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.
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.
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.
The study presented in this paper has shown that the generation of hard x rays and highenergy 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 estimates have been obtained for the kinetic energy of the ions generated by the Coulomb explosion.A Z pinch is an electrical discharge in plasma, which is compressed under the action of the magnetic pressure produced by the intrinsic discharge current [1][2][3][4][5][6][7]. Typical of Z pinches is the formation of hot spots due to the development of large-scale MHD instabilities [1,2,8-10].The discharge plasma column is deformed during compression, which is accompanied by the formation of necks smaller in radius than the main column. The magnetic pressure in the neck region increases, resulting, first, in faster compression and, second, in material outflow from the neck region in the axial direction. The final stage of the necking is a hot spot.Apparently, the axial jets were first detected in an experimental study [11] where the process of compression of deuterium pinches was investigated. As these jets were accounted for by an increase in neutron yield, the authors of Ref.[11] proposed a "noncylindrical Z pinch" in which the formation of the hot spot was predetermined by the geometry of the discharge.Subsequently, this configuration was called a plasma focus [12,13]. In subsequent experiments on a deuterium plasma focus at currents of about 1 MA, electron beams, hard x rays, and "epithermal" deuterons with energies up to 8 MeV were detected [14]. Even more energetic deuterons with energies of several tens of megaelectron-volts were detected in experiments with deuterium liners imploded at a current of 2.7 MA [15]. To explain the generation of high-energy ions and hard x rays, the so-called "target" mechanism was proposed [16]. It is assumed that in the final stage of formation of the hot spot, displacement currents occur that generate strong electric fields in which charged particles are accelerated.
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