S. Gleizer

Characterization of converging shock waves generated by underwater electrical wire array explosion

Results of ∼200 kbar pressure generation at 50 μm distance from the implosion axis of the converging shock wave produced by an underwater electrical explosion of a cylindrical wire array are reported. The array was exploded using a submicrosecond high-current generator (stored energy of ∼4.2 kJ, current amplitude of ∼325 kA, rise time of ∼1 μs). Multiframe shadow imaging of the shock wave was used to determine its time of flight. These data were applied for calculating the pressure at the vicinity of the implosion axis using one dimensional hydrodynamic calculations and the Whitham approach. However, it was found that in the case of wire array radius ≤5 mm, multiframe imaging cannot be used at the final stage of the shock wave implosion because of possible changes in the optical properties of the water. Optical and spectroscopic methods based on either the change in the refraction index of the optical fiber or spectroscopy of the plasma formed inside the capillary placed at the implosion axes were used fo...

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.

Underwater electrical wire explosion

The results of the investigation of the underwater electrical wire explosions using a high power sub-ns generator are reported. The spectroscopic analysis of the emitted radiation has unveiled no evidence for the formation of shunting plasma channel. The latter appears in vacuum and gas wire explosions and causes to the seizure of energy deposition into an exploding wire material. The combination of mechanism for the suppression of formation of shunting channel together with the increased energy deposition rate allows busting the efficiency of the energy deposition into the exploding wire. Estimated energy deposition into Cu and Al wire of up to 200 eV/atom was reported.

Extreme water state produced by underwater wire-array electrical explosion

The generation of an extreme water state (130 GPa, 5000 K, and 3.4 g/cm3) which is characterized as dense plasma at the axis of a converging shock wave is reported. A 4 kJ pulse generator was used to explode a 40 Cu-wire array, generating a cylindrical shock wave. The measured shock wave trajectory and energy deposited into the water flow were used in hydrodynamic simulations coupled with the equation of state to determine the water parameters. The temperature estimated using the emission data of water in the vicinity of the implosion axis agrees with the simulation results, indicating shock wave symmetry in such extreme conditions.

Experimental research of different plasma cathodes for generation of high-current electron beams

The results of experimental studies of different types of cathodes—carbon-epoxy rods, carbon-epoxy capillary, edged graphite, and metal-dielectric—under the application of high-voltage pulses with an amplitude of several hundreds of kV and pulse duration of several nanoseconds are presented. The best diode performance was achieved with the edged graphite and carbon-epoxy-based cathodes characterized by uniform and fast (<1 ns) formation of explosive emission plasma spots and quasi-constant diode impedance. This result was achieved for both annular cathodes in a strong magnetic field and planar cathodes of a similar diameter (∼2 cm) with no external magnetic field. The cathodes based on carbon-epoxy rods and carbon-epoxy capillaries operating with an average current density up to 1 kA/cm2 showed insignificant erosion along 106 pulses of the generator and the generated electron beam current showed excellent reproducibility in terms of the amplitude and waveform.

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.

Electron beam and plasma modes of a channel spark discharge operation

Parameters of a modified pulsed channel spark discharge (CSD), operating at a repetition rate up to 100 Hz at Ar gas pressures of 10−3 and 10−4 Torr and of the generated electron beam, were studied using different electrical, optical, and x-ray diagnostics. It was shown that efficient (up to ∼74%) transfer of the initially stored energy to the energetic electron beam is realized only at the pressure of 10−4 Torr. Conversely, at the pressure of 10−3 Torr, less than 10% of the stored energy is acquired by the energetic electrons. It was found that the energetic electron beam generation is limited by the expansion of the cathode and anode plasmas and by the formation of plasma inside the gap between the CSD capillary output and the anode. It was also found that the plasma, which acquires the hollow cathode potential, is already formed at the beginning of the CSD operation inside the capillary, and the electron emission occurs from the capillary output plasma boundary. Finally, it was shown that the electron ...

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.

Characterization of Deposited Films and the Electron Beam Generated in the Pulsed Plasma Deposition Gun

The channel spark discharge was used as a high-current density (up to 30 kA/cm2) relatively low-energy (<20 keV) electron beam source in a pulsed plasma deposition (PPD) gun. The PPD gun was used for the deposition of thin films by pulsed ablation of different target materials, at a background gas pressure in the 10-3–10-5 Torr range. The parameters of the electron beam generated in the modified PPD gun were studied using electrical, optical, and X-ray diagnostics. It was found that a higher background pressure stimulates a denser plasma formation between the gun output and the target, that restricts the energy delivery to the beam electrons. Namely, the efficient (up to ~74%) transfer of the initially stored energy to the electron beam is realized at the background gas pressure of 10-4 Torr. Conversely, at a pressure of 10-3 Torr, only ≤10% of the stored energy is acquired by the energetic electrons. It was shown that the modified PPD gun, owing to the extremely high energy density delivered by the electrons to the target, may be applied for the deposition of a wide variety of different insulators, semiconductors, and metals. A selection of materials such as diamond-like carbon (DLC), cadmium telluride (CdTe), cadmium sulphide (CdS), zinc oxide (ZnO), tungsten, and tungsten carbide (WC) have been deposited as thin films and the properties and deposition rates of the deposited thin films are discussed.

Generation of highly symmetric, cylindrically convergent shockwaves in water

We report on pulsed power driven, exploding copper wire array experiments conducted to generate cylindrical convergent shockwaves in water employing μs risetime currents >550 kA in amplitude and with stored energies of >15 kJ—a substantial increase over previous results. The experiments were carried out on the recently constructed Mega-Ampere-Compression-and-Hydrodynamics facility at Imperial College London in collaboration with colleagues of Technion, Israel. 10 mm diameter arrays consisting of 60 × 130 μm wires were utilized, and the current and voltage diagnostics of the load region suggested that ∼8 kJ of energy was deposited in the wires (and the load region close to the wires) during the experiments, resulting in the formation of dense, highly resistive plasmas that rapidly expanded driving the shockwaves in water. Laser-backlit framing images of the shockfront were obtained at radii <0.25 mm for the first time, and there was strong evidence that even at radii <0.1 mm this front remains stable, resu...