This paper describes a theoretical and experimental study of the implosion of heavy copper liners shaped as hollow cylindrical tubes having an outer diameter of 3 and 4 mm and a wall thickness of 500 mu m; the tube linear mass was 0.35 and 0.5 g/cm, respectively. The experiment was carried out on the GIT-12 pulsed-power generator (5 MA, 2 mu s). Under these experimental conditions, a skin effect occurred in an imploding tube. The implosion process was numerically simulated based on a radiative magnetohydrodynamic model. Both the experiment and the simulation have shown a fluctuating voltage across the tube. According to the simulation, the first fluctuation peak, followed by a sharp decrease in voltage, is associated with the "collapse" of the tube on the axis and the formation of a strong shock wave. The times at which first voltage peaks were detected in the experiment and the first peak occurrence times obtained in the simulation coincided to within 5-10%, and the experimentally obtained and the calculated voltage amplitudes differed by about 20-30%. Thus, the results of the experiment suggest that using oscilloscopic measurements of the voltage across a heavy metal tube, it is possible to detect the shock wave generated in the conductive material of the tube and to determine the collapse time.

Conditions for the generation of runaway electrons (RAEs) in a magnetically insulated coaxial air diode with graphite cathodes of different geometries - needle and conical with a Taylor opening half angle of 49.3 degrees - are compared. The axial magnetic field allows the RAE beam to be focused on a current probe collector, thereby increasing the sensitivity of the recording technique in use. The threshold RAE generation voltage for the Taylor cone is found to be lower than that for the needle (i.e., a cone with a small opening angle), which indicates its nonmonotonic angular dependence with a minimum at an angle not exceeding the Taylor angle. According to our estimates, the dynamics of free electrons change qualitatively at the Taylor angle. At large angles, they accelerate throughout the entire gap; at smaller angles, they accelerate near the cathode and then slow down at the periphery.

The conditions for the generation of runaway electrons in an air gap are compared at different degrees of inhomogeneity of the electric field distribution provided by varying the opening angle of the conical cathode: in the range 40 degrees-120 degrees in experiments and 0 degrees-180 degrees in calculations. It is demonstrated that, in a weakly inhomogeneous electric field (according to the proposed classification, this corresponds to cones with angles greater than the Taylor angle of 98.6 degrees), the runaway condition has a local character. The transition of free electrons into the runaway mode is determined by the local distribution of the electric field near their starting point-the tip of the cone. The local electric field strength must exceed a threshold value comparable to the strength critical for the runaway of electrons in a uniform field. In a strongly inhomogeneous field (cones with angles less than 98.6 degrees), this condition is not sufficient for electrons to run away throughout the gap. Electrons accelerating in the near-cathode region may begin to slow down in a weak field at a distance from the cathode. In this case, the runaway condition becomes nonlocal. It is determined by the dynamics of electrons in the entire gap, primarily in the near-anode region, and reduces to the requirement that the potential difference applied to the gap exceeds a certain threshold value.

A model of the phenomenon of explosive electron emission based on its similarity to the electrical explosion of conductors is presented. With this model, the microexplosive processes occurring on a cathode surface due to the action of the explosive emission current have been simulated. The simulation results have been used to analyze explosive emission processes caused by the operation of unipolar arcs in thermonuclear reactors with magnetic plasma confinement and by the initiation of radiofrequency vacuum breakdowns in the accelerating structures of linear electron-positron colliders. The structure of the arc discharge cathode spot and the erosion characteristics have been investigated for nanostructured tungsten (W-fuzz) surfaces formed in thermonuclear reactors with magnetic plasma confinement. For radiofrequency vacuum breakdowns, the initiating parameters have been estimated, and prebreakdown and microexplosive processes have been simulated.

An experimental study of the explosion of cylindrical copper rods in the current skinning mode was performed using the GIT-12 high-current pulse generator with currents of amplitude up to 5 MA and rise time about 2 μs. It was observed that the waveforms of the electrical impedance of the exploded rods exhibited low-frequency oscillations with a period of several hundreds of nanoseconds. The results of the experiment with exploded solid copper rods carried out on the GIT-12 facility were interpreted by performing a series of magnetohydrodynamic calculations. It was shown that the experimentally observed low-frequency oscillations of the rod impedance were associated with a fast magnetosonic wave excited in the material of the rod.

We report the first experimental results on the generation in a magnetically isolated air-filled diode of the flow of sub-relativistic runaway electrons (RAEs) with a duration of ≈ 20 ps and a charge of ≈1 nC resembling a slightly curved disk with a transverse size (≈20 mm) noticeably higher than that in the propagation direction. RAEs originate from the boundary of plasma blobs near electric field enhancers protruding at the graphite cathode end face. Accounting for the transformation of the incident voltage front permits profiling the edges of concentric protrusions so that the field on them synchronously reaches the critical value for RAE appearance. It is shown that ≈45% of RAEs have energies >125 keV. Emission moment shifts of the bunch radial fractions, their rms spread (≈1.2 ps), and the total bunch current (≈ 50 A) were determined.

The divergence of the runaway electron flow generated in an air-filled discharge gap with a sharp conical cathode can be essentially reduced by applying a guiding axial magnetic field, which opens up prospects for the practical use of formed dense paraxial bunches of fast electrons. In the present work, we consider factors that determine the radial scale of the bunch. Our analysis shows that the main factor is the diffusion of electrons across the magnetic field lines due to collisions with gas molecules. Calculations of the dependence of the runaway electron beam radius on the magnitude of the applied magnetic field, taking this phenomenon into account, agree with the experimental data.

This letter reports new results on the formation of a paraxial flow of runaway electrons emitted in an elongated air gap near the tip of a conical steel cathode. In the axial magnetic field > 4 T, the peak current density of ≈ 0.6 kA/cm 2 has been achieved in the bunch core with a 0.7 mm diameter. Similarly to the vacuum case, dependence of the bunch parameters on the magnetic field was obtained and interpreted. Analysis of recorded current waveforms and oscilloscope transient response shows that the original current pulse rise time and width did not exceed 5 and 10 ps, respectively. Using the time-of-flight method, we have estimated the particle kinetic energy (≈200 keV) at the peak of the distribution function.

A nanosecond electric discharge in air in submillimeter gaps at the overvoltages up to 15-fold is under investigation. The initial electric field in such discharges is up to 106 V/cm. This field is sufficient for the electrons that initiate the discharge, as well as those obtained during the discharge, to go into the runaway mode. These electrons are called runaway electrons (RE). REs create plasma in the period 10-9 s. When the plasma density is reached of 104 cm-3, a glow discharge (GD) is initiated. The process of transition of the discharge to the GD mode lasts 10-11 s. During this time, about 5·1010 pieces of runaway electrons are formed, which, falling on the anode, lead to the formation of an X-ray beam. This beam is due to a rapid increase in the plasma density during the discharge. In the GD stage, the electric field between the cathode and the anode is divided into two parts: the cathode layer (CL) and the plasma column. The electric field in the CL exceeds 106 V/cm. At such a field, field emission (FE) from microprotrusions on the cathode surface takes place on the cathode. It leads to the explosive emission of electrons and the formation of a cathode spot and the transition of the discharge to the arc mode.

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It is shown that, in addition to the well-known Townsend and streamer discharges in gas, there is a third type of discharge, namely nanosecond diffuse-channel discharge. It occurs in a highly overvolted gas gap. The study is carried out on the example of air at normal conditions in a uniform electric field. In this case, the ratio $$d{\text{/}}{{x}_{{\text{c}}}} \gg 1$$ , where d is the gap spacing and xc is the electron avalanche critical length. The electric field at the head of such an avalanche reaches 106 V cm–1 and higher, therefore, it emits runaway electrons, which create new electrons ahead of the old ones. An avalanche chain is formed, formally similar to a streamer but with low electrical conductivity. The runaway electrons and ultraviolet photoemission from the cathode contribute to the accumulation of secondary electrons in the gap. This leads to the appearance of a diffuse glow discharge, which then turns into a channel discharge and in an arc. The dependence of the overvoltage coefficient η on the product pd is calculated, where p is the gas pressure at d/xc = 10. It is compared with the well-known curve that separates Townsend and streamer discharges in air.

Flow of runaway electrons (RAEs) propagating in a radial, air-filled gap of coaxial line (CL) changes the dynamics of breakdown in the field of traveling voltage pulse. However, despite the effect of RAEs, breakdown does not occur if subnanosecond pulse is less in duration and amplitude than some values. In this work, we study the influence of an external axial magnetic field (B z) on the breakdown development. We demonstrate the transformation of the voltage pulse reflection from the ionized (breakdown) zone with changing B z. Due to gyration of fast electrons in an applied magnetic field, the gas region ionized by RAEs does not reach the anode. The ionized bridge between the cathode and anode is gradually replaced by a near-cathode plasma layer representing a discrete, reflecting/absorbing inhomogeneity in the CL.