S. Efimov

Nanosecond time scale, high power electrical wire explosion in water

Experimental and magnetohydrodynamic simulation results of nanosecond time scale underwater electrical explosions of Al, Cu, and W wires are presented. A water forming line generator with current amplitude up to 100kA was used. The maximum current rise rate and maximum Joule heating power achieved during wire explosions were dI∕dt⩽500A∕ns and 6GW, respectively. Extremely high energy deposition of up to 60 times the atomization enthalpy was registered compared to the best reported result of 20 times the atomization enthalpy for energy deposition with a vacuum wire explosion. Discharge channel evolution and surface temperature were analyzed by streak shadow imaging and by a fast photodiode with a set of interference filters, respectively. A 1D magnetohydrodynamic simulation demonstrated good agreement with experimental parameters such as discharge channel current, voltage, radius, and temperature. Material conductivity was calculated to produce the best correlation between the simulated and experimentally o...

Underwater electrical explosion of a Cu wire

Results and analysis of a microsecond time scale underwater electrical wire explosion are presented. Experiments were carried out with a Cu wire exploded by a current pulse ⩽100kA with microsecond time duration. The analysis is based on shadow and spectrally resolved streak photography which were used to monitor the evolution of the discharge channel and the shock wave. The obtained data were used for hydrodynamic calculation of the generated water flow parameters, such as pressure and flow velocity distribution between the discharge channel and the shock wave. In particular, the pressure at the discharge channel boundary and the energy transferred to the water were estimated. The results of the calculation have been verified by comparing the measured and calculated trajectories of the shock wave. Based on the results of the calculation the energy transferred to the water was estimated. In addition, the analysis shows that the energy initially deposited in the discharge channel continues to produce mechan...

Characterization of the plasma on dielectric fiber (velvet) cathodes

An investigation of the properties of the plasma and the electron beam produced by velvet cathodes in a diode powered by a ∼200kV, ∼300ns pulse is presented. Spectroscopic measurements demonstrated that the source of the electrons is surface plasma with electron density and temperature of ∼4×1014cm−3 and ∼7eV, respectively, for an electron current density of ∼50A∕cm2. At the beginning of the accelerating pulse, the plasma expands at a velocity of ∼106cm∕s towards the anode for a few millimeters, where its stoppage occurs. It was shown by optical and x-ray diagnostics that in spite of the individual character and nonuniform cross-sectional distribution of the cathode plasma sources, the uniformity of the extracted electron beam is satisfactory. A mechanism controlling the electron current-density cross-sectional uniformity is suggested. This mechanism is based on a fast radial plasma expansion towards the center due to a magnetic-field radial gradient. Finally, it was shown that the interaction of the elec...

Efficiency of the shock wave generation caused by underwater electrical wire explosion

Shearing interferometry, together with shadowgraph and Schlieren photography techniques, has been applied for the visualization of the cylindrical water flow behind the shock wave generated by high-power 6 GW nanosecond time-scale underwater electrical discharge. The flow was visualized during the first microsecond of the wire explosion process in the region between the expanding exploding wire discharge channel and the shock wave. The optical methods, combined with the hydrodynamic calculation, enable an accurate estimation of the energy transferred from the discharge to the water flow. The estimated efficiency of the transformation of the electrical dissipated energy to the mechanical energy of the generated compressed water flow is ∼15%.

Analysis of shock wave measurements in water by a piezoelectric pressure probe

Investigation of underwater electrical wire explosions occurring in the time scale of few microseconds requires a measurement of pressure waves with nanosecond rise time and microsecond fall time. Various types of pressure gauges are used for this purpose, however, none of them seems to be suitable for the task since the frequency range of the pressure waves lies between 107 and 109 Hz, whereas all types of mechanical gauges have a bandwidth below 107 Hz. Therefore, a mathematical processing of measurements is required for reconstruction of the actual pressure wave forms. In this article, a signal processing algorithm, based on energy conservation requirements and Fourier analysis, for reconstruction of the wave form of the pressure wave generated under water by electrical explosion of wires is proposed. The gauge used in the experiments is a PCB 119A12 type pressure gauge with a bandwidth below 1 MHz produced by Piezo-Electronics, Inc. Pressure waves were produced by underwater electrical explosion of a ...

Addressing the efficiency of the energy transfer to the water flow by underwater electrical wire explosion

Experimental and hydrodynamic simulation results of submicrosecond time scale underwater electrical explosions of planar Cu and Al wire arrays are presented. A pulsed low-inductance generator having a current amplitude of up to 380 kA was used. The maximum current rise rate and maximum power achieved during wire array explosions were dI/dt≤830 A/ns and ∼10 GW, respectively. Interaction of the water flow generated during wire array explosion with the target was used to estimate the efficiency of the transfer of the energy initially stored in the generator energy to the water flow. It was shown that efficiency is in the range of 18%–24%. In addition, it was revealed that electrical explosion of the Al wire array allows almost double the energy to be transferred to the water flow due to efficient combustion of the Al wires. The latter allows one to expect a significant increase in the pressure at the front of converging strong shock waves in the case of cylindrical Al wire array underwater explosion.

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...

Spectroscopic research of underwater electrical wire explosion

Results of spectroscopic research in the visible range of light of the radiation generated by underwater electrical wire explosions (UEWE) are presented. A pulsed generator with an output voltage of ∼110kV, current of ∼70kA, and rise time of ∼60ns was used for electrical explosion of Cu wires 0.1mm in diameter and 50mm in length. It was shown that UEWE is not governed by the “polarity” effect, which plays an important role in electrical wire explosions in vacuum. The results of detailed space- and time-resolved spectroscopic measurements show that the radiation spectrum differs significantly of the spectrum expected from the exploding wire. A model is suggested based on the formation of a few μm “water” plasma layer in the vicinity of the exploding wire plasma which efficiently absorbs the radiation of the exploding wire.

Generation of converging strong shock wave formed by microsecond timescale underwater electrical explosion of spherical wire array

A study of generation of converging strong shock wave using microsecond underwater electrical explosion of spherical Cu-wire array is presented. Hydrodynamic simulations coupled with the equation of state for Cu and water, deposited energy, and the magnetic pressure were used to calculate the water parameters in the vicinity of the implosion origin. The results of simulations agree with the shock wave time-of-flight and energy delivered to the water flow and show that in the vicinity (diameter of ∼12 μm) of an implosion one can expect water pressure of ∼6 TPa, temperature of ∼17 eV, and compression of ∼8.

Generation of a 400 GPa pressure in water using converging strong shock waves

Results related to the generation of an extreme state of water with pressure up to (4.3 ± 0.2)·1011 Pa, density up to 4.2 ± 0.1 g/cm3, and temperature up to 2.2 ± 0.1 eV in the vicinity of the implosion axis of a converging strong shock wave are reported. The shock wave was produced by the underwater electrical explosion of a cylindrical Cu wire array. A ∼8 kJ pulse generator with a current amplitude ≤550 kA and rise time of 350 ns was used to explode arrays having varying lengths, radii, and number of wires. Hydrodynamic numerical simulations coupled to the experimental data of the shock wave propagation in water, rate of energy deposition into the array, and light emission from the compressed water in the vicinity of the implosion axis were used to determine the pressure, density, and temperature profiles during the implosion. Results of a comparison between these parameters obtained with the SESAME and quantum molecular dynamics data bases of equation of state for water are reported as well. Also, the ...