Bremsstrahlung of fast electrons in long air gaps

Abstract

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.

Breakdown features of a high-voltage nanosecond discharge initiated with runaway electrons at subnanosecond voltage pulse rise time

“…Currently the studies of formation and propagation of streamer at breakdown of elevated pressure gases are conducted actively [17][18][19][20][21][22][23][24][25]. The streamer propagation velocity was shown to increase with applied voltage [17][18][19].…”

“…In [23] streamer discharge operated at applying two consecutive positive highvoltage pulses to a point-plane gas-filled gap was investigated. The appearance of runaway electrons during streamer propagation was discussed in [24,25]. At REP DD formation, the overlap of many electrons avalanches occurs and an ionization wave with transverse dimensions of the order of a centimeter forms [3,5,8,9].…”

Simulation of the runaway electron beam formed in a discharge in air at atmospheric pressure

“…[39][40][41][42] An x-ray flash occurred within about 280 ns from the beginning of the voltage pulse; the flash duration was a few nanoseconds. Its occurrence was associated with the evolution of both cathode-initiated and anode-initiated streamers.…”

“…The streamer length was measured by the discharge cutoff method, 43 and at the instant that an x-ray flash occurred the characteristic dimensions of the anode-directed streamers were: radius r s no more than 0.5 mm and length L s $ 1.5-2.5 cm. A numerical simulation [40][41][42] has shown that with these streamer dimensions an electron with energy of several electron-volts injected in a gap from the streamer surface can switch in the continuous acceleration mode. For this 043105-2…”

“…[39][40][41][42] The model proposed, similar to the majority of models dealing with the dynamics of RE, 25,32,33,35,[44][45][46] uses some simplifying assumptions and, therefore, it can only a qualitative description of the process of formation of an RE beam.…”