A. Yu. Labetsky

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.

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

Electrical explosion of conductors in the high-pressure zone of a convergent shock wave

The effect of the environmental pressure on the electrical explosion of a conductor (fine tungsten wire of diameter 30 μm) in an insulating liquid (distilled water) is studied. The pressure in the water is produced by exploding a multiwire array with the test conductor on its axis. Along with the experiment, the magnetohydrodynamic simulation of the explosion is carried out. It is shown that a high pressure produced in the explosion zone retards the electrical explosion of the conductor and, consequently, increases the explosion energy.

On stabilization of gas puff implosion: experiment and simulation

A double gas puff was used to study the mitigation of the magneto-Rayleigh-Taylor (RT) instabilities for long implosion times (up to 250 ns). The experiments have been performed on the inductive storage GIT-4 (1.7 MA, 120 ns) generator. Current division between the outer and inner shells was controlled using magnetron-discharge preionization. The implosion of the a double gas puff, with the improved preionization, results in the formation of a uniform plasma column. The results of two-dimensional (2-D) radiation-magnetohydrodynamic simulations support the experimental results: a double gas puff implosion mitigates the RT instabilities, leading to the development of only small-amplitude waves. The 2-D simulation allowed us to explain the halo effect seen in the experiments: the use of the low hybrid conductivity in the calculation demonstrated the existence of the high density plasma core surrounded by a low density plasma halo.

Study of the current-sheath formation during the implosion of multishell gas puffs

Results are presented from experimental studies of the dynamics of large-diameter multishell gas puffs imploded by microsecond megampere current pulses. The experiments were conducted on the GIT-12 generator in the regime of microsecond implosion (timp = 1.1–1.2 μs, I0 = 3.4–3.7 MA). The influence of the load configuration on the dynamics of current losses and gas-puff radiative characteristics was studied. The correlation between the radial compression ratio (the ratio between the initial and final Z-pinch radii) and the magnitude of the current flowing at the plasma periphery was investigated. The experiments show that, in a multishell gas puff, large-scale instabilities insignificantly affect the gas-puff implosion even over microsecond time intervals and that a compact dense pinch with a relatively high average electron temperature (400–600 eV) forms at the Z-pinch axis. The diameter of the plasma column radiating in the K-shell lines of neon is about 3–4 mm, the K-shell radiation yield being 5–11 kJ/cm. In the final stage of implosion, only a small portion of the current flows through the high-temperature central region of the pinch plasma, whereas the major part of the generator current flows through the residual peripheral plasma.

Formation of tight plasma pinches and generation of high-power soft x-ray radiation pulses in fast Z-pinch implosions

Argon K-shell plasma radiation source experiments were carried out on the GIT-12 generator [Bugaev, S.P. et al., 1997, Russian Phys. Journal, 40, 38] for a long (300 ns) implosion regime. The performance of a shell-on-solid-fill double gas puff was characterized in the experiments with and without an external axial magnetic field. The maximum Ar K-shell radiation yield registered in the experiments without an axial magnetic field was at the level of 1100 J/cm. This yield is consistent with the theoretically predicted yield for a short (100 ns) implosion regime. The experiments showed that the initial magnetic field which provides stabilization of the shell-on-solid-fill double gas puff was lower than that required for stabilization of a single annular gas puff. Satisfactory stabilization of the double gas puff was observed at an initial axial magnetic field of 1.4 kG. The maximum Ar K-shell radiation yield registered in the experiments with the axial magnetic field did not exceed 400 J/cm. A sharp reduction of the K-shell yield cannot be explained only by taking into account the energy losses associated with the compression of the axial magnetic field.

Experimental study of an argon-hydrogen Z pinch plasma radiation source

Experiments with Ar-H/sub 2/ double gas puffs have been conducted on the GIT-12 generator at the current level of 2.1-2.4 MA and 250-350 ns implosion times. The argon-hydrogen mixture was used as a working medium in the outer shell of a double gas puff. The goal of the experiments was to verify whether the use of the argon-hydrogen mixture in the outer shell can improve the stability of a double gas puff implosion and provide increased argon K-shell radiation yields. The experiments showed that hydrogen admixture results in the change of implosion dynamics: decreased implosion velocities and compression ratios. The experimental data does not allow a conclusion that the use of an Ar-H/sub 2/ mixture in the outer shell of a double gas puff significantly improves the implosion stability. An increase in the hydrogen percentage in the outer shell leads to a decrease in plasma density and temperature, and as a result, reduced K-shell radiation yields and powers.

Investigation of the mechanism for current switching from the outer gas puff to the inner wire array in a structured Z-pinch

Experiments are reported on the implosion of structured loads with outer argon, krypton, and xenon gas puffs and an inner tungsten multiwire array. Experiments were carried out in the GIT-12 generator with a current of 2.6 MA and a current rise time of 270 ns. It is shown that the current successfully switches to the wire array only when the gas puff is sufficiently light. The total implosion time is 300 ns, and the implosion time of a wire array, determined from streak camera images, is 50–70 ns. It is suggested that the switching is efficient only when the active impedance of the gas puff is higher than the transitional resistance of the electrically exploded wires.

Experimental verification of the effect of different preionization systems on the implosion dynamics of an argon double gas puff

Experiments are reported on the implosion of argon double gas puffs in the GIT-12 current generator (Tfr=0.25 µs, Im=2.3 MA). The gas-puff medium was preionized by different methods. The experimental data provide evidence for a strong effect of the initial conditions for the formation of the current-carrying shell on the implosion process. Emphasis is given to a discussion of the following issues: the enhanced scatter in both the emission power and X-ray yield in the Ar K-lines, the existence of a large number of current filaments, the uncertainty in the process by which the generator current is redistributed among a progressively smaller number of current filaments, and the redistribution of the generator current between the inner and outer gas-puff shells.