D. V. Rybka

The amplitude and current pulse duration of a supershort avalanche electron beam in air at atmospheric pressure

The results of experimental studies of the parameters (amplitude and duration) of a supershort avalanche electron beam (SAEB) generated in air at atmospheric pressure are presented. It is shown that the pulse duration of the beam current behind the foil from the entire area of the anode foil is larger than from small areas and depends on the cathode design. The number of electrons that are detected behind the 10-μm-thick Al foil is ∼6 × 1010 electrons, which corresponds to a SAEB amplitude of ∼100 A at a FWHM of the current pulse of ∼100 ps. An X-ray exposure dose per pulse of ∼1.8 mR was obtained using a 20-μm-thick copper foil. It was confirmed that the FWHM of a SAEB pulse is within ∼50 ps from small foil areas (with diameters of ∼7 mm or smaller).

Supershort Avalanche Electron Beams in Discharges in Air and Other Gases at High Pressure

In this paper, the generation of supershort avalanche electron beams (SAEBs) from the plasmas of nanosecond diffuse discharges in air and other gases at atmospheric pressure was investigated. It is shown that, in the mode of SAEB generation, the plasma in the discharge gap with an inhomogeneous electric field can be produced in a time no greater than 100 ps with the charged-particle density sufficient to force out the electric field from the discharge-gap region occupied by the dense plasma. It is demonstrated that decreasing the voltage pulsewidth in the discharge gap (to ~100-ps FWHM) reduces the optimum gap for SAEB generation. It is also found that the difference in electron path toward the foil center and the foil edge affects the SAEB current pulsewidth. For lower voltages across the gap (~25 kV), the SAEB pulsewidth at half maximum is ~200 ps.

SLEP-150m compact supershort avalanche electron beam accelerator

A SLEP-150M gas-diode runaway electron accelerator was designed to produce supershort avalanche electron beams (SAEBs) at increased gas pressures. The cathode and gas-diode used in the design made it possible to greatly increase the current amplitude of the runaway electron beam produced in atmospheric pressure air. The number of electrons downstream of the gas diode foil was ~5 × 1010 electrons, and this corresponds to a SAEB amplitude of ~80 A at a FWHM of ~100 ps.

Application of dynamic displacement current for diagnostics of subnanosecond breakdowns in an inhomogeneous electric field.

The breakdown of different air gaps at high overvoltages in an inhomogeneous electric field was investigated with a time resolution of up to 100 ps. Dynamic displacement current was used for diagnostics of ionization processes between the ionization wave front and a plane anode. It is demonstrated that during the generation of a supershort avalanche electron beam (SAEB) with amplitudes of ~10 A and more, conductivity in the air gaps at the breakdown stage is ensured by the ionization wave, whose front propagates from the electrode of small curvature radius, and by the dynamic displacement current between the ionization wave front and the plane electrode. The amplitude of the dynamic displacement current measured by a current shunt is 100 times greater than the SAEB. It is shown that with small gaps and with a large cathode diameter, the amplitude of the dynamic displacement current during a subnanosecond rise time of applied pulse voltage can be higher than 4 kA.

High-pressure runaway-electron-preionized diffuse discharges in a nonuniform electric field

High-pressure nanosecond diffuse (volume) discharges in a nonuniform electric field are studied experimentally using a recording system with a ?100-ps time resolution. As the voltage pulse shrinks to a width of ≈100 ps, the initiation of a diffuse discharge without a source of additional ionization is facilitated; specifically, a runaway-electron-preionized diffuse discharge is ignited in atmospheric-pressure air in the case of short interelectrode gaps. It is found that a major energy deposit into the plasma of this discharge is from an abnormal glow discharge following a maximum of the gap voltage.

Generation and measurement of subnanosecond electron beams in gas-filled diodes

The results of experimental studies of generation of supershort avalanche electron beams (SAEBs) in gas-filled diodes and the analysis of the techniques for measurements of their amplitude-time characteristics are presented. The optimal conditions for obtaining the maximum SAEB amplitudes are described. It is shown that, at a 6-mm-diameter cathode and a 10-mm interelectrode distance, a beam is detected over the entire foil area, the diameter of which is 50 mm, and equals the inner diameter of the gas diode. A half-height duration of the beam-current pulse shorter than 90 ps was measured with the use of a collector with a 3-mm-diameter receiving element. The SAEB amplitude measured behind the 10-μm-thick Al foil at this pulse duration was ∼50 A.

Generation of runaway electron subnanosecond pulses in nitrogen and helium at a voltage of 25 kV across the gap

The formation of a runaway electron beam in helium and nitrogen at a generator voltage of 25 kV is studied experimentally. At low generator voltages, an ultrashort avalanche electron beam (UAEB) is shown to form at the flat top of the voltage pulse and its delay time relative to the leading edge of the pulse may attain several tens of nanoseconds. The conditions of runaway electron beam generation depend on the pressure in the gas-filled diode. The FWHM of the beam current varies from 200 ps to several nanoseconds. Beam electron energy distributions at different pressures are obtained. It is found that, if the gap is preionized by an additional source, the UAEB generation conditions break.

Note: Measurement of extreme-short current pulse duration of runaway electron beam in atmospheric pressure air

This note reports the time-amplitude characteristic of the supershort avalanche electron beam with up to 20 ps time resolution. For the first time it is shown that the electron beam downstream of small-diameter diaphragms in atmospheric pressure air has a complex structure which depends on the interelectrode gap width and cathode design. With a spherical cathode and collimator the minimum duration at half maximum of the supershort avalanche electron beam current pulse was shown to be ~25 ps. The minimum duration at half maximum of one peak in the pulses with two peaks can reach ~25 ps too.

Effect of gas pressure on amplitude and duration of electron beam current in a gas-filled diode

The parameters of an electron beam generated in helium in the pressure range p = 10−4−12 atm are studied. Nanosecond high-voltage pulses are applied to a gap between a tubular cathode and planar anode, which is made of 45-μm-thick AlBe foil. Behind the anode, an electron beam is detected at a helium pressure of 12 atm. The pressure dependence of the beam current amplitude shows three peaks at p ≈ 0.01, ≈ 0.07, and ≈ 3 atm. The beam-induced glow of a luminescent film placed behind the foil and the discharge glow at different helium pressures in the gas-filled diode are photographed.

Luminescence of Crystals under the Action of a Subnanosecond Electron Beam

Subnanosecond avalanche electron beams formed in air at atmospheric pressure ensure intense luminescence of synthetic ruby and natural spodumene crystals.