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 (t _imp = 1.1–1.2 μs, I ₀ = 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.     

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.     

Distribution of leakage currents in the cylindrical and conical sections of the magnetically insulated transmission line of the Angara-5-1 facility in experiments with wire arrays

Current leakages in the magnetically insulated transmission lines (MITL) impose restrictions on the transmission of electromagnetic pulses to the load in high-power electrophysical facilities. The multimodule Angara-5-1 facility with an output electric power of up to 6 TW is considered. In this work, the experimental and calculated profiles of leakage currents in two sections of the line are compared when the eight-module facility is loaded by a wire array. The azimuthal distribution of the current in the cylindrical section of the MITL is also considered.     

Propagation of current pulses with an amplitude of up to 85 kA in soil over distances of several tens of meters

Conditions for the propagation in soil of current pulses with an amplitude of up to 85 kA and temporal characteristics typical of a lightning stroke are studied with the help of a specially designed mobile test complex on the basis of a 4-MJ capacitive energy storage with an output voltage of up to 2 MV. In contrast to the conventional opinion that the ionization processes in highly conductive soils are weakly pronounced, a dramatic reduction in the grounding resistance at a resistivity of about 100 Ω m and currents above 10 kA was observed. A time interval in which the grounding resistance is determined by the skin effect in soil is revealed. It is shown that the grounding resistance continues to decrease behind the front of the current pulse due to the continuous growth of spark channels in soil. Time variations in the grounding resistance cannot be related to the formation of a continuous ionization zone near the grounding electrodes and are explained only by the simultaneous growth of several long spark channels extending from the grounding device.     

Comparison of the measured and calculated time profiles of the leakage current in the magnetically insulated transmission line of the angara-5-1 facility

One of the factors limiting the transmission of the electromagnetic pulse to the load in high-power electrophysical facilities is the current leakage in magnetically insulated transmission lines (MITLs). In this paper, the Angara-5-1 eight-module facility with an output power up to 6 TW is considered. The experimental and calculated time profiles of the leakage current for eight-module shots with a dynamic load (cylindrical arrays made of 40 tungsten wires) and single-module shots with a solid cylindrical metal load are compared. When interpreting the results, the contribution of vacuum electrons to the leakage current at the transition from the cylindrical to the conical section of the MITL is taken into account.     

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.     

Electrophoretic mobility of bimolecular lipid membranes

The floating membrane vesicle is fixed by the counter solution flow in different points along the radius of a cylinder electrophoretic chamber, which permits to measure the vesicle electrophoretic mobility (EM). Close state condition of the chamber is provided for by the capillary system successively connected with the chamber. Relationship between EM of bimolecular lipid membranes (BLM) and pH and ionic concentration of aqueous solution qualitatively coincides with similar relationship for liposomes. The EM value of BLM essentially decreases in solution containing polyene antibiotics nystatine and levorin when derivative of cholesterol having 3betaOH-groups is present in the membrane.     

Effect of the high-voltage electrode polarity and wire preheating on the energy characteristics of electric explosion of fine tungsten wires in vacuum

Results are presented from experiments on the explosion of 30.5-μm tungsten wires at a current density of up to 140 MA/cm² and resistive-heating time of 40–100 ns. The experiments were performed both with and without preheating of wires and at different polarities of the high-voltage electrode. The effect of plasma production at the electrodes on the initiation of breakdown along the exploding wire was investigated by using a frame camera. It is shown that, when the polarity of the high-voltage electrode is positive, breakdown begins with the formation of a bright spot on the wire surface near the cathode, whereas at the negative polarity, breakdown begins with the formation of bright spots on the cathode surface. A comparative analysis of the main characteristics of wire explosions is performed. It is shown that preheating of the conductor increases the resistive-heating time and, accordingly, the energy deposited in the wire core. This effect takes place during explosions of both single wires and wire arrays. The evolution of the state of a metal during the explosion (including melting and evaporation) is studied by one-dimensional simulations by using a semiempirical equation of state describing the properties of tungsten over a wide range of parameters.     

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 (T _fr=0.25 μs, I _m =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. __________ Translated from Fizika Plazmy, Vol. 27, No. 7, 2001, pp. 584–591. Original Russian Text Copyright ? 2001 by Rousskikh, Baksht, Labetsky, Shishlov, Fedyunin.     

Simulations of the Evolution of the Heterophase Electrode of a Vacuum Transmission Line During the Passage of a High Current Pulse

Plasma Physics Reports - In electromagnetic generators of the terawatt range with current pulse of 1–30 MA, electric explosion of the surface of current-carrying electrodes takes place. One...