, 5.93 km/s, powdered dolomite taxget) conducted at 15 ø and 30 ø , respectively. The largest change occurs between 15 ø and 30 ø. At 15 ø, both signals are usually weaker and the downrange signal generally precedes the uprange signal (though both signals start within microseconds of the time of impac0. At 30 ø, however, as the degree of axial symmetry increases, the uprange signal (which starts ~300 • after impac0 typically precedes the downrange signal (which starts ~800 [xs after impac0. At higher impact angles, the difference between the arrival time of the two signals decreases. Since the search coils are at slightly different radial distances and depths within the target, the signals from a 90 ø impact arrive -200 • apart. The long delay from impact to arrival of the magnetic signals strongly suggests that the evolution of the plasma cloud is not related to the jetting process.

Impact-generated magnetic fields should depend on the initial distribution and subsequent evolution of impact-generated plasma which in turn should depend on the style of the early time projectile/target interaction. Consequently, the significant change in the observed magnetic signals occurring between impact angles of 15 ø and 30 ø should relate to a change in the ha-

40

30

20

10 0

-10

-20

i i i i -30 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0

Time after impact (ms) Time after impact (ms)

Fig. 3. Average of the five data sets of Figure I (solid line) + su•ndard error (dashed lines). The data have been shifted for maximum correlation using cross correlation as described in the text.

Uprange Downrange 4

3' ß

: .•

ß -•. i "-"\ .-, •:•/k4 ,- . ,. : ¾' ," x .- ,- -x._

. .77• •~..---7..- ..... .- .........

l..,x :v v

I .... I, , , , I ..... 3t .... I .... I , , , , I .... I .... 2.5

CRAWFORD AND SCHULTZ: LABORATORY INVESTIGATIONS OF IMPACr-GENERATED PLASMA 18,811

Uprange

C ,

lO

20 - -

15

Downrange .30 .......................

-1

15

-0.50 0.00 0.50 1.00 1.50 2.00 2.5 --0.50 0.00 0.50 1.00 1.50 2.00 2.5


a) 5.41 km/s 0.64 cm AI, dolomite

b) 6.05 km/s 0.64 cm nylon, dolomite

5.91 km/s 0.32 cm Fe, dolomite

d) 5.29 lcm/s 0.32 cm Cu, dolomite

e) 6.02 km/s 0.32 cm Fe, silica sand

Fig. 4. Vertical component of the magnetic field observed uprange (left) and downrange (righ0 of hypervelocity impacts (low ambient field, impact angle 30 ø from horizontal) conducted with a variety of projectile/target materials (Figures 4a-4e). The impact velocity and projectile/target materials (e.g., 0.64-crn AI, dolomite) are indicated to the fight of the figures. Impact occurs at time zero.

Uprang½ 4.0

2'0 I 0.0 •

--20

-4'0 ' ß ß

20 - -

Downrange 1.0

0.0 I -1.0

-2.0

10

............

15

.............

a) 5.65km/s 15 ø

b) 5.91 km/s 30 ø

c) 5.65 km/s 60 ø

2.0 • 0.0

--2.0

--4.0 ........

d) 5.66 km/s 90 ø

e) 5.22 km/s 15 ø

2o

--0.50 0.00 0.50 1.00 1,50 2.00 2.5


2C -- - -0.50 0.00 0.50 1.00 1.50 2.00 2.5

f) 5.93 km/s 30 ø

Fig. 5. Vertical component of the magnetic field observed uprange 0eft) and downrange (fight) of hypervelocity impacts 0ow ambient field, 0.32-cm Fe, 0.64-cm AI projectiles, dolomite target) conducted at various impact angles (Figures 5a-Sd: 0.32 cm Fe, Figures 5e and 5f.. 0.64-cm AI). The impact angle and velocity a• indicated to the right of the figures. Impact occurs at time zero.

18,812 CRAWFORD AND SCHt,•TZ: LABORATORY [NVESTIOATIONS OF IMPAC'I'-GENFJ•TED •MA

ture of the early time projectile/target coupling. This is consistent with the experimental evidence of Schultz and Gault [1990a]. At low impact angles (<15ø), large portions of the projectile survive as ricochet fragments created by spallation of the upper portion of the projectile, which subsequently impact downrange [Schultz and Gault, 1990b]. Most of the impact- generated vapor (and probably plasma) comprises an expanding cloud whose center of mass moves rapidly downrange leading to the early arrival of the downrange magnetic signal. As the projectile penetrates the target at higher impact angles, an early time, asymmetric cavity is produced which contains and redi- rects the impact-generated plasma uprange analogous to a jet [Schultz and Gault, 1979, 1982], thereby leading to the early ar-

plasma conditions with the production of magnetic fields. We have conducted nine oblique impact experiments with combined electrostatic and magnetic probes under various ambient magnetic field conditions in order to determine the plasma expansion velocity as well as the plasma parameters mentioned above and to correlate these measurements with the production of impact-generated magnetic fields. This section first describes electrostatic probe measurements and their theoretical analysis and compares these results with magnetic probe measurements. Finally, preliminary evidence of impact-generated magnetic fields produced by aluminum impacting aluminum at vertical incidence is presented.

Our initial oblique impact electrostatic probe studies used rival of the uprange magnetic signal. At 90 ø the uprange and standard aluminum (2024-TS) projectiles and aluminum plate downrange magnetic signals arrive nearly simultaneously due to (~1 cm thick) targets to reduce the amount of neutral vapor pro- axial symmetry. Future experiments can be designed to explore duction which can complicate analysis of electrostatic probe the evolution of asymmetries in the plasma cloud at different data. Two types of electrostatic probes allowed characterizing impact angles.

Certain characteristics of crater-related magnetic anomalies due to impact-generated magnetic fields can be predicted in laboratory cratering experiments. Relatively little impact melt is generated, and the cooling times are relatively long; consequently, impact-related magnetic remanence of transient magnetic fields most likely results from passage of the shock wave in the target. The spatial wavelength of crater-related paleomagnetism due to shock remanence can be estimated by dividing the shock velocity in the target by the characteristic temporal frequency of the observed magnetic fields. For a shock velocity of ~1 km/s [Gault, 1974] and characteristic frequencies between 10 and 50 kHz (at laboratory scale), crater-related paleomagnetism due to shock remanence of a transient impact-gen-

the impact-generated plasma. Probe 1, a large solid almninmn cylinder (2 cm diameter, 15.7 cm long, 102 cm 2 surface area), provided a sensitive indication of the presence of plasma in the probe's immediate vicinity; whereas probe 2, a much smaller copper cylinder with tin plate (0.028 cm diameter, 11.4 cm long, 1.0 cm 2 surface area) was used as a Langmuir probe to directly measure the plasma's electron number density (n) and temperature (T) (see Figure 6 for probe placement). The current flowing between the probe and plasma as a function of an applied, varying probe potential was measured for both probes and es- tablished the probe "characteristic". The probe circuit used the same amplifier and digitizer electronics as the magnetic detectors with the grounded target plate providing a reference potential and current sink for the plasma. Since plasma generated in

erated magnetic field should exhibit large intensity variation at these laboratory hypervelocity impacts lasts only a few mil- the 2-10 cm scale (a fraction of the crater radius). If the charac- liseconds, the proper choice of a probe potential sweep rate is teristic temporal frequencies of impact-generated magnetic crucial in order to obtain an undistorted Langmuir probe charac- fields are inversely proportional to the crater radius, as would teristic yet achieve high sample resolution. On the basis of the- occur if they were primarily due to density and temperature oretical considerations and exploratory experiments a 5 kHz, variations in the plasma, and the shock velocity in the target is +10 V sinusoidal potential provided a relatively undistorted approximately constant, then the estimated spatial wavelength of Langmuir probe characteristic and about 50 data points per crater-related paleomagnetism due to shock remanence will be a probe sweep. relatively constant fraction of the crater radius up to the scale at Figures 7 and 8 show the current measured by probes 1 and 2, which ambient field interactions play a dominant role. At respectively, during the oblique (15 ø from horizontal, 5.44 km/s) broader scale, where relatively large amounts of impact melt are impact of a 0.64-cm aluminum projectile into a solid aluminum generated and the duration of impact-generated magnetic fields may approach a fraction of a second or more, thermal remanence as well as shock remanence may play a role. Future experimental work with widely varying projectile sizes and impact velocities should allow assessing scaling effects on the characteristic frequencies of impact-generated magnetic fields, thereby characterizing spatial wavelengths of crater-related paleomagnetic anomalies.

Complementary electrostatic and magnetic probe experiments

While magnetic probes provide valuable information on the nature of the early time projectile/target interaction and the dynamics of impact-generated plasma (and may ultimately yield some of this information for ancient impact sites), electrostatic probes allow direct laboratory measurement of certain plasma parameters such as ion and electron number densities and electron temperature. Such data are crucial for testing quantitative models of transient magnetic fields produced by major meteoroid impact (see, for example, Hood and Vickery [1984]). In- cluding both magnetic and electrostatic probes in laboratory hypervelocity impact experiments allows direct correlation of

target. Probe 1 was located 42 cm downrange and 8 cm below,

Probe 1 [ Ground Plone

Fig. 6. Configuration of the combined electrostatic and magnetic probe experiments. The oblique impact produces a plasma that expands downrange, immediately engulfing the Langrnuir probe (probe 2) and eventu- ally expanding to engulf probe 1. The projectile, target and ground plane were made of 2024-TS aluminum. The oscillating probe potentials and the resulting probe currents (It,) were measured by the circuit, shown schematically, within the box.

CRAWFORD AND SCHULZ: LABORATORY INVF3TIGATIONS OF IMPACr-G•TED PLAS• 18,813

15

-5 -

-10

-'15

Time (ms)

Fig. 7. Current measured by probe 1, located 42 cm downrange and 8 an below the oblique impact (15 ø from horizontal, 5.44 km/s) of a 0.64-cm aluminum projectile into a solid aluminum target. The plot has been oriented so that the probe current is positive when the probe potential is positive. The impact occurred at approximately 0.5 ms.

2.0

1.0

-1.0

-2.0

-3.0 -8.0

i

i

i

2 B C I _

_

-A _

-6.0 -4.0 -2.0 0 V 2.0 4.0 6.0 a

v

Fig. 9. The Langmuir probe (probe 2) characteristic. Point A is the ion saturation current, point B is the probe's floating potential, and point C is the plasma potential.

and probe 2 was located 7 cm downrange and 15.5 cm above, the impact point. The plots have been oriented so that the probe current is positive when the probe potential is positive. The positive probe current is predominantly due to electrons and, to a lesser extent, negative ions, whereas the negative probe current is predominantly due to positive ions. The 300-1as delay from the time of impact to the first signal on probe 1 corre- sponds to a time-averaged plasma expansion velocity of ~1

The signal on probe 2 (Figure 8) arises from two distinct plasma pulses. The first, lasting about 1 ms, coincides with the time of impact (to within _+50 I. ts); the second, lasting several

plasma pulse from the chamber walls; however, this is unlikely due to the geometry of the impact chamber. In any case, the late time plasma pulse is a good example of plasma with an overall density not much greater than the ambient atmospheric density as there has been ample time for the plasma to disperse.

In order to use conventional probe theory for low pressure pla.sma, the characteristic dimension of a Langmuir probe must be larger than the Debye length

4•:e2n (5)

milliseconds, occurs much later. The reason for the late time and smaller than the electron mean free path, • [Swift and plasma pulse is unknown but may be due to incorporation of Schwar, 1970]. We conducted several experiments to determine ionized gas, originally ejected uprange of the impact, into the the optimum probe size. dominant downrange, moving fluid system or from ionization of Figure 9 is a plot of the Langmuir probe characteristic ob-

tained from the second probe potential sweep of the early-time ricocheted debris impacting downrange. It is also possible that the late-time plasma pulse arises from reflection of the original plasma pulse of Figure 8. The curve has been shifted such that

4.0 , ,,, [, [, i i i i i, i.[ i i , , , , i,, [ i, i

3.0

2.0

1.0

0 •

the probe's floating potential (at which no current flows) corre- sponds to zero volts. The probe current when the probe potential is slightly positive (section B-C on the characteristic) is mostly due to the electron current which can be given by

where

I• = I o exp 0 < W < V• (6)

-1.0 .... I .... I .... I .... [ .... !,,,, 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Time (ms)

Fig. 8. Current measured by probe 2 (the Langmuir probe), located 7 ½rn downrange and 15.5 ½m above the oblique impact (15 ø from horizontal, where n+ is the ion number density and m+ is the ionic mass. 5.44 kin/s) of a 0.64 cm aluminum projectile into a solid aluminum target. The kink in the plot of ln(/v) versus W (Figure 10) most likely The impact occurred at approximately 0.5 ms. occurs at the plasma potential (V,,); hence the linear slope in the

•kTrn • (7) and/• is electron current, W is probe potential, V,, is plasma po- tentiM, m e is mass of an electron, and At, is area of the probe (1.0 cm2). The ion saturation current (point A on the characteristic) can be represented by an equation of the form

I+ =- 0.6 n+ e At, •/• (8)

18,814 C•wn•m• A•rn SonmTz: LAaO•?OR¾ INVESTIOATIONS OF IMPA•-G••D PLASMA

2.0• o

-1.o

-3.0 V' 0.0 a 1.0 2.0 3.0

V

Fig. 10. Plot of In(It,) versus Vt, for section B-C of the Langrnuir probe characteristic. The s-lope of the linear portion gives an electron temperature of 4500 K and the kink locates the plasma potential (V a ~ 0.9 V).

plot yields an electron temperature (T) of approximately 4500 K. Equation (7) then yields a minimum electron number density (n) of approxirrtately i 09 cm '3 for I o ~1 mA (from Figure 9). From the ion saturation current (~1 mA), however, equation (8) yields an ion number density (n+) of approximately 101! cm '3. The two order-of-magnitude disparity between the observed electron and ion number densities (which should be nearly equal) can possibly be explained by (1) the plasma density steadily de- creased during the time of the probe sweep (due to cloud expansion); (2) an additional nonthermal ion current was induced by the directed motion of the plasma flow [Fuhs, 1965]; (3) the probe characteristic is incomplete and the plasma potential is at a much higher value; or (4) the plasma had a high neutral number density at these early times in the form of vapor or small (~1-10 I. un) droplets (and consequently, a low electron

mean free path) that strongly modified the observed electron current. The first three explanations Can account for only about 10% of the disparity with reasonable values for the expansion velocity and plasma potential. The last explanation requires that the electron mean free path of the first plasma pulse was re- duced relative to the second plasma pulse by at least a factor of 2. As a result, at least 10 '3 projectile mass was vaporized or 10 '2- 10 '1 projectile mass was melted (and entrained in the plasma cloud) during the impact.

If estimates of the electron number density made with the Langmuir probe bracket the actual value, then between 10 '7 and 10 '5 of the impact-generated vapor cloud is ionized for an overall plasma density only slightly greater than that of the residual atmosphere in the impact chamber (about 1016 If, at the time of the measurement• the overall plasma density is much greater than that of the residual atmosphere then the ionized fraction will be much smaller. Under these conditions, the

Debye length (•'a) is between 0.002 and 0.02 cm and the electron mean free path (•) is between 0.1 and 1.0 cm.

Magnetic probes simultaneously recorded impact-generated magnetic fields in the vicinity of the Langmuir probe during these experiments. Figure 11 compares the horizontal component of impact-generated magnetic fields observed (at x=--5, y=-16, z=17 cm) above the impact point of two oblique aluminum into aluminum impacts that were conducted within opposite polarity ambient field environments. Figure 1 la is the field produced by an impact under ambient field conditions within the impact chamber (30 }xT, horizontal, 5.6 kin/s), and Figure 1 lb is the field produced by an identical impact under reversed ambient field conditions (-30 }aT, horizontal, 5.5 kin/s). The strong correlation between the two signals implies that most of the impact-generated field observed under these conditions is spontaneous. Adding the two signals and dividing by two yields an estimate of the contribution from spontaneous magnetic fields due to nonaligned electron density and temperature gradients (Figure 12a). Subtracting the two (and dividing by 2), yields a residual signal (Figure 12b) which, in the absence of noise, would reveal the contribution from interactions of the

-10

10_

-15

+30 }tT ambient field

-20 0.0

ß

0.5 1.0 1.5 2.0

-30 }tT ambient field 10

5

0

-5

-10

-15

b

-20 !.5 0.0 0.5 1.0 1.5 2.0 2.5

Time (ms) Time (ms)

Fig. 11. Horizontal component of impact-generated magnetic field measured above the impact point (at x•5, y=-16, z=17 cm). The fieldlt were produced by oblique impacts of 0,64-an aluminum projectiles into solid aluminum targets in opposite ambient field environments. (a) Ambient field: 30 lIT, horizbntal, impact velocity 5.6 km/s; (b) ambient field: -30 [aT, horizontal, impact velocity 5.5 km/s.

CRAWFORD AND SCHULTZ: LABORATORY [NVBSTIGATION$ OF IMPACr-GENBRATBD PLASlVIA 18,81:5

10 Spontaneous field

-10

-15

-20

0.0

-, , , , I • , , , I , , , , I , , , , I , , , ,

0.5 1.0 1.5 2.0 2.5

Residual signal

b

.... I .... I .... I,,, : l, ,,,

0.0 0..5 1.0 1.5 2.0 2.5

Time (ms) Time (ms)

Fig. 12. Contribution of (a) spontaneous magnetic field generation and (b) residual signal m the fields shown in Figure 11.

plasma with the ambient magnetic field. The relatively small signals of • Figure 12b highlight the reproducibility of the spontaneous magnetic fields shown in Figure 12a.

Figure 13 shows the vertical component of the magnetic field observed below the impact point of a vertical impact (0.64-cm aluminum, 5.68 kin/s, into solid •mninum target) conducted within the normal ambient field. The Langmuir probe, adjacent to the magnetic search coil, did not detect the presence of impact-generated plasma. Mo•t likely, the impact-generated plasma was confined above the target plane as revealed in a time exposure photograph. The relative proportion of spontaneous magnetic fields and ambient field interactions to the observed impact-generated magnetic field is unknown as only one experiment of this kind has been conducted to date; nevertheless, the

presence of an impact-generated magnetic field due to either mechanism suggests that at least some plasma was generated.

Other impact experiments indicate that higher velocities or

the use of other projectile/target materials such as dry ice, water ice, calcium carbonate, and powdered dolomite substantially increase the amount of vaporization at low impa.et angles down to 15 ø [Schultz, 1988]. As indicated by the degree to which the ambient magnetic field is excluded from • the impact region [Crawford and Schultz, 1988a], the electrical conductivity during these impacts is comparable to, if not greater than, the ana- lyzed case of an aluminum projectile impacting into solid aluminum. Hence the degree of i9nization is probably greater in these cases than in the example discussed here. The large amount of neutral vapor produced by such vapor-generating impacts, however, tends to reduce the electron mean free path, thereby reducing the effectiveness of the electrostatic probe method to obtain meaningful values of the plasma parameters. Careful probe design and placement should permit characterizing such parameters in the future.

CONCLUDING REMARKS

15 Observed. Magnetic Field, Vertical Impact

-15

0.0

, I , , , , I , ,, , ,

0.5 1.0 1.5

We have demonstrated that impact-generated magnetic fields exhibit complex spatial and temporal distributions dependent on impact angle, velocity, and projectile/target composition. Al- though it is unknown how this complexity will be affected by scaling to larger sizes and longer event times relevant to hypervelocity cratering on planetary surfaces, crater-related palcomagnetic anomalies should exhibit similar complexity, depend- ing on the nature of the remanence mechanism. Observations at laboratory scales suggest that crater-related palcomagnetism should exhibit spatial wavelengths of fractions of crater radii, thereby implying that the vast majority of small craters on the lunar surface might produce magnetic anomalies with lower characteristic wavelength than that recognizable from lunar or- bit. Anomalies associated with large impact craters, however, might dis. play complexity dependent to some extent on impact angle, velocity and projectile/target composition. Future experimental and analytical work should improve our understanding of impact, generated plasma by isolating the several possible mechanisms for pfoducing ionization at relatively low impact

Time (ms) velocities; by determining, in more detail, the spatial distribu- Fig. 13. Vertical component of impact-generated magnetic field measured tion and dynamics of impact-generated plasma; and by charac- below the impact point of a 0.64-cm aluminum projectile impacting verti- terizing the scaling relationships of impact-generated magnetic cally at 5.68 km/s into a solid alum,mum target. fields.

18,816 CRAWFORD AND SCHULTZ: LABORATORY INVESTIGATIONS OF IMPACr-GENFJ•TED PLASMA

Impact-generated magnetic fields may have significance for the future study of paleomagnetism on a variety of cratered surfaces in the solar system, such as the Moon, Mercury, Phobos, asteroids, etc. Once we understand the complex dynamics of impact-generated plasma and how the laboratory observations can be extrapolated to larger scale, we may be able to determine parameters such as impact angle, velocity, and projectile/target composition by measuring crater-related paleomagnetic anomalies.

APPENDIX A: MAGNETIC PROBE THEORY

The magnetic flux (•) passing through a search coil can be represented by an inverse Fourier transform of the form

ß (t) = A c B(t) •Ac F'I[B(k)] (A1) where A c is the coil area and B(k) is the Fourier transform of the magnetic field intensity, B(t). The response of the search coil, V(t), is found from

d• F4 V(t) = C(k) '•' = C(k) A, [ik B(k)] (A2)

where C(k) is a calibration function dependent on frequency and proportional to the number of coil loops. Taking the Fourier transform of both sides and rearranging terms yields

and finally

where

B(k) = ik A• C(k) (A3)

B(t)=r l A(k) / (A4)

A(k) = ik A• C(k) (A5)

is the spectral response of a search coil to a spectrally flat magnetic field source. A(k) is experimentally found by recording the response, Vo(t ), of a search coil embedded in a known whim noise magnetic field source. A Helmholtz coil connected to a

digital whim noise current source provided Bo(t ) so that

1.0

0.8

0.6

0.4

0.2

!

0 50 100 150 200

Frequency OcHz)

Fig. A1. Power spectra of the coil response functions of type A (dashed line) and type B (solid line) search coils.

A(k) = F[Bo(t)] (A6) Figure A1 shows the spectral response of the two types of search coils used in our study. The spectral response functions actually consist of real and imaginary parts, although only the power spectra are shown here.

Acknowledgments. We would like to thank Len Smka for suggesting the Langmuir probe experiments and for many helpful discussions throughout this study. Additionally, we would like to acknowledge the constructive reviews from Lon Hood and an anonymous reviewer. We would like to give special thanks to the NASA Ames Vertical Gun Crew (John Vongray, Ben Langedyk and Wayne Logsdon) for their tireless efforts. One of us (DAC) was supported by a NASA Graduate Student Researchers Program fellowship. This research was supported by NASA Grant NAGW-705.

Bianchi, R., et al., Radiofrequency emissions observed during macroscopic hypervelocity impact experiments, Nature, 308, 830-832, 1984.

Cisowski, S. M., et al., Magnetic effects of shock and their implications for magnetism of lunar samples, Proc. Lunar Sci. Conf., 6, 3123-3141, 1975.

Cisowski, S. M., et al., Magnetic effects of shock and their implications for lunar magnetism (II), Proc. Lunar Sci. Conf., 7, 3299-3320, 1976.

Crawford, D. A., and P. H. Schultz, Conductivity of an expanding plasma cloud above a hypervelocity impact, Lunar and Planet. Sci., 19, 217- 218, 1988a.

Crawford, D. A., and P. H. Schultz, Laboratory observations of impact- generated magnetic fields, Nature, 336, 50-52, !988b.

Fuhs, A. E., Instrumentation for High Speed Plasma Flow, pp. 147-155, Gordon and Breach, New York, 1965.

Gault, D. E., Impact cratering, in A Primer in Lunar Geology, edited by 1L Greeley and P. Schultz, pp. 137-157, NASA Ames Research Center, Moffett Field, Califomia, 1974.

Gault, D. E. and J. A. Wedekind, Experimental studies of oblique impact, Proc. Lunar Sci. Conf, 9, 3843-3875, 1978.

Gold, T., and S. Sorer, Cometary impact and the magnetization of the Moon, Planet. Space Sci., 24, 45-54, 1976.

Hide, R., Comments on the Moon's magnetism, Moon, 4, 39, 1972. Hood, L. L., and Z. Huang, Formation of magnetic anomalies antipodal to

lunar impact basins: two-dimensional model calculations, J. Geophys. Res., 96, 9837-9846, 1991.

Hood, L. L., and A. Vickery, Magnetic field amplification and generation in hypervelocity meteoroid impacts with application to lunar palcomagnetism, Proc. Lunar Planet. Sci. Conf. 15th, Part 1, J. Geophys. Res., 89, suppl., C211-C223, 1984.

Ivanov, B. A., B. A. Okulessky, and A. T. Basilevsky, Shock wave, a possible source of magnetic fields?, in Impact and Explosion Cratering, edited by D. Roddy, R. O. Pepin, and R. B. Merrill, pp. 861-867, Perg- amon, New York, 1977.

Kissel, J., and F. R. Krueger, Ion formation by impact of fast dust particles and comparison with related techniques, Appl. Phys. A, 42, 69-85, 1987.

Martelli, G., and P. Cerroni, On the theory of radio frequency emission from macroscopic hypervelocity impacts and rock fracturing, Phys. Earth Planet. Inter., 40, 316-319, 1985.

Martelli, G., and G. Newton, Hypervelocity crateting and impact magneti- sation of basalt, Nature, 269, 478-480, 1977.

Nitsan, U., Electromagnetic emission accompanying fracture of quartz- bearing rocks, Geophys. Res. Lett., 4, 333-336, 1977.

Pert, G. J., Self-generated magnetic fields in plasmas, J. Plasma Phys., 18, 227-241, 1977.

Runcom, S. K., Lunar magnetism, polar displacements and primeval satellites in the Earth-Moon system, Nature, 304, 589-596, 1983.

Schultz, P. H., Impact vaporization O f easily volatized targets: experimental results and implications, Lunar Planet. Sci., 19, 1039-1040, 1988.

Schultz, P. H., and D. E. Gault, Atmospheric effects on martian ejecta emplacement, J. Geophys. Res. 84, 7669-7687, 1979.

CRAWFORD AND SCHULTZ: LABORATORY INVESTIGATIONS OF IMPA•-G••D PLAS• 18,817

Schultz, P. H., and D. E. Gault, Impact ejecta dynamics in an atmosphere: experimental results and extrapolations, in Geological Implications of Impacts of Large Asteroids and Comets on the Earth, edited by L. Sil- ver, and P. H. Schultz, pp. 153-174, Geol. Soc. of Am. Spec. Pap. 190, 1982.

Schultz, P. H., and D. E. Gault, Prolonged global catastrophes from oblique impacts, in Global Catastrophes in Earth History: An Interdis- ciplinary Conference on Impacts, Volcanism, and Mass Mortality, edited by V. L. Sharpton, and P. D. Ward, pp. 239-260, Geol. $oc. Am. Spec. Pap. 247, 1990a.

Schultz, P. H., and D. E. Gault, Decapitated impactors in the laboratory and on the planets, Lunar Planet. Sci., 21, 1099-1100, 1990b.

Schultz, P. H., and L. J. Smka, Cometary collisions on the Moon and Mer- cury, Nature, 284, 22-26, 1980.

Smka, L. J., Spontaneous magnetic field generation in hypervelocity impacts, Proc. Lunar Sci. Conf., 8, 893-895, 1977.

Smka, L. J., et al., Magnetic field and shock effects and remanent magnetization in a hypervelocity impact experiment, Earth Planet. Sci. Lett., 42, 127-137, 1977.

Stamper, J. A., et al., Spontaneous magnetic fields in laser-produced plasmas, Phys. Rev. Lett.,26, 1012-1015, 1971.

Swift, J. D., and M. J. Schwar, Electrical Probes for Plasma Diagnostics, Iliffe, London, 1970.

D. A. Crawford and P. H. Schultz, Department of Geological Sciences, Box 1846, Brown University, Providence, RI 02912.

(Received July 11, 1990; revised July 12, 1991;

accepted July 31, 1991)

laboratory investigations of impact&generated plasma · moffett field, california. high frame...

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