The x-ray and gamma-ray flashes observed in the Earth atmosphere during a thunderstorm are usually associated with the generation of runaway electrons (RE) in atmospheric electric fields. It is supposed that runaway electron avalanches initiated by cosmic rays play the main role in high-altitude discharges observed in a thunderstorm atmosphere. We have performed three-dimensional numerical calculations to investigate the mechanism of the development of a critical avalanche and to determine its parameters. It has been shown that the number of electrons in a critical avalanche occurring in air under conditions characteristic of thunderstorm discharges can reach a value of the order of 10 18 .
Filamentation is a type of magnetohydrodynamic instability that may develop in a currentcarrying plasma. It is supposed that filaments, individual current channels, are formed due to thermal instabilities. The growth of these instabilities is determined by the behavior of the electrical conductivity of the material depending on its thermodynamic parameters. If the conductivity increases with temperature, as is the case in a plasma, thermal instabilities should give rise to the formation of separate current channels. This paper presents an analysis of the development of thermal instabilities in imploding plasma liners performed in terms of small perturbation theory. The theoretical predictions are compared with the results of experiments conducted on the IMRI-5 facility at a current of amplitude up to 450 kA and rise time about 500 ns.
The results of experiments with exploding copper conductors, performed on the MIG facility (providing currents of amplitude of about 2.5 MA and rise time of 100 ns), are analyzed. With an frame optical camera, large-scale instabilities of wavelength 0.2–0.5 mm were detected on the conductor surface. The instabilities show up as plasma “tongues” expanding with a sound velocity in the opposite direction to the magnetic field gradient. Analysis performed using a two-dimensional MHD code has shown that the structures observed in the experiments were formed most probably due to flute instabilities. The growth of flute instabilities is predetermined by the development of thermal instabilities near the conductor surface. The thermal instabilities arise behind the front of the nonlinear magnetic diffusion wave propagating through the conductor. The wavefront on its own is not subject to thermal instabilities.
To gain a better understanding of the operation of atmospheric pressure air discharges, the formation of a runaway electron beam at an individual emission site on the cathode has been numerically simulated. The model provides a description of the dynamics of the fast electrons emitted into an air gap from the surface of the emission zone by solving numerically two-dimensional equations for the electrons. It is supposed that the electric field at the surface of the emission zone is enhanced, providing conditions for continuous acceleration of the emitted electrons. It is shown that the formation of a runaway electron beam in a highly overvolted discharge is largely associated with avalanche-type processes and that the number of electrons in the avalanche reaches 50% of the total number of runaway electrons.
The results of an experiment on discharges in long atmospheric pressure air gaps at a pulsed voltage of amplitude up to 800 kV and risetime 150–200 ns have been analyzed. In the experiment, a radiation pulse of photon energy >5 keV and duration 10–20 ns was observed. In analyzing the experimental data it was supposed that a streamer is a plasma protrusion whose surface is equipotential to the cathode surface. It has been shown that the x-ray pulse results from the switch of electrons into the mode of "runaway" from the head of anode-directed streamers. For the electrons injected in the electrode gap from the streamer head, conditions for their switching into the mode of continuous acceleration are realized due to the enhanced electric field at the head. The predicted maximum of the spectrum of the bremsstrahlung generated by the runaway electron beam is around 15 keV. The presence of a maximum in the bremsstrahlung spectrum is due to that the photons emitted by electrons are absorbed by atoms of the gas in which the discharge operate.
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