A. P. Artyomov

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.

Coulomb explosion of “hot spot”

The study presented in this paper has shown that the generation of hard x rays and high-energy ions, which are detected in pinch implosion experiments, may be associated with the Coulomb explosion of the hot spot that is formed due to the outflow of the material from the pinch cross point. During the process of material outflow, the temperature of the hot spot plasma increases, and conditions arise for the plasma electrons to become continuously accelerated. The runaway of electrons from the hot spot region results in the buildup of positive space charge in this region followed by a Coulomb explosion. The conditions for the hot spot plasma electrons to become continuously accelerated have been revealed, and the estimates have been obtained for the kinetic energy of the ions generated by the Coulomb explosion.

Temporal response of silicon EUV and soft X-ray detectors

A small-sized generator of picosecond electron beams is used to measure the temporal resolution of EUV and soft X-ray silicon detectors produced by the Ioffe Physical Technical Institute, Russian Academy of Sciences. The temporal resolution of the EUV and soft X-ray detectors based on silicon photodiodes is shown to be ∼1 ns. Preliminary experiments have been performed using these detectors with the aim of investigating the radiation characteristics of a soft X-ray source based on an X-pinch driven by a small-sized highcurrent generator with a current pulse amplitude of 250 kA. This source is used at the Institute of High Current Electronics for soft X-ray backlighting diagnostics of various plasmas.

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.

Simulation of the radiation from the hot spot of an X-pinch

The results of X-pinch experiments performed using a small-sized pulse generator are analyzed. The generator, capable of producing a 200-kA, 180-ns current, was loaded with an X-pinch made of four 35-μm-diameter aluminum wires. The analysis consists of a one-dimensional radiation magnetohydrodynamic simulation of the formation of a hot spot in an X-pinch, taking into account the outflow of material from the neck region. The radiation loss and the ion species composition of the pinch plasma are calculated based on a stationary collisional-radiative model, including balance equations for the populations of individual levels. With this model, good agreement between simulation predictions and experimental data has been achieved: the experimental and the calculated radiation power and pulse duration differ by no more than twofold. It has been shown that the x-ray pulse is formed in the radiative collapse region, near its boundary.

Capacitive energy stores with nanosecond energy transfer

The paper reports on recent developments in the field of design, manufacturing, and testing of compact capacitive energy stores with an energy transfer time of 100 ns, namely high-voltage pulse capacitors, capacitor-switch assemblies, and artificial forming lines, at the High Energy Density Department of the Institute of High Current Electronics. A characteristic feature of the developments is the possibility to create variously shaped capacitive stores with an energy density of up to 350 J/dm3, low inductance of 10-20 nH, and capability of withstanding pulse currents of up to 100 kA. The devices find application in drivers of hard1 and soft2,3 X-ray sources and can be used in pulse power supply systems of different accelerators.

X-pinch soft x-ray source dynamics at a subnanosecond time resolution

The paper reports on an experimental study of the X-pinch soft x-ray source dynamics at a subnanosecond time resolution with the use of an x-ray imaging technique based on an AXIS-NX streak camera. The study was performed on a compact generator with a current amplitude of 300 kA to a short-circuit load and current rise time of 180 ns. It is shown that in the spectral range 1-1.55 keV, the X-pinch soft x-ray source in whole represents a set of sources which can be radially offset by ~ 10 microns about the X-pinch axis. Each of the sources generates a pulse of duration 0.2-0.7 ns. The interval between the formation of the sources and hence between their radiation pulses is 0.5 ns and longer.