Preliminary Study on Magnetic Induction Intensity Induced by Plasma During Hypervelocity Impact

Transcription

1 Chinese Journal of Aeronautics 22(2009) Chinese Journal of Aeronautics Preliminary Study on Magnetic Induction Intensity Induced by Plasma During Hypervelocity Impact Tang Enling a,b, *, Zhang Qingming b, Zhang Jian a a School of Equipment Engineering, Shenyang Ligong University, Shenyang , China b State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing , China Received 10 August 2008; accepted 17 November 2008 Abstract An experimental system has been built to produce and measure the magnetic field in the backward ejected matter during hypervelocity impact. The designs of measurement system and coil, the choice of associated equipment, and the system calibration are also described in detail. The measurement of magnetic induction intensity for different given coil positions and azimuth angles are performed with two-stage light-gas gun. On condition that impact velocities are approximately equal and incidence angles are 45, 60 and 90 respectively, the relationship between average magnetic induction intensity and impact angle at different time spans is obtained. Experimental results show that the average magnetic induction intensity with incidence angle of 90 is larger than those with incidence angles of 45and 60. Keywords: hypervelocity impact; plasma; magnetic fields; measurement system; magnetic induction intensity 1. Introduction 1 The plasma was considered to be generated by partial vaporization and ionization of the projectile and target material under the extreme conditions of hypervelocity impact [1-8]. R. Hide [8] suggested that hypervelocity impacts could produce magnetic fields through the hydromagnetic interaction of impact-generated electrically conductive plasma. The electromagnetic properties of plasma produced by hypervelocity impact have been exploited by researchers as a diagnostic tool to explain the jumbled state of potential magnetic field on the lunar surface [9-11] and the loss of Olympus experimental communication satellite [12]. Ionization and formation of plasma due to impact of fast dust particles have been investigated extensively in order to study cometary dust particles encountered during space flight [13]. This article aims at establishing the measurement system of magnetic field induced by plasma which is *Corresponding author. Tel.: address: tangenling@126.com Foundation items: National Natural Science Foundation of China ( ); Talent Resources Development Special Funds of Shenyang ( ); Doctoral Initiation Special Fund of Shenyang Ligong University generated during hypervelocity impact, and acquiring the relational feature of average magnetic induction intensity to impact angle with different given coil positions, azimuth angles and time spans during early impact stage. 2. Theory Refs.[14],[15] considered that thermally driven electrical currents and their associated magnetic fields could be generated in plasma clouds during the early stage of hypervelocity impacts. He developed a simple theoretical model of impact-generated magnetic fields. The model describing the rate of magnetic induction intensity changing with time (B/t) is derived from generalized Ohm s law 1 nk T J E ub (1) c en where J is the electric current distribution; E and B are the electric and magnetic field intensities, u and the plasma s fluid velocity and electrical conductivity; c is the speed of light, k Boltzmanns constant; and n, T and e are the electron number density, temperature and charge, respectively. Applying Maxwell s equation to Eq.(1) obtains Elsevier Ltd. Open access under CC BY-NC-ND license. doi: /s (08)

2 388 Tang Enling et al. / Chinese Journal of Aeronautics 22(2009) No.4 2 B ck c 2 ( T n) B ( u B) (2) t en 4 The first term on the right part of Eq.(2) represents source term which arises from drift current due to nonzero pressure gradients [14] ; the second term represents the diffusion of B through the electrically conductive plasma; the third term represents advection of B due to fluid motion. Refs.[9],[10],[14],[15] also considered that the maximum magnetic field intensity could be estimated from Eq.(2). The distribution of ionized fraction within the vapor cloud may be considerable complexity due to jetting, entrained or ricocheted fragments and the internal shear heating of the projectile and target [16]. If the diffusion term in Eq.(2) can be neglected [14], the order-of-magnitude estimate of the maximum magnetic induction intensity B max coming from impact-generated plasma can be obtained by setting B/t =0 max ~ ck T n B (3) eu r n where u is the magnitude of the plasma velocity (usually approximated by the gas expansion velocity) and T/r and n/n represent temperature and density gradients being considered as the functions of position within plasma cloud. Eq.(3) shows that the spontaneous impact-generated magnetic field is not strongly affected by the absolute level of electron number density n but relies on the fluctuations of electron number density n/n. 3. Experiment The experimental investigation of magnetic field production is performed with the two-stage light-gas gun installed in the Laboratory of High Pressure Physics of Southwest Jiaotong University. The gas gun is capable of launching macroscopic projectiles with the velocity up to 7 km/s, and the target chamber is vacuumized to achieve its vacuity being less than 100 Pa for the superhigh velocity impact experiment Experimental basic supposition In the experiment, the shock-induced ionization of the air ahead of the projectile may exist and form a plasma layer near the surface of spherical metal projectile. The effect of plasma on the measurement of magnetic induction intensity is supposed to be negligible, because the residual gas pressure of target chamber is less than 100 Pa and the hydrogen gas after the projectile is expanded into the front half chamber, which is effectively separated from the behind half chamber Experimental system Two-stage light-gas gun launching macroscopic projectile is a metal sphere. After flying certain distance in target chamber, the metal sphere is separated from the sabot, and then impacts the target. Coils are located at the downrange positions of backing plate through support setting, which is located far away from the mainstream of ejectors generated by hypervelocity impact. The measured signals in expanding plasma cloud are obtained by search coil and input measuring system which transports voltage signals by coaxial cables. Weak voltage signals collected by coils are amplified by differential operational amplifier and input to oscilloscope. Fig.1 is the block diagram of system construction. Fig.1 Block diagram of system construction Experimental system layout In order to position the coils, a Cartesian coordinates system (X, Y, Z) with the origin at the impact point is used, where +Z denotes the height above the target surface, +Y the distance from the impact point to the upper range, and +X the distance away from the projectile flying line which meets the right-handed rule. Coil space layouts for three sets of experiments are the same that each set of experiment is equipped with two coils symmetrically arranged on both sides of the plane which is perpendicular to the target and trajectory. The coordinates of the centers of coil 1 and coil 2 are (50, 175, 185) and (50, 175, 185), respectively. Coil plane is perpendicular to backing plate with an angle of 45 between flight line and coil center line. Fig.2 shows the experimental system layout Design of coil measurement system The rapidly changing magnetic field produced during hypervelocity impact in the laboratory is measured with search coils. The electromotive force (EMF) induced within each coil is proportional to the rate-ofchange of the magnetic field and is changing with time (B/t). The voltage is damped by 10 k resistors and amplified by a differential operational amplifier. After being amplified, the signals are input to the digital acquisition system. Fig.3 is the diagrammatic sketch of coil measurement system, where R L is self-integral resistance which determines the smooth degree of measured signal. The coils consist of 300 turns of 30 gauge (American Wire Guide) copper magnet wire wound helically on a plastic coil form approximately 6 cm in diameter.

3 No.4 Tang Enling et al. / Chinese Journal of Aeronautics 22(2009) Fig.2 Experimental system layout. Fig.3 Diagrammatic sketch of coil measurement system. All the coils are shielded by grounded aluminum foil with thickness of 2 mm. Due to the grounded aluminum foil, the electrostatic component of the coil signals is negligible. The signal coming from each coil is routed through an amplifier with the gains of 10, 100, 300, 1 000, , and input to two channels of digital oscilloscope. Four-channel voltage preamplifier is chosen for all the experiments. The model number of the differential operational amplifier is HB-854, which is developed by Weak Signal Detection Center of Nanjing University of China Experimental parameters (1) Basic impact parameters The materials for both projectile and target are all LY12-Al. Projectile is a solid sphere with a diameter of 6.4 mm. Thickness of LY12-Al targets is 23 mm. The basic impact parameters are listed in Table 1. Table 1 Basic impact parameters d 1 V( t) C( k) C( k) Ac F (i kb ( k)) (5) dt where C(k) is a calibration function being dependent on frequency and proportional to the number of coil turns. Applying the Fourier transforms of both sides and rearranging them yields F( V ( t)) B ( k) (6) i kacc ( k) and finally it yields 1 F( V ( t)) B () t F Ak ( ) (7) where A(k)=ikA c C(k) is the spectral response of a search coil to a spectrally flat magnetic field source. A(k) can be found by recording the response V 0 (t) of a search coil embedded in a known white noise magnetic field source. A Helmholtz coil is connected to a digital white noise current source to provide B 0 (t), so that F( V0 ( t)) Ak ( ) (8) F( B0 ( t)) Fig.4 shows the spectral response of the search coils used in our study. The average search coil response is about 0.4 mv/nt for 300 turns coil when the frequency is between 1 khz and 150 khz. Experimental code Impact velocity /(kms 1 ) Impact angle /() Chamber pressure/pa (2) System calibration [9] The magnetic flux passing through a search coil can be represented by an inverse Fourier transform c c 1 () t A B() t A F ( B ( k)) (4) where A c is the coil area and B(k) the Fourier transform of the magnetic field intensity B(t). The response of the search coil V(t) is given by Fig.4 Spectrum of coil response function. 4. Experimental Results and Analysis 4.1. Typical original data Electromagnetic pulse excited by magnetic speed

4 390 Tang Enling et al. / Chinese Journal of Aeronautics 22(2009) No.4 measurement system of projectile is used as the outer trigger signal of oscilloscope, then starts recording the signal when being triggered. One can well understand the working process from Fig.2. Fig.5 shows the results of a series of hypervelocity impacts (6.4 mm aluminum projectiles, 300 turns, mean velocity 5.94 km/s) are conducted. The distance between velocity measuring coil center and impact point is mm for all the experiments. Experimental results show that the starting times of signals for two coils of each set of experiment have very good synchronism and approximate equal amplitude values of signal Analysis of magnetic induction intensity According to Fig.6, one can obtain the results that the order of magnitude and amplitude value for the Fig.5 Variation of induced electromotive force with time. Fig.6 Variation of magnetic induction intensity with time.

5 No.4 Tang Enling et al. / Chinese Journal of Aeronautics 22(2009) magnetic induction intensity are congruent with those obtained by the others [10-11]. Due to the fact that the voltage signals of every coil are amplified by differential operational amplifier and its dynamic range is finite, some of the high frequency signals are clipped and the low pass cut-off frequency is 150 khz for all the experiments Average magnetic induction intensity at different time spans Fig.7 is the average magnetic induction intensity at different time spans during early impact stage. According to Fig.7, one can know that when impact velocities are close, the average magnetic induction intensity is very sensitive to the impact angles at different time spans during early impact stage. Average magnetic induction intensity of upright impact is larger than those of the impact angles being 45 and 60. The above mentioned results may be explained that the Fig.7 Average magnetic induction intensity at different impact angles and time spans. T n product for large impact angle (counted from r n the plane of target) is large. 5. Conclusions Experiments indicate that the coil measurement system can give credible measuring results for magnetic induction intensity induced by plasma during hypervelocity impact. Experimental results show that the average magnetic induction intensity at incidence angle of 90 is larger than those at incidence angles of 45and 60 at different time spans during early impact stage. This is only preliminary work in the field and there is a long way to go in the future. Acknowledgments These experiments are performed with the support and help of Professor Liu Fusheng, Engineer Zhang Mingjian and Xue Xuedong in the Laboratory of High Pressure Physics of Southwest Jiaotong University, so the authors of the article thank them devoutly. References [1] Tang E L, Zhang Q M, He Y H, et al. Preliminary study on diagnostic techniques for transient plasma generated by hypervelocity impact. Plasma Science and Technology 2008; 10(6): [2] Tang E L, Zhang Q M, Huang Z P. Electron temperature diagnosis of plasma generated during hypervelocity impact. Transactions of Beijing Institute of Technology 2007; 27(5): [in Chinese] [3] Tang E L, Zhang Q M, Ouyang J T. Fast diagnosis of transient plasma by Langmuir probe. Journal of Beijing Institute of Technology 2007; 16(3): [4] Tang E L, Zhang Q M, Zhang J. Characteristic parameter measurement of plasma generated during hypervelocity impact on LY12 aluminum target. Journal of Projectiles, Rockets, Missiles and Guidance 2008; 28(4): [in Chinese]

6 392 Tang Enling et al. / Chinese Journal of Aeronautics 22(2009) No.4 [5] Hood L L, Huang Z. Formation of magnetic anomalies antipodal to lunar impact basins: two-dimensional model calculations. Journal of Geophysics Research 1991; 96(B6): [6] Schultz P H. Impact vaporization of easily volatized targets: experimental results and implications. Lunar Planet Science 1988; 19: [7] Schultz P H, Srnka L J. Cometary collisions on the Moon and Mercury. Nature 1980; 284: [8] Hide R. Comments on the Moon s magnetism. Moon 1972; 4(1-2): 39. [9] Crawford D A, Schultz P H. Laboratory observations of impact-generated magnetic fields. Nature 1988; 336: [10] Crawford D A, Schultz P H. Laboratory investigations of impact-generated plasma. Journal of Geophysics Research 1991; 96(E3): [11] Crawford D A, Schultz P H. The production and evolution of impact-generated magnetic fields. International Journal of Impact Engineering 1993; 14: [12] Caswell R D, McBride N, Taylor A. Olympus end of life anomalya perseid meteoroid impact event. International Journal of Impact Engineering 1995; 17 (1-3): [13] Kissel J, Krueger F R. Ion formation by impact of fast dust particles and comparison with related techniques. Applied Physics A: Materials Science & Processing 1987; 42(1): [14] Srnka L J. Spontaneous magnetic field generation in hypervelocity impacts. Proceeding of Lunar Science Conference. 1977; [15] Stamper J A. Spontaneous magnetic fields in laserproduced plasmas. Physical Review Letters 1971; 26(17): [16] Schultz P H, Gault D E. Decapitated impactors in the laboratory and on the planets. Abstracts of the Lunar and Planetary Science Conference. 1990; 21: Biography: Tang Enling Born in 1971, he received Ph.D. degree from Beijing Institute of Technology in His main research interests are hypervelocity impact, plasma diagnostic, impact light flash and pulsed plasma thruster (PPT). tangenling@126.com