A. S. Zhigalin

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.

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.

Metastable states and their disintegration at pulse liquid heating and electrical explosion of conductors

The regularities of formation of metastable states and their disintegration under pulse liquid heating and electrical heating and explosion of conductors are studied. With a high energy flux density, the phase transitions occur with a high intensity of heat and mass fluxes, leading to spontaneous generation of a new phase and to phase explosion. The basic features of bubble-like disintegration in not uniformly superheated water and alcohol layers on the microheater are found. Regularities of matter disintegration with electrically exploded conductors are obtained. The metastable liquid disintegration is experimentally investigated for characteristic times of matter transfer to a metastable state of 1 to 4 µs; phase transitions during electric conductor explosion are studied at characteristic times of transfer to a metastable state to 200 ns. A common approach to describing the effects with radically different characteristic times of transfer of the matter to a metastable state is developed.

Stratification in Al and Cu foils exploded in vacuum

An experiment with exploding foils was carried out at a current density of 0.7 × 108 A/cm2 through the foil with a current density rise rate of about 1015 A/cm2 s. To record the strata arising during the foil explosions, a two-frame radiographic system was used that allowed tracing the dynamics of strata formation within one shot. The original striation wavelength was 20–26 μm. It was observed that as the energy deposition to a foil stopped, the striation wavelength increased at a rate of ∼(5–9) × 103 cm/s. It is supposed that the most probable reason for the stratification is the thermal instability that develops due to an increase in the resistivity of the metal with temperature.

Study of the stability of Z-pinch implosions with different initial density profiles

Stability of metal-puff Z pinches was studied experimentally. Experiments were carried out on a facility producing a load current up to 450 kA with a rise time of 450 ns. In a metal-puff Z pinch, the plasma shell is produced due to evaporation of the electrode material during the operation of a vacuum arc. In the experiment to be reported, a single-shell and a shell-on-jet pinch load with magnesium electrodes were used. Two-dimensional, 3 ns gated, visible-light images were taken at different times during the implosion. When the shell was formed from a collimated plasma flow with small radial divergence, Rayleigh–Taylor (RT) instability typical of gas-puff implosions was recorded. The RT instability was completely suppressed in a mode where the initial density distribution of the shell approached a tailored density profile [A. L. Velikovich et al., Phys. Rev. Lett. 77, 853 (1996)].

A double-frame nanosecond soft X-ray backlighting system based on X-pinches

The operation of a double-frame soft X-ray backlighting system on the basis of two table-top pulsed power generators and X-pinches is described. The system allows one to obtain two backlighting frames of a fast process with a nanosecond exposure (2–3 ns), a micrometer spatial resolution, and an adjustable delay between the frames. Backlighting registration of electrically exploding single aluminum wires was performed in the spectral range hν > 0.8 keV of the backlighting source using the double-frame system. The jitter time between X-ray pulses was within ±18 ns.

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.

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.

A synchronized X-pinch driver

The efficiency of the X-ray point-projection radiography technique has been demonstrated on a terawatt pulse power generator using a detached compact current generator driving an X-pinch load. This technique has been approved in an experiment on the multiwire-array implosion performed at the Angara-5-1 generator with a peak power as high as 6 TW. The advantage of this experiment over earlier experiments on terawatt generators is in the use of a separate X-pinch driver, which makes it possible to arbitrarily vary the sample probing time. The X-pinch driver is connected to the load unit by means of a flexible low-inductance transmission line. The flexibility of the transmission line is an additional advantage of this technique, since it allows the accuracy of the X-ray radiography system adjustment to be improved and the X-pinch to be located near the plasma load. When compared to the laser method for producing a probe radiation source, the proposed technique features a smaller size, a lower cost of the facility, the absence of high-price optical elements, and a higher efficiency of X-ray generation. Owing to the small size of the synchronized X-pinch driver, it can be transported for use in experiments performed at other research organizations.

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.