G. V. Ivanenkov

Analysis of the discharge channel structure upon nanosecond electrical explosion of wires

The structure of the discharge channel during nanosecond wire explosions has been studied using laser probing. Wires of 25μm diameter and 12mm length were exploded in air and vacuum by 10kA current pulse having a 50A∕ns rate of rise. Upon electrical explosion of thin wires in the air, the development of shock waves was observed. The propagation of shock waves was analyzed, and it was possible to draw conclusions on the location of the flow of most of the current in the volume of the discharge channel. This permitted distinguishing between two scenarios (shunting and internal) of the interelectrode gap breakdown development. The scenario depends to a large extent on the properties of the exploding wire material. The same two scenarios are valid upon electrical explosion of wire in vacuum. Moreover, if secondary breakdown develops in the internal scenario, the value of the energy deposition in the wire material during explosion in vacuum may be comparable with that found during explosion in air.

X-pinch source of subnanosecond soft X-ray pulses based on small-sized low-inductance current generator

For the first time, the regime of a micrometer-size hot spot formation is impemented for an X-pinch in a plasma, which is fed from a current generator based on low-inductance capacitors and rapid current switches. The configurations of X-pinches, which can be used effectively as point sources of soft X-rays with this type of current generator, are determined. A prototype of a small-size radiation source for high-resolution point projection X-ray radiography has been constructed. The main parameters of X-pinch as a radiation source are analyzed and compared with X-pinch parameters in high-voltage setups with shaping lines. An analysis of the data on the operation of X-pinches in generators with different parameters has led to simple relations that can be used to select optimal initial X-pinch parameters.

Electric explosion of fine wires: Three groups of materials

Experimental data demonstrating differences in the structures of channels formed during nanosecond discharges through fine wires made of different materials are presented. In addition to the traditional two classes of metals and alloys (the copper and tungsten groups), a new class is proposed to which materials of the nickel type belong. Their properties combine the characteristic properties of the two traditional groups, due to which they occupy an intermediate position between the latter. This manifests itself in the unstable character of explosion, the type of which can change drastically when changing the ambient medium or other conditions. Most of the reported results were obtained at a small setup with maximum values of the current and voltage of 10 kA and 20 kV, respectively, the current rise time being about 300 ns. An attempt is made to construct a scenario of the development of a nanosecond explosion that would make it possible to qualitatively describe the formation of the discharge channel structure. The analysis is based on the recent experimental results indicating that the cores formed in the course of the discharge have a tubular structure.

Microexplosion of a hot point in an X-pinch constriction

The dynamics of an X-pinch in the diode of a high-power nanosecond current generator is studied experimentally and theoretically. The X-ray backlighting technique with subnanosecond time resolution and micron space resolution made it possible to trace both the formation of the constriction before the X-ray burst and the subsequent breaking and decay of the constriction. The radiative MHD model allowed simulation of the main characteristics of the process, including the formation of a minidiode and constriction, microexplosion of the hot point, and the generation of shock waves, followed by breaking of the constriction.

Dynamics of thin exploded-wire plasma with a cold dense core

Ideas are put forward regarding the possibility of a cold dense core, surrounded by a plasma corona, forming near the axis at the initial stage of a nanosecond electric explosion of metal wires, and the influence of such a radial structure on the plasma compression dynamics is discussed. Experimental evidence supporting these suppositions is presented. It includes both indirect confirmations, based on optical and x-ray diagnostics data, and direct observations of the core by new means of x-ray probing employing an X pinch as a source of radiation.

Time-resolved X-ray spectroscopy of hot spots in an X-pinch

The plasma parameters in hot spots of an X-pinch are determined by using time-resolved data from X-ray spectroscopy in experiments on the implosion of crossed Ti wires in the XP device with a current of 480 kA and pulse duration of 100 ns. The electron densities and temperatures calculated from these data are in the ranges (0.8–3)×1023cm–3 and 1–2.5 keV, respectively. An analysis performed shows that the plasma processes are highly nonequilibrium.

Dynamics of hybrid X-pinches

The dynamics of a new type of pinches—hybrid X-pinches (HXPs)—has been studied experimentally and numerically. The initial configuration of an HXP consists of a high-current diode with conical tungsten electrodes separated by a 1- to 3-mm-long gap and shunted with a 20- to 100-μm diameter wire. It was shown earlier that a hot spot (HS) with high plasma parameters also formed in the HXP, although its initial configuration is simpler than that of a standard X-pinch. Although details of the HXP dynamics still remain insufficiently studied, the main factors governing the HXP formation were investigated both experimentally and using magnetohydrodynamic simulations. The formation of a specific pressure profile in the electrode plasma after the wire explosion was investigated both experimentally and theoretically. It is shown that the effect of the pressure profile on the expanding wire plasma is similar for both standard X-pinches and HXPs, which allows one to assign them to the same class of loads of pulsed facilities. It is also established that the final stages of HS formation and the parameters of the HS plasma in standard X-pinches and HXPs are practically identical.

Formation, cascade development, and rupture of the X-pinch neck

Data from high-resolution x-ray shadow photography of an X-pinch in the diode of a high-power dense-plasma generator are presented. The processes leading to the formation of a minidiode, the compression of the neck arising in it, and the cutoff and subsequent emptying of the neck are studied. Cascade formation of short-lived structures, which consistently reproduce the form of the minidiode on small scales before the x-ray burst, is observed in the course of the implosion. The position of the x-ray emission points is determined.

On the phase state of the core matter in a high-power discharge through a wire

Experimental facts attesting to a two-phase (liquid-metal-vapor) state of the matter in the core produced in a high-power discharge through thin wires are presented. The modern technique of multiframe x-ray shadow photography has made it possible to observe vapor bubbles in the core and their interaction with the shock wave penetrating from the corona.

MHD processes during the cascade development of the neck and hot spot in an X-pinch

Results are presented from two-dimensional MHD simulations of X-pinch implosion. The simulations were performed in the (r, z) and (x, y) geometries for homogeneous (dense plasma) and heterogeneous (core-corona) loads. The formation of a minidiode, the development of a neck and an X-radiating hot spot, and the influence of the plasma corona on the implosion dynamics of the dense X-pinch plasma were investigated. For through simulations, the conical neck model was used, whereas a detailed analysis of the X-ray burst was performed in the parabolic neck model. The MHD processes occurring during the implosion of oblique shock waves and the onset of instability of the plasma column were examined. It is found that, due to the quasi-periodic character of these processes, the neck compression proceeds in a cascade fashion. The plasma state in a hot spot just before the break of the neck is analyzed, and the possibility of generating fast particle beams is considered.