N. A. Labetskaya

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.

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.

Skin explosion of double-layer conductors in fast-rising high magnetic fields

An experiment has been performed to study the electrical explosion of thick cylindrical conductors using the MIG pulsed power generator capable of producing a peak current of 2.5 MA within 100 ns rise time. The experimental goal was to compare the skin explosion of a solid conductor with that of a double-layer conductor whose outer layer had a lower conductivity than the inner one. It has been shown that in magnetic fields of peak induction up to 300 T and average induction rise rate 3 × 109 T/s, the double-layer structure of a conductor makes it possible to achieve higher magnetic induction at the conductor surface before it explodes. This can be accounted for, in particular, by the reduction of the ratio of the Joule heat density to the energy density of the magnetic field at the surface of a double-layer conductor due to redistribution of the current density over the conductor cross section.

Multichannel vacuum arc discharge used for Z-pinch formation

Results are presented from experimental studies of the implosion dynamics and radiative characteristics of an aluminum Z-pinch formed from a plasma shell (PS). The PS with an initial diameter of 4 cm was produced with the help of a multichannel vacuum arc discharge and formed due to the evaporation of the electrode material in ten parallel arc discharges. The PS composition depended on the electrode material in the arc discharge. The described experiments were performed with aluminum electrodes. The total arc current was 80 kA. The PS implosion was provided by an IMRI-5 high-current generator with a current amplitude of 450 kA and rise time of 500 ns. The PS implosion resulted in the formation of a 0.2-cm-diameter plasma column with an electron temperature of 700–900 eV and average ion density of (5–8) × 1017 cm−3. The maximum radiation power per unit length in aluminum K-lines reached 300 MW/cm, the duration of the radiation pulse being 20 ns.

Experimental study of the nonlinear diffusion of a magnetic field and skin explosion of cylindrical conductors

The paper presents the results of an experimental study of the skin explosion of cylindrical conductors of diameter 1–3 mm (copper, aluminum, titanium, steel 3, and stainless steel) at a peak magnetic field of 200–600 T. The experiments were carried out on the MIG pulsed power generator at a current of up to 2.5 MA and a current rise time of 100 ns. The surface explosion of a conductor was identified by the appearance of a flash of extreme ultraviolet radiation. A minimum magnetic induction has been determined below which no plasma is generated at the conductor surface. For copper, aluminum, steel 3, titanium, and stainless steel, the minimum magnetic induction has been estimated to be (to within 10%) 375, 270, 280, 220, and 245 T, respectively.

MHD instabilities developing in a conductor exploding in the skin effect mode

The results of experiments with exploding copper conductors, performed on the MIG facility (providing currents of amplitude of about 2.5 MA and rise time of 100 ns), are analyzed. With an frame optical camera, large-scale instabilities of wavelength 0.2–0.5 mm were detected on the conductor surface. The instabilities show up as plasma “tongues” expanding with a sound velocity in the opposite direction to the magnetic field gradient. Analysis performed using a two-dimensional MHD code has shown that the structures observed in the experiments were formed most probably due to flute instabilities. The growth of flute instabilities is predetermined by the development of thermal instabilities near the conductor surface. The thermal instabilities arise behind the front of the nonlinear magnetic diffusion wave propagating through the conductor. The wavefront on its own is not subject to thermal instabilities.

X-pinch dynamics: Neck formation and implosion

We propose a model that describes the neck formation and implosion in an X-pinch. The process is simulated to go in two stages. The first stage is neck formation. This stage begins with an electrical explosion of the wires forming the X-pinch, and at the end of the stage, a micropinch (neck) is formed in the region where the wires are crossed. The second stage is neck implosion. The implosion is accompanied by outflow of matter from the neck region, resulting in the formation of a “hot spot”. Analytical estimates obtained in the study under consideration indicate that these stages are approximately equal in duration. Having analyzed the neck implosion dynamics, we have verified a scaling which makes it possible to explain the observed dependences of the time of occurrence of an x-ray pulse on the X-pinch current and mass.

Suppression of Rayleigh-Taylor instabilities in Z-pinches

Experiments on studying the stability of Z-pinch compression were carried out at a current of 450 kA with a build-up time of 450 ns. The plasma shell of the pinches was formed by evaporating the electrode material in the process of vacuum arc burning. The Rayleigh–Taylor (RT) instabilities were suppressed using the regime of arc combustion on the surface of one of the electrodes in the high-voltage gap in which the pinch was positioned. As a result of free plasma discharge, the radial density distribution was formed such that the plasma concentration increased from the outer boundary to the shell axis. The experiments demonstrated that such an initial radial density distribution almost completely suppresses of the RT instability.

Influence of surface finish on the plasma formation at the skin explosion

The paper reports on experiments to investigate how the quality of surface finish, i.e., surface roughness, influences the plasma formation in a skin explosion of conductors. The experiments were performed on a MIG terawatt generator with a current amplitude of up to 2.5 MA and current rise time of 100 ns. The plasma formation at the conductor surface and the evolution of the plasma boundary was recorded using a four-frame optical camera with an exposure time of 3 ns per frame. It is shown that the quality of surface finish little affects the onset of plasma formation in a skin explosion of stainless steel and St3 steel conductors at a magnetic field of up to 400 T.

Metal-puff Z-pinch implosions on generator MIG with current level up to 2.3 MA

Summary form only given. Two experimental test sets on the compression of metal-puff Z-pinches [1-3] were carried out on generator MIG at current level up to IMIG ~ 2.3 MA and rise time trt ~ 80 ns. The first set has been devoted to the study of the Al metal-puff Z-pinches. The second set was devoted to the study of the Bi metal-puff Z-pinches. The main objectives of these experiments are: 1) to investigate an implosion dynamic of metal-puff Z-pinches and 2) to optimize the plasma shell formation assembles from the point of view a radiation yield. Namely, we varied the amplitude and rise time of the plasma gun current Igun, a plasma density distribution inside the plasma shell, and the initial diameter of the plasma shell. To measure the K-shell Al radiation power, a pinhole camera and Diamond Radiation Detector (DRD) filtered with 18-μm thick polypropylene film were used. We recorded also the radiation energy (nickel bolometer) and the radiation power (XRD with bare Al and Cu photocathode). The metal puffs were produced by plasma guns powered by a capacitive power supply with quarter period Tgun/4 = 6.6 and 9.3 μs. The amplitude of current Igun was 60 and 70 kA correspondingly. The time tdel between the operation of the plasma guns and the operation of the MIG generator was controlled by using a delay generator; tdel = 4.7 - 10.5 μs. Based on the analysis of the results and comparison of experimental data with simulation the recommendations on metal-puff Z-pinches optimization were given.