The results of an experimental, analytical, and numerical study of the cylindrical shock wave generated by the underwater electrical explosion of copper and aluminum wires are reported. Experiments were conducted using a microsecond timescale generator delivering ∼180 kA pulses with a 1.2 μs rise time. Shadow streak images were used to study the radial expansion of the exploding wire and the generated shock wave. It was found that the shock wave expansion velocity decreases to the velocity of sound in two stages: a fast stage and then a gradual stage. The fast stage occurs during ∼1.5 μs after the maximum of the resistive voltage is reached, and then, a gradual decrease occurs during several tens of microseconds. It was shown that the duration of the fast stage corresponds to the period of time when the main energy deposition into the wire occurs. Hydrodynamic simulations show that the fast decrease in the shock velocity is related to the evolution of the exploded wire's subsonic expansion, which leads to time/spatial compression of the adjacent water layer. For the gradual decrease stage of the shock wave velocity, we developed a simplified model, which considers uniform water density between the wire boundary and the shock wave front. The results of this model agree satisfactorily with the experimentally obtained shock wave trajectory and radial expansion of the wire.
We present the measurements of the development of striation like instabilities during the electrical driven explosions of wires in a water bath. In vacuum based wire explosion experiments, such instabilities have long been known. However, in spite of intense research into the explosion of wires in liquids, the development of these instabilities has either not been observed or has been assumed to play a minor role in the parameters of the exploding wire due to the tamping of the wire's explosion. Using synchrotron based multiframe radiography, we have seen the development of platelike density structures along an exploding copper wire. Our measurements were compared to a 2D magnetohydrodynamics simulation, showing similar striation formation. These observed instabilities could affect the measurements of the conductivity of the wire material in the gas-plasma state-an important parameter in the warm dense matter community. The striations could also act as a seed for other instabilities later in time if the wire is in a dense flow of material or experiences a shock from an adjacent wire-as it would do in experiments with arrays of wires.
We present the first use of synchrotron-based phase contrast radiography to study pulsed-power driven high energy density physics experiments. Underwater electrical wire explosions have become of interest to the wider physics community due to their ability to study material properties at extreme conditions and efficiently couple stored electrical energy into intense shock waves in water. The latter can be shaped to provide convergent implosions, resulting in very high pressures (1-10 Mbar) produced on relatively small pulsed power facilities (100s of kA-MA). Multiple experiments have explored single-wire explosions in water, hoping to understand the underlying physics and better optimize this energy transfer process; however, diagnostics can be limited. Optical imaging diagnostics are usually obscured by the shock wave itself; and until now, diode-based X-ray radiography has been of relatively low resolution and rather a broad x-ray energy spectrum. Utilising phase contrast imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility, we were able to image both the exploding wire and the shock wave. Probing radiation of 20-50 keV radiographed 200 μm tungsten and copper wires, in ∼2-cm diameter water cylinders with resolutions of 8 μm and 32 μm. The wires were exploded by a ∼30-kA, 500-ns compact pulser, and 128 radiographs, each with a 100-ps X-ray pulse exposure, spaced at 704 ns apart were taken in each experiment. Abel inversion was used to obtain the density profile of the wires, and the results are compared to two dimensional hydrodynamic and one dimensional magnetohydrodynamic simulations.
The results of experiments and one-dimensional (1D) hydrodynamic (HD) simulations of electrical explosions of spherical Cu wire arrays in water and glycerol for various stored energy of the pulse generator and sphere diameters are presented and discussed. It was found that the convergence of the shock wave generated by an exploding spherical wire array in glycerol is significantly faster than in water. The resulting pressure in the vicinity of the implosion center is several times larger in glycerol than in water. Increasing the initially stored energy from 3.6 to 6.1 kJ (for identical array diameters) or decreasing the sphere diameter from 30 to 20 mm (for identical stored energy) leads to an increase in the pressure, temperature, and density in the vicinity of the implosion center. The pressure in a spherical volume of ∼0.2 mm in diameter at the origin of the sphere is estimated by 1D HD simulations to be in excess of 1012 Pa.
Using streak images of underwater electrically exploding copper, aluminum, and tungsten wires (current densities of 10 7-10 8 A/cm 2 and energy density deposition of 10-50 kJ/g) and generated weak shocks, the onset of each phase transition, its duration, and the time when the wire explosion occurred were determined. The measured discharge current and resistive voltage were used to calculate the energy and energy density deposition. Using the discharge current waveform and the onset of the strong shock wave, the specific action integral was calculated and compared with published data. The thermodynamic parameters during the wire explosion were calculated using one-dimensional magneto-hydrodynamic simulations coupled with equations of state for water, copper, and aluminum. It was shown that the onset times of weak shocks, in general, cannot be related to the melting or the evaporation of the entire wire.
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