D. Shafer

Underwater Electrical Explosion of Wires and Wire Arrays and Generation of Converging Shock Waves

A brief review of the results obtained in recent research of underwater electrical explosions of wires and wire arrays using microsecond-, submicrosecond-, and nanosecond-timescale high-current generators is presented. In a microsecond-timescale wire explosion, good agreement was attained between the results of experiments and the results of magnetohydrodynamic calculations coupled with equations of state (EOS) and modern conductivity models. Conversely, in a nanosecond-timescale wire explosion, the wire resistance and the EOS were modified in order to fit experimental data. In experiments with cylindrical and spherical wire arrays, generation of a converging shock wave (SW) was demonstrated allowing formation of an extreme state of water in the vicinity of either the axis or the origin of the SWs implosion. In addition, it is shown that SW convergence in superspherical geometry allows one to achieve larger values of pressure, density, and temperature of water in the vicinity of the axis of convergence than in the case of a spherical implosion. The results of experiments and numerical analysis showed that a cylindrical SW keeps its symmetry along the main path of its convergence. In addition, it is shown that underwater electrical explosion of an X-pinch wire configuration and a cone wire array allows one to generate fast jets of metal and water, respectively, without using chemical explosions.

Peculiarity of convergence of shock wave generated by underwater electrical explosion of ring-shaped wire

Nanosecond timescale underwater electrical wire explosions of ring-shaped Cu wires were investigated using a pulsed generator with a current amplitude up to 50 kA. It was shown that this type of wire explosion results in the generation of a toroidal shock wave (SW). Time- and space-resolved optical diagnostics were used to determine azimuthal uniformity of the shock wave front and its velocity. It was found that the shock wave preserves its circular front shape in the range of radii 50μm<r<5 mm. At r≤15μm, azimuthal irregularities of the SW front were obtained indicating the appearance of azimuthal instability. A surprising finding is that the shock wave propagates with a constant velocity of vsw=1.2M, where M is the Mach number. The dynamics of the leading part of the shock wave, based on the oblique shock wave theory, is presented, explaining the constant velocity of the shock wave.

Diagnostics of a converging strong shock wave generated by underwater explosion of spherical wire array

The results of experimental studies of the convergence of shock waves (SWs) generated by the underwater electrical explosion of a spherical wire array supplied by a current pulse with an amplitude ∼300 kA and rise time ∼1.1 μs are reported. In the experiments, the power and spectrum of the light emission from an optical fiber, the explosion of a copper tube, and the time-dependent resistance of a resistor placed in the equatorial plane of the spherical wire array were measured. A comparison of the experimental data with the results of numerical simulations of SW convergence shows that the SW keeps its uniformity along the major part of the convergence towards the implosion origin.

Evaluation of electrical conductivity of Cu and Al through sub microsecond underwater electrical wire explosion

Sub-microsecond timescale underwater electrical wire explosions using Cu and Al materials have been conducted. Current and voltage waveforms and time-resolved streak images of the discharge channel, coupled to 1D magneto-hydrodynamic simulations, have been used to determine the electrical conductivity of the metals for the range of conditions between hot liquid metal and strongly coupled non-ideal plasma, in the temperature range of 10–60 KK. The results of these studies showed that the conductivity values obtained are typically lower than those corresponding to modern theoretical electrical conductivity models and provide a transition between the conductivity values obtained in microsecond time scale explosions and those obtained in nanosecond time scale wire explosions. In addition, the measured wire expansion shows good agreement with equation of state tables.

Diagnostics of underwater electrical wire explosion through a time- and space-resolved hard x-ray source

A time- and space-resolved hard x-ray source was developed as a diagnostic tool for imaging underwater exploding wires. A ~4 ns width pulse of hard x-rays with energies of up to 100 keV was obtained from the discharge in a vacuum diode consisting of point-shaped tungsten electrodes. To improve contrast and image quality, an external pulsed magnetic field produced by Helmholtz coils was used. High resolution x-ray images of an underwater exploding wire were obtained using a sensitive x-ray CCD detector, and were compared to optical fast framing images. Future developments and application of this diagnostic technique are discussed.

Shock wave convergence in water with parabolic wall boundaries

The convergence of shock waves in water, where the cross section of the boundaries between which the shock wave propagates is either straight or parabolic, was studied. The shock wave was generated by underwater electrical explosions of planar Cu wire arrays using a high-current generator with a peak output current of ∼45 kA and rise time of ∼80 ns. The boundaries of the walls between which the shock wave propagates were symmetric along the z axis, which is defined by the direction of the exploding wires. It was shown that with walls having a parabolic cross section, the shock waves converge faster and the pressure in the vicinity of the line of convergence, calculated by two-dimensional hydrodynamic simulations coupled with the equations of state of water and copper, is also larger.

Extreme states of water obtained by generating converging shock waves by underwater electrical explosions of wire arrays

Summary form only given. Underwater electrical explosion of wire arrays is a promising method of obtaining extreme states of water (T > 1 eV, P > 1010 TPa). These extreme states are obtained in the vicinity of implosion of strong converging shock waves generated by the array explosion. Explosions of cylindrical and spherical wire arrays were studied in which the latter results in higher water parameters due to the shock wave convergence to a smaller region (origin of a sphere as opposed to an axis of a cylinder). Latest results obtained in our laboratory show that one can achieve even higher water parameters with the same amount of stored energy than with the spherical wire array. This is achieved by introducing parabola shaped boundaries (super-spherical geometry) in the space where the generated strong shock wave is propagating. The experimental and numerical results of this recent research will be reported.

Generation of fast cumulative water jets by underwater electrical explosion of conical wire arrays

The results of experiments with underwater electrical explosion of conical arrays of copper wires are presented. A pulsed generator producing a 300 kA-amplitude current with a 1.2 μs rise time was used in the explosion of the arrays. As a result of the explosion, fast-moving water jets, with velocities of up to 1200 m/s, were observed being ejected from the surface of the water covering the wire array. The position of the water jets was measured by multiple-exposure fast framing imaging. The apex angle of the array or the thickness of the water layer above the arrays was altered from shot to shot, which changed the resulting velocities and shapes of the emitted jets. A numerical model, based on the models of cumulation and penetration of a jet through material of similar density, is suggested. The velocities of jets obtained by this model, agree well with the experimentally observed jet velocities.

Shock wave implosion in water with different boundary conditions

Summary form only given. The convergence of shock waves in water with either conical or parabolic walls boundaries is studied. The shock waves are generated by underwater electrical explosions of planar Cu wire arrays using a high-current generator with stored energy of ~260 J, rise time of ~80 ns and peak current of ~45 kA. The boundaries of walls are symmetric along the z axis which is also the direction of the exploding wires. It is shown that the shock waves converge faster with the parabola shaped walls. Also the pressure and temperature in the vicinity of the line of convergence, calculated by 2D hydrodynamic simulations coupled with equations of state of water and copper, are larger.

Generation of ultra-fast cumulative water jets by sub-microsecond underwater electrical explosion of conical wire arrays

The results of experiments with underwater electrical explosion of modified conical arrays of copper and aluminum wires are presented. A pulsed generator producing a 550 kA-amplitude current with a 400 ns rise time was used in the explosion of the arrays. The array explosion generates water flows converging at the axis of the cone. This flow generates a fast-moving water jet with a velocity exceeding 1.8 × 105 cm/s, which was observed being ejected from the surface of the water covering the array. The positions of the water jet were measured by multiple-exposure fast framing imaging. In experiments, the apex angle of the array, the thickness of the water layer above the arrays, or the material of the wires was altered, which changed the resulting velocities and shapes of the emitted jets. A model that considers the converging stationary flow of a slightly compressible fluid is suggested. The velocities and shapes of the jets obtained by this model agree well with the experimentally measured jet velocities.