Generation regimes for the runaway-electron beam in gas

The generation of the runaway-electron beam in nitrogen and helium is studied at an oscillator voltage of about 25 kV. Various regimes of the electron beam generation with a pulse duration ranging from 200 ps to several nanoseconds are realized depending on the pressure in the gas diode. An ultrashort avalanche electron beam (UAEB) with a low (10–20%) variation in the voltage across the gap is obtained at a relatively high pressure in the gas diode. The UAEB generation can be delayed relative to the leading edge of the voltage pulse by tens of nanoseconds at low oscillator voltages.     

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.     

Effect of the amplitude and rise time of a voltage pulse on the formation of an ultrashort avalanche electron beam in a gas diode

The effect of the amplitude and rise time of a voltage pulse from a RADAN-303 pulser on the formation of an ultrashort avalanche electron beam (UAEB) in a gas diode is experimentally investigated. It is shown that, when the open-circuit voltage of the pulser exceeds an optimum value, the beam current amplitude and the gap voltage under which the UAEB is generated decrease.     

Effect of a transverse magnetic field on the generation of electron beams in the gas-filled diode

The effect of a transverse magnetic field (0.080 and 0.016 T) on generation of an electron beam in the gas-filled diode is experimentally investigated. It is shown that, at voltage U = 25 kV across the diode and a low helium pressure (45 Torr), the transverse magnetic field influences the beam current amplitude behind a foil and its distribution over the foil cross section. At elevated pressures and under the conditions of ultrashort avalanche electron beam formation in helium, nitrogen, and air, the transverse magnetic field (0.080 and 0.016 T) has a minor effect on the amplitude and duration of the beam behind the foil. It is established that, when the voltage of the pulse generator reaches several hundreds of kilovolts, some runaway electrons (including the electrons from the discharge plasma near the cathode) are incident on the side walls of the diode.     

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.     

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.     

纳秒脉冲气体放电中逃逸电子束流的研究

经典的放电理论(Townsend和流注理论)不能很好地解释纳秒脉冲放电中的现象,近年来基于高能量电子逃逸击穿的纳秒脉冲气体放电理论研究受到广泛关注,有研究发现,高能逃逸电子是纳秒脉冲气体放电中的新特征参数,本文研制了用于测量纳秒脉冲放电中逃逸电子束流的收集器,并对脉宽3—5ns、上升沿1.2—1.6 n8激励的大气压纳秒脉冲气体放电中逃逸电子束流进行了测量,收集器采用类似法拉第杯的原理,利用金属极收集纳秒脉冲放电中的高能电子,并转换为电信号后由示波器采集,为了获得更好的逃逸电子束流波形,对逃逸电子束流收集器进行了优化设计,提高了收集器的阻抗匹配特性,基于上述的逃逸电子束流收集器,研究了纳秒脉冲气体放电中逃逸电子的特征,实验结果表明,所设计的收集器可以有效地测量到逃逸电子束流,改进设计后收集器测得的逃逸电子柬流的时间分辨率和幅值均得到提高,施加电压约80 kV时,大气压空气中的逃逸电子束流幅值可达160 mA,脉宽小于1ns,多个脉冲激励放电的结果表明逃逸电子束流收集器具有较好的可靠性,其瞬态响应与时间分辨率比较稳定。     

Generation of runaway electrons in a nonuniform electric field by applying nanosecond voltage pulses with a frequency of 100–1000 Hz

Generation of runaway electrons and X-ray radiation in helium and air under the action of a pulsed-periodic discharge in a nonuniform electric field is studied. Positive and negative voltage pulses with a repetition rate of up to 1 kHz, a duration on the order of 1 ns, and an incident wave amplitude of 12.5 kV are applied to a needle-plane gap. For both polarities of the main voltage pulse and a helium pressure from several Torr to several tens of Torr, the arrival of negative reflected voltage pulses at the gap is shown to be accompanied by an electron beam generation. X-ray radiation is detected in a wide range of pressure, including under normal pressure of helium and air.     

Spectra of electrons and X-ray photons in a diffusive nanosecond discharge in air under atmospheric pressure

The spectra of electrons and X-ray photons generated in nanosecond discharges in air under atmospheric pressure are investigated theoretically and experimentally. Data for the discharge formation dynamics in a nonuniform electric field are gathered. It is confirmed that voltage pulses with an amplitude of more than 100 kV and a rise time of 1 ns or less causing breakdown of an electrode gap with a small-radius cathode generate runaway electrons, which can be divided into three groups in energy (their energy varies from several kiloelectronvolts to several hundreds of kiloelectronvolts). It is also borne out that the formation of the space charge is due to electrons appearing in the gap at the cathode and a major contribution to the electron beam behind the foil comes from electrons of the second group, the maximal energy of which roughly corresponds to the voltage across the gap during electron beam generation. X-ray radiation from the gas-filled diode results from beam electron slowdown both in the anode and in the gap. It is shown that the amount of group-3 electrons with an energy above the energy gained by runaway electrons (in the absence of losses) at a maximal voltage across the gap is much smaller than the amount of group-2 electrons.     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U _cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U _cr , pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.     

Experimental and numerical investigation of two mechanisms underlying runaway electron beam formation

The electrical breakdown of a gas-filled diode with a highly nonuniform electric field is studied in the case when a 25-kV voltage pulse generates runaway electron beams with time-separated maxima of different duration behind anode foil. Experimental data are analyzed and numerically simulated using the PIC/MC code OOPIC-Pro. It is shown that, in terms of the model used, both beams arise at the cathode but their formation mechanisms differ. The first runaway electron beam no longer than 500 ps is attributed to the ionization mechanism; the second one, which may last several nanoseconds, is due to emission.     

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.     

Generation of dual pulses of the runaway electron beam current during the subnanosecond breakdown of atomic and molecular gases

With a diaphragm placed behind the anode foil, dual runaway electron beams have been provided in helium, hydrogen, nitrogen, and air under a pressure of several torrs to several dozen torrs and a high-voltage pulse amplitude of about 250 kV. These beams consist of two pulses with commensurable amplitudes with a time interval between them of several dozen picoseconds to several hundred picoseconds. It has been shown that the breakdown of the interelectrode gap at pressures from several torrs to several dozen torrs may occur in different regimes and dual pulses of the electron beam current are registered when the initial current through the gap is below 1 kA. It has been found that a supershort avalanche electron beam that consists of one pulse is generated when the delay of breakdown equals several hundred picoseconds. It has been shown that, when the gas pressure reaches several hundred Torr, including atmospheric pressure, the runaway electrons are detected behind the foil after the termination of the supershort avalanche electron beam pulse.     

Inflections of spark leaders in elevated-pressure nanosecond gas discharges

The formation of spark leaders and a diffuse discharge due to the breakdown of gaps with a non-uniform electric field is studied in different elevated-pressure gases under the conditions of runaway electron beam and X-ray generation. Negative voltage pulses are applied to an electrode with a small radius of curvature. For pulse rise times of 0.5 and 1.5 ns, inflections on the spark leaders were observed. In a number of cases, two inflections appear making a right angle to the longitudinal axes of the leaders. It is shown that the formation of spark leaders is preceded by the appearance of diffuse tracks, which bridge the gap for no longer than 1 ns.     

Study of Intense Electron Beams Produced by High-Voltage Pulsed Glow Discharges

We report the generation of high-current-density (20 A/cm2) pulsed electron beams from high-voltage (48-100 kV) glow discharges using cathodes 7.5 cm in diameter. The pulse duration was determined by the energy of the pulse generator and varied between 0.2 ?s and several microseconds, depending on the discharge current. The largest electron beam current (900 A) was obtained with an oxidized aluminum cathode in a helium-oxygen atmosphere. An oxidized magnesium cathode produced similar results, and a molybdenum cathode operated at considerably lower currents. A small-diameter (<1 mm) well-collimated beam of energetic electrons of very high current density (>1 kA/cm2) was also observed to develop in the center of the discharge. Electrostatic probe measurements show that the negative glow plasma density and the electron beam current have a similar spatial distribution. Electron temperatures of 1-1.5 eV were measured at 7 cm from the cathode. The plasma density (8.5 · 1011 cm-3 at 450 A) was found to depend linearly on the discharge current. In discharges at high currents a denser and higher temperature plasma region was observed to develop at approximately 20 cm from the cathode. We have modeled the process of electron beam generation and predicted the energy distribution of the electron beam. More than 95 percent of the electron beam energy is calculated to be within 10 percent of that corresponding to the discharge voltage.     

Numerical investigation of the methods for reducing the runaway electron beam divergence

We have performed a comparative numerical analysis of two methods for reducing the runaway electron beam divergence using an external magnetic field or a dielectric tube. The generation of runaway electrons takes place in an inhomogeneous medium that consists of a hot channel (spark channel, laser torch, etc.) surrounded by air under normal conditions. The model makes it possible to consistently calculate the formation of a subnanosecond gas discharge and the generation of accelerated electrons under these conditions. The possibility of effectively decreasing the runaway electron beam divergence using an external magnetic field, as well as a dielectric tube, has been demonstrated. However, the number of runaway electrons in the case with the tube is considerably smaller than in the case with the magnetic field due to the fact that some runaway electrons settle on the tube walls. The energy spectra of the runaway electrons significantly differ in these cases, which can be explained by the differences in the dynamics of the discharge formation.     

Production of powerful electron beams in dense gases

Subnanosecond electron beams with the record current amplitude (～70 A in air and ～200 A in helium) were produced at atmospheric pressure. The optimal generator open-circuit voltage was found for which the electron-beam current amplitude produced in a gas diode was maximal behind a foil. It was established that the electron beam was produced at the stage when the cathode plasma closely approaches the anode. It was shown that a high-current beam can be produced at high pressures because of the presence of the upper branches in the curves characterizing the electron-escape (runaway) criterion and the discharge-ignition criterion (Paschen curve).     

Energy distribution of runaway and fast electrons upon nanosecond volume discharge in atmospheric-pressure air

Nanosecond space discharge in a gas-filled diode is promising for pumping of lasers and high-power lamps. The space charge formed in the absence of an additional preionization source has a few advantages. The energy distributions of the beam electrons and the X-ray spectrum are determined. It is demonstrated that several high-energy electron bunches are formed in such a discharge. The main contribution to the beam current measured behind the foil is related to the runaway electrons, which have energies of tens or hundreds of kiloelectronvolts (supershort avalanche electron beam (SAEB)). Fast electrons with energies of several or tens of kiloelectronvolts are responsible for the generation of the soft X rays in the discharge gap. Anomalous electrons whose energy is higher than the voltage across the gap provide for a minor (less than 5%) contribution to the beam current. The generation time of these electrons is equal to the SAEB generation time accurate to 0.1 ns. It is demonstrated that the anomalous electrons can be generated owing to the acceleration in the presence of the field in front of the moving background-electron multiplication wave. The spectra of the X-ray radiation generated by the fast electrons in the volume are calculated.     

长脉冲高阻抗强流电子束二极管

 介绍了强光一号加速器长脉冲功率系统；分析了强光一号加速器长脉冲轴向绝缘高阻抗电子束二极管的管绝缘体和真空磁绝缘传输线的结构与绝缘性能；阐述了二极管工作阻抗和阴阳极的设计原则，给出了设计参数。实验研究表明：二极管电压0.75~2.6MV，电流65~85kA，电压幅值对应的工作阻抗14~44Ω，输出轫致辐射脉冲宽度100~400ns，100 cm²输出窗口的轫致辐射剂量率10⁸~2×10⁹Gy/s。     

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.