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.     

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.     

Nanosecond discharge in sulfur hexafluoride and the generation of an ultrashort avalanche electron beam

A discharge in the presence of a nonuniform electric field and the generation of an ultrashort avalanche electron beam (UAEB) are studied in the insulating gas SF₆ at the pressures 0.01–2.50 atm. High-voltage nanosecond pulses (about 150 and 250 kV) and the voltage pulses with an amplitude of 25 kV and a duration of tens of nanoseconds are applied across the gap. An electron beam is obtained behind the AlBe foil with a thickness of 45 μm at a sulfur hexafluoride pressure in a gas-filled diode of up to 2 atm. It is demonstrated that, at relatively high pressures (greater than 1 atm) and in the presence of high-voltage nanosecond pulses across the gap, the UAEB pulse FWHM increases. The spectra of the diffuse and contracted discharges in sulfur hexafluoride are measured.     

Effect of insulating films on the current of an electron beam from gas-filled diodes exposed to voltage pulses with a nanosecond rise time

The parameters of subnanosecond electron beams generated in gas-filled diodes are studied. When the voltage pulse rise time is short (≈1 ns or less) and the electron beam is extracted from the diode through insulating films, the amplitude of the signal measured at the collector depends not on the electron beam current but on the electromagnetic radiation arising in the diode and the capacitive current from the collector. If the electron beam is extracted through thin metallic foils and fine metallic grids, the FWHM of the runaway electron pulsed beam is ≈0.1 ns and its amplitude reaches several hundreds of amperes.     

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.     

Electron flux spatial distribution in an ultrashort avalanche electron beam generated at atmospheric air pressure

The results of experimental study on generation of ultrashort avalanche electron beams (UAEB) in gas-filled diodes are considered. The spatial distribution of the flux of runaway electrons and X-rays generated in the gas diode fed by nanosecond high-voltage pulses was studied. It was shown that the UAEB in the gas-filled diode (at an air pressure of 1 atm) with sharply nonuniform electric field is generated from the interelectrode region into a solid angle exceeding 2π sr. Narrowing of the cathode-anode gap results in a decrease in the current amplitude of the beam generated to side walls of the gas diode and an increase in the beam current pulse duration in both axial and radial directions. Current pulses of the beam initiated from the side surface of the tubular cathode were detected.     

Electron beam formation in a gas diode at high pressures

Electron beam formation in krypton, neon, helium, and nitrogen at elevated pressures are experimentally investigated. It is shown that, when the krypton, neon, and helium pressures are varied, respectively, from 70 to 760 Torr, from 150 to 760 Torr, and from 300 to 4560 Torr, runaway electrons are beamed at the instant the plasma in the discharge gap approaches the anode and the nonlocal criterion for electron runaway is fulfilled. The fast-electron simulation of discharge gap preionization is performed. The simulation data demonstrate that preionization in the discharge gap is provided if the voltage pulse rise time is shorter than a nanosecond under atmospheric pressure.     

Formation of a volume discharge at a subnanosecond rise time of the voltage pulse

A study is made of the formation of a volume discharge in atmospheric-pressure air in a nonuniform electric field without additional preionization. It is shown that the spatial distribution of the plasma glow between a plane and a spherical (as well as a point) electrode at a subnanosecond rise time of the high-voltage pulse is volumetric in character. The change of the voltage polarity does not qualitatively affect the character of the glow. The propagation of a spherical ionization wave in nitrogen is calculated in the drift-diffusion approximation. The fact that the character of the discharge glow is essentially independent of the voltage polarity is explained by the multiplication of the background electrons in the dense working gas.     

Formation of ultrashort electron pulses on scattering of an electron beam by a standing laser wave of ultrashort duration

The scattering of a well collimated electron beam by a strong standing laser wave of ultrashort duration, giving rise to a great number of scattered photons, is considered. This type of scattering is found to cause an electron beam to divide effectively into two parts. Ultrashort laser pulses are shown to be capable of forming ultrashort electron bunches whose length is governed by the laser beam diameter.     

Measurement of the superluminal group velocity of an ultrashort Bessel beam pulse

The superluminal group velocity of an ultrashort optical Bessel beam pulse is measured over its entire depth of field, corresponding to approximately 2x10(4) optical wavelengths. The method used is to measure the traveling ionization front induced by the pulse.     

Analysis of parameters of an electron beam from a plasma-filled diode

We analyze the properties of a high-current electron beam formed in an electron source based on a plasma-filled diode and a linear pulsed transformer. The beam parameters are determined by measuring bremsstrahlung X-rays and the beam current, as well as the photographs of the diode gap in the optical range, of the anode in X-rays, and beam autographs. A beam with a current of ～100 kA and a mean electron energy exceeding 0.7 MeV for an accelerating voltage amplitude of ～1 MV is obtained. The diameter of the generated beam is ～1 cm. The electron beam from the plasma-filled diode makes it possible to attain a high anode power density (>10¹⁰ W/cm²) for exciting shock waves, for obtaining high pressures, and for generating powerful X-rays.     

超短电子脉冲在聚焦强激光场中的三维有质动力散射

采用三维测试粒子模拟(test-padicle simulation)的方法，研究了聚焦的飞秒脉冲强激光与超短电子束进行有质动力的散射过程.详细讨论了超短电子束经过不同激光延迟下有质动力散射的空间分布特征和能谱特征.提出了通过测量电子束的空间分布和能谱分布实现对超短脉冲电子束的脉冲宽度测量的方法. 关键词： 有质动力散射 聚焦的飞激光 飞秒电子束     

强流脉冲电子束二极管等离子体漂移速度的研究

强流相对论电子束二极管阴阳极等离子体的形成和漂移,是二极管工作状态研究的重要组成部分。根据Child-Langmuir定律和二极管导流系数,结合二极管阴极电子发射面积的变化模型,给出了二极管阴阳极等离子体漂移所导致的阴阳极间隙闭合速度。     

强流脉冲电子束二极管等离子体漂移速度的研究

 强流相对论电子束二极管阴阳极等离子体的形成和漂移,是二极管工作状态研究的重要组成部分。根据Child-Langmuir定律和二极管导流系数,结合二极管阴极电子发射面积的变化模型,给出了二极管阴阳极等离子体漂移所导致的阴阳极间隙闭合速度。     

On the self-consistent states of a planar vacuum diode with an electron beam

The self-consistent stationary solutions for a planar vacuum diode with a monoenergetic electron beam are fully classified. Formulas are presented to calculate potential distributions of all types and the maximum electric current that can pass through the diode under steady-state conditions at an arbitrary applied voltage. The stability of the solutions is studied.     

Effect of the current rise rate in a relativistic electron beam pulse propagating in the ion focusing regime on the dynamics of ion hose instability

The effect of the current rise rate in a relativistic electron beam pulse propagating in the ion focusing regime on the spatial dynamics of the ion hose instability is considered. Numerical analysis of the formulated equations shows that the lower rate of current rise in the beam pulse at the linear stage of evolution of the instability noticeably reduces the amplitude of hose oscillations.