A. G. Rousskikh

Study of the strata formation during the explosion of a wire in vacuum

The formation of strata during fast electrical explosions of aluminum wires at current densities of (1–1.4)×108 A/cm2 has been studied experimentally. To observe the strata, the soft x radiation generated at the hot point of an x-pinch was used. It has been revealed that strata are formed before the voltage collapse, that is, at the stage of heating of the wire metal. Two wire explosion modes were realized: with and without cutoff of the current carried by the exploding wire. Analysis of the experimental results shows that the stratification is most probably due to the thermal instability that develops as a consequence of the increase in metal resistivity with temperature.

Study of metal conductivity near the critical point using a microwire electrical explosion in water

Electrical explosion of aluminum and tungsten microwires in water was studied both experimentally and numerically. The experimental range of currents through the wire was 0.1–1 kA for explosion times of 40–300 ns and current densities up to 1.5×108 A/cm2. The experimental results were interpreted on the basis of magnetohydrodynamical simulation with various metal conductivity models. A comparison of the experimental and numerical results allows the conclusion to be drawn that the metal conductivity models used in this work are adequate.

The K-shell radiation of a double gas puff z-pinch with an axial magnetic field

A double shell z -pinch with an axial magnetic field is considered as a K -shell plasma radiation source. One-dimensional radiation-hydrodynamics calculations performed suggest that this scheme holds promise for the production of the K -shell radiation of krypton ( h ν ≈ 12–17 keV). As a first step in verifying the advantages of this scheme, experiments have been performed to optimize a neon double-shell gas puff with an axial magnetic yield for the K -shell yield and power. The experiments show that the application of an axial magnetic field makes it possible to increase the K -shell radiation power and reduce the shot-to-shot spread in the K -shell yield. Comparisons between the experiments and modeling are made and show good agreement.

Use of vacuum arc plasma guns for a metal puff Z-pinch system

The performance of a metal puff Z-pinch system has been studied experimentally. In this type of system, the initial cylindrical shell 4 cm in diameter was produced by ten plasma guns. Each gun initiates a vacuum arc operating between magnesium electrodes. The net current of the guns was 80 kA. The arc-produced plasma shell was compressed by using a 450-kA, 450-ns driver, and as a result, a plasma column 0.3 cm in diameter was formed. The electron temperature of the plasma reached 400 eV at an average ion concentration of 1.85 · 1018 cm−3. The power of the Mg K-line radiation emitted by the plasma for 15–30 ns was 300 MW/cm.

An experimental study of the effect of Rayleigh-Taylor instabilities on the energy deposition into the plasma of a Z pinch

The mechanism for the heating of the plasma of a Z pinch due to the generation of toroidal magnetic structures (magnetic bubbles) which are formed in the plasma as a result of the penetration of the azimuthal magnetic field into the gas puff plasma was investigated experimentally. The experiments were performed with single-shell and double-shell gas puffs (60/30 mm in diameter) on the IMRI-4 generator (I/sub max/=350 kA, T/4=1.1 /spl mu/s). The gases used for the gas puff material were neon, argon, and krypton. Electrical investigations have shown that the final resistance of the plasma depends on the linear mass of the gas puff and equals to /spl sim/0.06/spl divide/0.1 /spl Omega/, which coincides in the order of magnitude with the prediction of the theory of an enhanced energy deposition into the plasma of a Z pinch. Probing of the plasma was carried out with a YAG:Nd/sup 3+/ laser with a wavelength of 532 nm, a pulse energy of the order of 30 mJ, and a pulse full-width at half-maximum of /spl sim/5 ns. Polarimetry has shown that at the stage of stagnation of a Z pinch, there are regions inside the plasma column where the radial distribution of the electron density has a local minimum. The rotation of the polarization plane of the electromagnetic wave probing the plasma suggests that some portion of the azimuthal magnetic field of the Z pinch is captured by the current loop (a magnetic bubble is formed). The magnetic field inside the magnetic bubble is, on the average, 600/spl divide/800 kG and coincides in the order of magnitude with the magnetic field near the pinch boundary.

The Effects of Preheating of a Fine Tungsten Wire and the Polarity of a High-Voltage Electrode on the Energy Characteristics of an Electrically Exploded Wire in Vacuum

Results obtained from experimental and numerical studies of tungsten wires electrical explosion in vacuum are presented. The experiments were performed both with and without preheating of the wires using positive or negative polarity of a high-voltage electrode. Preheating is shown to increase energy deposition in the wire core due to a longer resistive heating stage. The effect was observed both in single wire and wire array experiments. The evolution of the phase state of the wire material during explosion was examined by means of a one-dimensional numerical simulation using a semiempirical wide-range equation of state describing the properties of tungsten, with allowance made for melting and vaporization

Electric explosion of fine tungsten wires in vacuum

A study is made of the breakdown of a fine wire during its electric explosion in vacuum. The problem of how the wire diameter, the rate of energy deposition in the wire, and the insulation of the electrode surface near the electrode-wire contact influence the wire explosion and the accompanying breakdown is investigated experimentally. The wire explosion was performed at a positive polarity of the high-voltage electrode. A current density growth rate of 6×1011–5×1016 A/(s cm2) is achieved. It is shown that the breakdown along a wire is similar in many respects to the gas breakdown. The insulation of the wire surface makes it possible to avoid breakdown and to increase the deposited energy to values sufficient for the wire sublimation.

Expansion of the plasma corona from a wire exploded in vacuum

An experiment was performed with the aim to determine the expansion velocity of the corona that is formed around a wire exploded in vacuum. The corona expansion velocity was found for Al and W wires as the wire current density was increased to 1×108–1.4×108 A/cm2. It was estimated by the time at which current started flowing through auxiliary electrodes separated from the wire axis by a certain distance. The measurements were performed with preliminary heated and unheated wires. It has been demonstrated that for unheated wires the expansion velocity of the plasma corona is determined by the expansion velocity of the desorbed gas and approximately equals (7±0.5)×106, (9±0.5)×106, and (1.1±0.6)×107 cm/s at a generator charge voltage of 10, 20, and 30 kV, respectively. For preliminary heated tungsten wires the metal vapor expansion velocity was (4.2±0.5)×106, (7±0.5)×106, and (9±0.6)×106 cm/s at a charge voltage of 10, 20, and 30 kV, respectively.

Investigation of the transport properties of metals in the biphase region

Results of experiments on electrical wire explosion are presented and processes of stratum formation and decay are analyzed in this paper. A procedure of calculating the transport coefficients from the rate of stratum damping is described. It is demonstrated that values of the transport coefficients for metals are not an unambiguous function of the material state in the biphase region for characteristic times of ∼10−7 s but depend on the process prehistory.

Strata formation at fast electrical explosion of cylindrical conductors

The process of electrical explosion of aluminum and tungsten wires at current densities above 108 A/cm2 (the fast electrical explosion regime) is investigated. Within the frame of 2D magnetohydrodynamic calculations based on the Particle-in-Cell technique with realistic equations of state of the metals, the processes of strata formation in the plasma are considered. In the fast electrical explosion regime, strata formation is shown to take place due to the overheat instability. The strata occurrence is caused by the character of the conductivity change near the critical point of the liquid-vapor phase transition, that is, by metal conductivity decrease with a temperature increase and a density decrease. To provoke strata formation, the energy deposited into the wire substance should be of about the sublimation energy.