Supershort Avalanche Electron Beam Research Articles

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.

Breakdown of gas gaps in a nonuniform electric field at a subnanosecond voltage pulse rise time

The influence of the nitrogen pressure on the breakdown voltage in a nonuniform electric field is studied. Voltage pulses with nanosecond and subnanosecond rise times are applied to the gas gap. Simultaneously with the application of voltage pulses, supershort avalanche electron beam pulses are observed behind a foil anode. It is found that, when a runaway electron beam is generated and voltage pulses have a subnano-second rise time, the breakdown voltage rises as the nitrogen pressure decreases from 9 × 104 to 1 × 102 Pa. Experimental data are in good agreement with pulsed breakdown analytical curves.

X-ray radiation and runaway electron beam spectra at a nanosecond discharge in atmospheric-pressure air

The dependences of the electron beam intensity and X-ray dose on the thickness of metal foils (Al, Cu) in a nanosecond discharge initiated in atmospheric-pressure air are studied theoretically and experimentally. Calculated curves of electron beam attenuation in aluminum and X-ray dose attenuation in copper agree well with experimental data. It is found that the amplitude of a super-short avalanche electron beam and the X-ray exposure dose reach maximal values at different values of the interelectrode gap. When the length of the cathode’s edge with a small radius of curvature increases, an interelectrode gap maximizing the amplitude of the runaway electron current shrinks.

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.

The temporal structure of a runaway electron beam generated in air at atmospheric pressure

Supershort avalanche electron beams (SAEBs) generated in air at atmospheric pressure have been studied with picosecond time resolution. It is established that an SAEB has a complicated structure that depends on the interelectrode gap width and cathode design. In a gas-filled diode with a small gap width, an SAEB current pulse with a full width at half maximum (FWHM) of ∼25 ps has been observed behind a collimator with a hole diameter of 1 mm. As the gap width is increased or decreased relative to the optimum value that corresponds to the maximum beam current, the SAEB current pulse shape changes and pulses with two peaks are more likely detected. The two-peak SAEB current pulse shape is retained behind aluminum foil with a thickness of 60 and 110 μm.

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).

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.

On the influence of the voltage pulse rise time and cathode geometry on the generation of a supershort avalanche electron beam

The influence of the voltage pulse rise time on the amplitude of a runaway electron beam and X-ray generation in air and nitrogen under atmospheric pressure is studied experimentally and theoretically. Generalization of the whistle criterion for the case of a nonuniform field is suggested. It is shown that the maximal energy of beam electrons and the beam current amplitude grow when the voltage pulse rise time decreases. It is found that the amplitude of the runaway electron current reaches a maximum at a certain curvature of the cathode. The maximal energy of electrons increases when the radius of curvature of the cathode exceeds the value at which the beam current amplitude is the highest. If the field is nonuniform, its critical value at which many electrons run away is more than an order of magnitude lower than in the uniform field.

Parameters of a supershort avalanche electron beam generated in atmospheric-pressure air

Conditions under which the number of runaway electrons in atmospheric-pressure air reaches ∼5 × 1010 are determined. Recommendations for creating runaway electron accelerators are given. Methods for measuring the parameters of a supershort avalanche electron beam and X-ray pulses from gas-filled diodes, as well as the discharge current and gap voltage, are described. A technique for determining the instant of runaway electron generation with respect to the voltage pulse is proposed. It is shown that the reduction in the gap voltage and the decrease in the beam current coincide in time. The mechanism of intense electron beam generation in gas-filled diodes is analyzed. It is confirmed experimentally that, in optimal regimes, the number of electrons generated in atmospheric-pressure air with energies T > eUm, where Um is the maximum gap voltage, is relatively small.

超高功率体放电的形成及其应用

The experimental results on study of characteristics of the high-pressure volume(diffuse) discharges in nitrogen at applying to the interelectrode gap voltage pulse with the amplitude of hundreds kV,duration of several ns and the rise-time of fraction ns are presented.At the pressure range of nitrogen in(0.2-4)×105 Pa,the discharge transformation from the diffuse form to the spark is investigated.Dependencies of the current amplitude and FWHM of the runaway electron beam current on nitrogen pressure are determined.It is shown that the generation of the Supershort Avalanche Electron Beam(SAEB) leading to the formation of diffuse discharge in the gap has a significant influence on the development of the discharge.At the pressures indicated above,the dependence of the time delay of SAEB's moment generating relatively discharge current beginning on pressure are obtained.According to this dependence,the time delay changes as pressure increases and it is minimal at pressure of 2×105 Pa.Also,it is shown that the maximum peak value of diffuse discharge current pulse decreases with increasing pressure.At a pressure of 0.5×105 Pa with the use of blade electrodes and the N2∶SF6=10∶1 active medium of length 6 cm,the output laser energy of 2 mJ is achieved for the pulse power of 0.55 MW.It is reported that during the treatment of the AlBe foil by the Runaway Electron Preionized(REP) discharge in atmospheric pressure air,its surface layer is cleaned from carbon,and atoms of oxygen penetrate into the foil(by 450 nm per 300 pulses).

Modes of Generation of Runaway Electron Beams in He, $ \hbox{H}_{2}$, Ne, and $\hbox{N}_{2}$ at a Pressure of 1–760 Torr

In this paper, the characteristics of runaway electron beams downstream of a foil anode were studied at a pressure of helium, hydrogen, neon, and nitrogen of 1-760 torr. High-voltage pulses (~150 and ~250 kV) with pulse rise times of ~300 and ~500 ps were applied to the tubular cathode-plane anode gap. It is shown that the highest amplitudes of a supershort avalanche electron beam (SAEB) of 100-ps pulse duration are attained in helium, hydrogen, and nitrogen at pressures of ~60, ~30, and ~10 torr, respectively. It is demonstrated that further decreasing the pressure changes the mode of generation of runaway electron beam and increases the beam current amplitude and the voltage pulse duration across the gap. It is found that increasing the pressure of helium, hydrogen, and nitrogen to hundreds of torr decreases the delay time between the instants the voltage pulse is applied to the gap and the SAEB is generated, as well as the maximum voltage across the gap.

An efficient cathode for generating an supershort avalanche electron beam in air at atmospheric pressure

The cathodes of accelerators intended for generating an ultrashort electron beam in air at atmospheric pressure were studied. A cathode allowing an increase in the amplitude of the beam current behind the foil by several times has been developed. The beam current amplitude obtained at a half-height pulse duration of ∼100 ps is ∼80 A.

Effective regimes of runaway electron beam generation in helium, hydrogen, and nitrogen

Runaway electron beam parameters and current-voltage characteristics of discharge in helium, hydrogen, and nitrogen at pressures in the range of several Torr to several hundred Torr have been studied. It is found that the maximum amplitudes of supershort avalanche electron beams (SAEBs) with a pulse full width at half maximum (FWHM) of ∼100 ps are achieved in helium, hydrogen, and nitrogen at a pressure of ∼60, ∼30, and ∼10 Torr, respectively. It is shown that, as the gas pressure is increased in the indicated range, the breakdown voltage of the gas-filled gap decreases, which leads to a decrease in the SAEB current amplitude. At pressures of helium within 20–60 Torr, hydrogen within 10–30 Torr, and nitrogen within 3–10 Torr, the regime of the runaway electron beam generation changes and, by varying the pressure in the gas-filled diode in the indicated intervals, it is possible to smoothly control the current pulse duration (FWHM) from ∼100 to ∼500 ps, while the beam current amplitude increases by a factor of 1.5–3.

Supershort Avalanche Electron Beams and X-rays in Atmospheric-Pressure Air

The conditions for the generation of runaway electron beams with maximum amplitudes and soft X-rays with maximum exposure doses in a nanosecond discharge in atmospheric-pressure air were determined. A supershort avalanche electron beam (SAEB) with a current of amplitude ~50 A, a current pulse of full-width at half-maximum (FWHM) ~ 100 ps, and a current density up to 20 A/cm2 was recorded downstream of the gas diode foil. It is shown that the maximum of the SAEB current amplitude shifts in time relative to the voltage pulse rise as a collector is displaced over the foil surface. A source of soft X-rays with an FWHM of less than 200 ps and an exposure doze of ~3 mR per pulse was designed based on a SLEP-150 pulser (maximum voltage amplitude ~140 kV, FWHM ~1 ns, and pulse rise time ~0.3 ns). It is demonstrated that X-ray quanta with an effective energy of ~9 keV make a major contribution to the exposure dose.

On the generation mechanism of supershort avalanche electro beams during a nanosecond discharge in high-pressure gases

The results of an investigation of the runaway electron beams in gases of an elevated pressure are presented. It is shown that at a given gas pressure, maximal amplitudes of supershort avalanche electron beams (SAEB) are attainable in the case of diffuse discharge and electron acceleration between the anode and the ionization wave front, which propagates at a rate of about 10 cm/ns.

Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm2, and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.

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.

Generation of supershort avalanche electron beams and formation of diffuse discharges in different gases at high pressure

This paper presents the results of experimental investigations on the generation of supershort avalanche electron beams (SAEB) at nanosecond discharges initiated in gas-filled diodes. In air, at a pressure of 1 atm with a sharply non-uniform electric field, an SAEB is generated from the inter-electrode gap at a solid angle exceeding 2π sr. Reduction of gap d between the cathode and the anode leads to a decrease in the current amplitude of the beam incident on the lateral walls of the gas-filled diode and in an increase in the pulse duration of the beam current both in the axial and radial directions. Pulses of the beam current initiated on the lateral surface of a tube cathode have been recorded. It is shown that to generate electrons with an ‘anomalous’ energy, cathodes with an increased radius of curvature should be used.

Supershort avalanche electron beam generation in gases

AbstractThis paper reports on the properties of a supershort avalanche electron beam generated in the air or other gases under atmospheric pressure and gives the analysis of a generation mechanism of supershort avalanche electron beam, as well as methods of such electron beams registration. It is reported that in the air under the pressure of 1 atm, a supershort (<100 ps) avalanche electron beam is formed in the solid angle more than 2π steradian. The electron beam has been obtained behind a 45 µm thick Al-Be foil in SF6 and Xe under the pressure of 2 atm, and in He, under the pressure of about 15 atm. It is shown that in SF6 under the high pressure (>1 atm) duration (full width at half maximum) of supershort avalanche electron beam pulse is about 150 ps.

X radiation from a laser-triggered spark gap

Laser triggering of spark gaps is now widely used for low-jitter firing of one or several spark gaps [1–5]. The most important advantage of laser-triggered high-voltage spark gaps is no galvanic coupling between the gap electrodes and the laser. This allows low-jitter firing of one or several spark gaps to which electrodes a dc or a pulsed voltage is applied. Conduction between the spark gap electrodes is initiated by the plasma produced at one of the electrodes or in the gas-filled gap. For a laser-triggered spark gap, the trigger delay and jitter depend on the gas mixture composition and pressure, laser beam focusing site, electric field strength, laser wavelength, radiation power density and pulse duration, and laser beam divergence. For low radiation pulse energies, the shortest trigger delays are achieved for systems with uv lasers, though in some cases, pulsed lasers operating in other spectral ranges are used. Elegas (SF6) or its mixtures with nitrogen are generally used for the working gas in high-voltage spark gaps. SF6 is electronegative gas possessing high electric strength. In optimum conditions (high electric fields, optical breakdown of the gas in a gap at the cathode) the delay time of the spark gap triggering after application of a laser pulse is a few nanoseconds, and the jitter is no more than 1 ns even for a rather short length of the laser spark (about ten gap spacings). Recently it has been shown [6] that the spark preionization systems that are widely used to initiate a volumetric discharge in pulsed lasers generate not only uv and vuv radiation, but also x rays. This makes preionization more uniform and improves the lasing performance for some lasers. There are many reports on x radiation and runaway electron beams in nanosecond discharges initiated in a nonuniform electric field (see, e.g., [7, 8–14]). Supershort avalanche electron beams (SAEBs) where produced in gas diodes at a pressure of 12 atm for helium, 4 atm for nitrogen, and 2 atm for xenon and elegas [8, 10, 14]. It was natural to suggest that runaway electrons and x radiation are generated in a laser-triggered discharge gap as well, and it is just the runaway electrons and x radiation that provide low-jitter laser triggering. However, we have found no information on this subject in the available literature. Note that in conditions of laser triggering, the threshold of optical breakdown for gases used in spark gaps is in the range 5⋅10– 5⋅10 W/cm [1, 15], which is several orders of magnitude lower than the threshold for the appearance of runaway electrons and x radiation in a laser plasma [16]. The goal of the work under discussion was to detect x radiation from a laser-triggered spark gap and to demonstrate that the generation of x radiation in the gap is due to the electric field enhancement by the laser-produced plasma.