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Research Papers

Interaction Between Solid Copper Jets and Powerful Electrical Current Pulses

[+] Author and Article Information
Patrik Appelgren1

 Swedish Defence Research Agency (FOI), SE-164 90 Stockholm, Swedenpatrik.appelgren@foi.se

Torgny E. Carlsson, Andreas Helte, Tomas Hurtig, Anders Larsson, Patrik Lundberg, Melker Skoglund, Lars Westerling

 Swedish Defence Research Agency (FOI), SE-164 90 Stockholm, Sweden

1

Present address: Division of Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden.

J. Appl. Mech 78(2), 021006 (Nov 08, 2010) (7 pages) doi:10.1115/1.4002568 History: Received September 09, 2009; Revised February 25, 2010; Posted September 16, 2010; Published November 08, 2010; Online November 08, 2010

The interaction between a solid copper jet and an electric current pulse is studied. Copper jets that were created by a shaped-charge device were passed through an electrode configuration consisting of two aluminum plates with a separation distance of 150 mm. The electrodes were connected to a pulsed-power supply delivering a current pulse with amplitudes up to 250 kA. The current and voltages were measured, providing data on energy deposition in the jet and electrode contact region, and flash X-ray diagnostics were used to depict the jet during and after electrification. The shape of, and the velocity distributions along, the jet has been used to estimate the correlation between the jet mass flow through the electrodes and the electrical energy deposition. On average, 2.8 kJ/g was deposited in the jet and electrode region, which is sufficient to bring the jet up to the boiling point. A model based on the assumption of a homogenous current flow through the jet between the electrodes underestimates the energy deposition and the jet resistance by a factor 5 compared with the experiments, indicating a more complex current flow through the jet. The experimental results indicate the following mechanism for the enhancement of jet breakup. When electrified, the natural-formed necks in the jet are subjected to a higher current density compared with other parts of the jet. The higher current density results in a stronger heating and a stronger magnetic pinch force. Eventually, the jet material in the neck is evaporated and explodes electrically, resulting in a radial ejection of vaporized jet material.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

The measured current pulse in the 96 kJ experiment

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Figure 2

X-ray pictures of jet disruption at different initial energy in the PPS. From top, 96 kJ, 39 kJ, and 0 kJ are stored in the PPS. The jet moves from left to right and the shadowed areas on the left hand side are the positions of the electrodes. The pictures are taken at 86 μs after contact is made by the tip with electrode 2, except for the lower picture taken at 81 μs after contact. The jet tip has moved a distance of 620 mm from the electrode 2 at time 86 μs.

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Figure 3

The jet radius and jet velocity distribution at the time the tip makes contact with electrode 2 and the current begins to flow, as functions of distance to the cone base of the shaped charge

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Figure 4

Position-time diagram for the jet used in the experiments. The sloped lines indicate the jet segment velocity for the tip and rear, given by the numbers in km/s. The jet shape at five times is shown where the stretching is clearly illustrated.

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Figure 5

Distribution of the jet mass for jet segments with an axial velocity above 2.1 km/s. The jet mass before (A), in between (B), and after (C) the electrodes summing up to the total mass indicated by line D.

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Figure 6

The jet radius at electrodes 1 and 2 during its passage between the electrodes, calculated using the data in Fig. 3

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Figure 7

Position-time diagram for the jet with the current pulse. The vertical dashed lines marked X-ray A and X-ray B marks the times (36 μs and 86 μs) when the X-ray pictures were taken. At time 36 μs, the part with velocity between 3 km/s and 4.7 km/s is between the electrodes and at time 86 μs, the part with velocity between 1.9 km/s and 3 km/s is between the electrodes.

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Figure 8

Same position-time diagram as in Fig. 7 but with the deposited energy. Almost all energy has been deposited in the jet and electrode region by the time the rear part of the jet leaves the electrode region.

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Figure 9

Same position-time diagram as in Fig. 7 but with the power deposition in the jet. Note the low power up to the time the 6.6 km/s fragment leaves the electrode region, which may explain the low radial dispersion velocity of the tip.

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Figure 10

The measured resistance of the jets in the 39 kJ and 96 kJ experiments

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Figure 11

The calculated resistance of the part of the jet that is between the electrodes and the energy deposited in the jet. The rise in resistance in the first 20 μs is due to heating. The resistance then drops due to the larger conducting area of the jet parts entering the electrode region and also due to decreasing current. The energy deposition is much lower than measured in the experiments.

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Figure 12

The axial velocity for individual fragments in the 39 kJ (diamonds), 96 kJ (circles), and for an unelectrified jet (squares). The axial velocity as function of fragment number is similar for the electrified and unelectrified jets and this shows that electrified jets break up into the same number of fragments and with similar velocity distribution as the unelectrified jet.

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Figure 13

The jet between the electrodes in experiments with energies of 0 kJ, 39 kJ, and 96 kJ stored in the capacitors. The 0 kJ experiment is depicted at time 31 μs and the 39 kJ and 96 kJ experiments are depicted at time 36 μs. The jet radius is scaled ×3. The white spots are markers for measurements.

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Figure 14

The jet between the electrodes in experiments with energies of 0 kJ, 39 kJ, and 96 kJ stored in the capacitors. The 0 kJ experiment is depicted at time 81 μs and the 39 kJ and 96 kJ experiments are depicted at time 86 μs. The jet radius is scaled ×3. In the 96 kJ experiment, the jet has begun to disintegrate, indicated by the arrows. The white spots are markers for measurements.

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Figure 15

The jet diameter along the jet in the 39 kJ experiments at 86 μs, extracted from Fig. 1. The solid, fat line is a second-order polynomial fitted to the diameter to obtain the mean diameter of the jet and the dashed lines are the mean diameter plus and minus the deviation shown in Fig. 1. The vertical dash-dotted lines mark the position of the white reference markers in Fig. 1.

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Figure 16

The deviation from the mean diameter of the jet in the 39 kJ experiments at 86 μs. The solid, fat line is the second-order polynomial fitted to the deviation from the mean diameter and indicates a clear growth of the instability from 0.07 mm to 0.5 mm.

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Figure 17

The deviation from the mean diameter of the jet in the 0 kJ experiments at 81 μs. The solid, fat line is the second-order polynomial fitted to the deviation from the mean diameter and indicates no growth of the instability and provides a reference for the natural deviation. Note that this reference level is around 0.1 mm all along the jet while in the 39 kJ experiment, the deviation grows from approximately this value. Note that the 0 kJ experiment is depicted at 5 μs earlier than the 39 kJ experiment in which the jet moves between 10 mm and 15 mm.

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Figure 18

Plot of the mean diameter along the jets at time 31/36 μs for the 0 kJ, 39 kJ, and 96 kJ experiments together with the diameter deviation from the mean diameter. At this time, the deviations are not significantly different and a normal deviation is about 0.1 mm.

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Figure 19

Plot of the mean diameter along the jets at time 81/86 μs for the 0 kJ, 39 kJ, and 96 kJ experiments together with the diameter deviation from the mean diameter. At this time, the effect of electric energy on deviations is strong. The circle and the square marks the interval of the jet used to calculated the growth velocity in Fig. 2.

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Figure 20

Plot of the deviation growth velocity along the jet at time 86 μs for the 39 kJ experiments together with the jet velocity. The velocity reaches about 10 m/s toward the end of the interaction, indicating an acceleration of the order of 105 m/s2.

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