EXPLODING-WIRE-DRIVEN SHOCK WAVES

G.L. Clark, J.J. Hickey, R. J. Kingsley, and R. F. Wuerker* Space Technology Laboratories Los Angeles, California

INTRODUCTION

The explosion of fine silver wires by the fast discharge of a low-inductance capacitor has been photographed with an STL Model C Image Converter Camera operated as a streak camera. All of the previously observed shock waves and contact surfaces have been clearly recorded [1-3]. In addition, the large effective aperture, f/O.5, of the camera, due to its fast optics, 50: 1 light gain, and 0.5-/Lsec phosphor persistence, has allowed the initial shock wave in air at atmospheric pressure to be photographed by its own lumi­nosity. Photographs of both radially and circumferentially propa­gating shock waves, depending upon dwell duration, have been recorded during the second conduction phase of the discharges.

APPARATUS

The exploding wire apparatus consists of an Axel Brothers 25-kv, 0.27 -/Lf capacitor, a triggered spark gap, and a wire support­ing frame, all arranged to minimize circuit inductance. Figure 1 shows the basic experimental arrangement. A 0.0025- or 0.005-in.­diameter pure silver wire, 11/2 in. long, was stretched between the top of the spark gap and the upper wire terminal and held at both ends by tapered pins. The current through the wire was monitored by a ferrite-core Rogowski coil having a sensitivity of 800 amp/v, mounted above the upper wire terminal. The circuit constants of the machine were found by discharging the 0.27 -/Lf capacitor through a lit 6 - in. nonexploding wire mounted between the terminals. The con­stants are L = 0.15 /Lh, "Vf/c=O. 75 ohm, f = 780 kc, and Q = wL/R = 18.

The electronic camera was triggered either directly from a derivative probe or through a 1/2- to 10-/Lsecvariable delay circuit. When directly triggered, the inherent camera delay is about 15 nanoseconds.

*Now at Quantatron, Inc .. Santa Monica, California.

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W. G. Chace et al. (eds.), Exploding Wires© Plenum Press New York 1962

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Fig. 1. The experimental setup consisting of the exploding wire apparatus and the camera.

EXPERIMENTAL RESULTS

Our observations indicate that two distinct regimes of exploding wire phenomena exist in our apparatus, depending upon the capacitor voltage. At high voltage, a discharge channel forms almost imme­diately after the current break. In this case, the dwell is almost imperceptible, but is evidenced by a sudden change in the current waveform.

An example of an explosion with a fast restrike is shown in Fig. 2. This is a 1-,usec streak photograph of a O.0025-in. silver wire with the capacitor charged to 25 kv. The circuit was under­damped in this case, decaying to lie in about 3,usec. The picture shows the expansion of the metal vapor as the boundary of the gray area, and the discharge path as the bright region starting at the axis and accelerating outward. The air shock wave is not visible in this picture and the contact surface is moving very slowly, be­cause very little energy was released during the brief metallic conduction phase. The expansion of the contact surface is not at all parabolic.

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Fig. 2. One-microsecond streak of a fast restrike; O.0025-in. silver wire, 25 kv initial capacitor voltage.

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The discharge establishes itself along a narrow path at the center because of the narrow region of low density at the axis. As the column expands and more energy is released, the conduction path expands rapidly to take in the whole column. More of the ex­pansion is seen in Fig. 3, which has the same radial scale. The length of the streak is 2 !-Lsec. The contact surface is no longer visible at the beginning, but the dim part which is concave outward at the left is the same as the bright portion of the previous figure. The bright, narrow band followed by the wider dark area has not been explained, but this occurs just as the discharge envelops the full width of the vapor column. No internal shock waves are visible in the picture.

The rate of energy dissipation in the wire may be increased by using a wire having an effective resistance which more closely matches the impedance of the resonant circuit. Figure 4 is a com­posite picture showing the explosion of a 0.005-in. silver wire at 25 kv. The circuit is so well matched in this case that the capacitor is discharged in 4,usec, or roughly one-quarter of the interval shown. Metallic conduction lasted only % !-Lsec. The air shock is not visible in the picture, but shows up faintly on the negative and was photographed under the same conditions by removing the 60: 1 neutral density filter and setting the line aperture at f/8. It will be

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Fig. 3. Two-microsecond streak showing metal vapor contact sur­face; O.0025-in. silver wire, 25 kv initial capacitor voltage.

noted that the contact surface again does not follow a parabolic law, but stops expanding soon after the heating ceases. In this case a bright, linearly expanding wedge of light is seen, starting at 8 ).Lsec after the initial conduction. This wedge has been explained [2,4] as the intersection of a radially converging cylindrical shock wave which is sometimes nonluminous.

At lower voltage, such as 20 kv, a short dwell is observed, as shown in Fig. 5. The outer front is the air shock, which expands

Fig. 4. Fifteen-microsecond composite streak photograph showing metal vapor contact surface and expanding radial shock wave; O.005-in. silver wire, 25 kv initial capacitor voltage.

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Fig. 5. Ten-microsecond streak photograph showing air shock and the radial converging shock wave which reflects at the center of the metal vapor column; O.OOS-in. silver wire, 20 kv initial capaci­tor voltage.

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initially according to a % power law but obeys a square law after cessation of the driving current at about 3 f.Lsec. The restrike occurs on the axis, for the reason outlined ear lier, and drives a shock wave radially outward, as described by Muller [1]. When it reaches the density discontinuity at the boundary of the metal vapor, it reflects and moves radially inward, as may be seen in the picture. It reflects at the center and expands linearly outward,as in the previous case.

An even lower voltage, such as 18 kv, produces a much longer dwell, as seen in Fig. 6. Because the air shock, seen at the left, was only one-thousandth the intensity of the restrike, beginning at 5 f.Lsec, a composite picture was required to display the entire event. In the case of a long dwell, the low-density region at the center of the vapor cloud expands, so that the cloud is actually a hollow cyl­inder, as shown by end-on photographs [1]. The position of the restrike is thus no longer confined to the axis as in all the previous examples. In fact, according to the theory of Wecken, the lowest pressure is found just inside the inward-facing shock wave near the outside of the hollow cylinder, and it is here that the restrike must take place. Once a filamentary path has been established, the channel expands in both directions circumferentially, driving shock waves ahead of it. These aZimuthal, or theta, waves meet on the

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Fig. 6. Fourteen-microsecond composite streak photograph show­ing air shock and the circumferentially propagating theta waves induced within the metal vapor core by the restrike; O.005-in. silver wire, 18 kv initial capacitor voltage.

opposite side of the cylinder, producing the intersection seen in the photograph. This mode of propagation explains why the inter­section invariably appears on the opposite side of the axis and the same distance from it as the restrike. A shock wave. propagating radially from an off-center restrike filament. could not come to a sharp focus on the other side after reflection from the cylindrical boundary and produce the distinct intersection which is consistently observed. Further verification of the model of circumferentially propagating linear waves is provided by the fact that the projection of the motion. as recorded on streak photographs. follows the ex­pected sinusoid.

The authors wish to acknowledge the assistance of Mr. Garland McKenna in the construction of the exploding wire apparatus. This work was carried out under the Space Technology Laboratories General Research Program.

REFERENCES

1. w. Miiller, Z. Physik Vol. 149, p. 397, 1957. 2. F.D. Bennett, Phys. Fluids Vol. I, p. 347,1958. 3. W.G. Chace and H.K. Moore [eds.], "Exploding Wires," Vol. I, Plenum Press, New

York, 1959. 4. C. A. Rouse, in "Exploding Wires," Vol. I, W. G. Chace and H. K. Moore [eds.]. Plenum

Press, New York, 1959, p. 227.