Wide-aperture high-current electron beam in a discharge with cathode plasma at elevated pressure

This work pursues investigations into the discharge with a cathode plasma in a cavity one wall of which is an insulating plate with a hole D in diameter (the cavity is 0.5 or 1.5 mm wide). This discharge is thoroughly analyzed in comparison with the high-voltage hollow-cathode discharge. Owing to the reduced emission of electrons from the cathode plasma, the discharge becomes more stable against transition to the low-voltage form, as a result of which an electron beam can be generated under higher gas pressures. Such a beam formed at the entrance to the cavity is used as an auxiliary one that propagates over the remaining (flat) surface of the cathode and adds to the gas ionization. Accordingly, the beam current from the main discharge to the anode rises (high-current regime). Wide-aperture (D = 22 mm) ≈1-μs-long pulsed beams with a current an order of magnitude higher than the total current of the equivalent anomalous discharge are obtained. Experiments are carried out at a helium pressure to 20 Torr and a voltage from 1 to 20 kV.     

空气中大气压下均匀辉光放电的可能性

利用介质阻挡电极结构,对101325×105Pa气压下空气间隙中的放电进行了实验研究,数值模拟计算了实验条件下电子雪崩的发展过程.结果表明:对于长度不大于2mm的空气间隙,可能实现辉光放电.对于长度不小于5mm的空气间隙,如果不能设法降低放电场强,放电必然是流注形式,不可能实现辉光放电.另外,实验结果未能验证“离子捕获”机理降低放电场强而实现辉光放电的正确性 关键词： 大气压辉光放电 电子雪崩 流注     

A change in the electro-physical properties of narrow-band CdHgTe solid solutions acted upon by a volume discharge induced by an avalanche electron beam in the air at atmospheric pressure

The effect of a nanosecond volume discharge forming in an inhomogeneous electrical field at atmospheric pressure on the CdHgTe (MCT) epitaxial films of the p-type conduction with the hole concentration 2·10¹⁶ cm³ and mobility 500 cm²·V^–1·s^–1 is studied. The measurement of the electrophysical parameters of the MCT specimens upon irradiation shows that a layer exhibiting the n-type conduction is formed in the near-surface region of the epitaxial films. After 600 pulses and more, the thickness and the parameters of the layer are such that the measured field dependence of the Hall coefficient corresponds to the material of the n-type conduction. Analysis of the preliminary results reveals that the foregoing nanosecond volume discharge in the air at atmospheric pressure is promising for modification of electro-physical MCT properties.     

X-ray radiation due to nanosecond volume discharges in air under atmospheric pressure

The formation of nanosecond discharges in atmospheric-pressure air versus the applied pulse polarity and discharge gap geometry is studied. It is shown that the polarity of high-voltage nanosecond pulses and the electrode configuration have a minor effect on the volume discharges under a variety of experimental conditions. When the spacing between needle-like electrodes is large, the discharge is asymmetric and its glow is weakly dependent on the sign of the potential applied to the electrode. Negative voltage pulses applied to the potential electrode generate X-ray radiation from both the surface and volume. For a subnanosecond rise time of the voltage pulse and diffusion character of the discharge, the X-ray radiation comes from the brightly glowing region of a corona discharge. The average values of the fast electron velocity and energy in nitrogen are calculated. At field strengths E/p < 170 kV/cm atm, the average velocity of a fast electron bunch is constant because of central collisions. At field strengths E/p > 170 kV/cm atm, fast electrons run away. Central collisions are the reason for X-ray radiation from the volume.     

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.     

大气压空气中重复频率纳秒脉冲气体放电模式研究

利用上升沿100ns、脉宽150ns的单级磁压缩纳秒脉冲电源,通过电压电流测量和放电图像拍摄实验,研究了大气压空气中极不均匀电场结构重复频率纳秒脉冲气体放电的放电模式。结果表明纳秒脉冲气体放电存在三种典型的放电模式:电晕放电、弥散放电和火花放电。施加的脉冲电压幅值对放电模式影响显著,随着电压幅值的增加,放电依次经历电晕、弥散和火花放电。固定电压幅值时,放电可能同时存在两种模式。重复频率加强了放电强度,弥散放电的激发电压随重复频率的增加变化不大,但火花放电的激发电压随着重复频率的增加而降低。因此降低重复频率有利于在较大电压范围获得大气压空气弥散放电。     

大气压空气中重复频率纳秒脉冲气体放电模式研究

利用上升沿100 ns、脉宽150 ns的单级磁压缩纳秒脉冲电源,通过电压电流测量和放电图像拍摄实验,研究了大气压空气中极不均匀电场结构重复频率纳秒脉冲气体放电的放电模式。结果表明纳秒脉冲气体放电存在三种典型的放电模式：电晕放电、弥散放电和火花放电。施加的脉冲电压幅值对放电模式影响显著,随着电压幅值的增加,放电依次经历电晕、弥散和火花放电。固定电压幅值时,放电可能同时存在两种模式。重复频率加强了放电强度,弥散放电的激发电压随重复频率的增加变化不大,但火花放电的激发电压随着重复频率的增加而降低。因此降低重复频率有利于在较大电压范围获得大气压空气弥散放电。     

Numerical simulation and experimental validation of direct current air corona discharge under atmospheric pressure

Air corona discharge is one of the critical problems associated with high-voltage equipment. Investigating the corona mechanism plays a key role in enhancing the electrical insulation performance. An improved self-consistent multi-component two-dimensional plasma hybrid model is presented for the simulation of a direct current atmospheric pressure corona discharge in air. The model is based on plasma hydrodynamic and chemical models, and includes 12 species and 26 reactions. In addition, the photoionization effect is introduced into the model. The simulation on a bar-plate electrode configuration with an inter-electrode gap of 5.0 mm is carried out. The discharge voltage– current characteristics and the current density distribution predicted by the hybrid model agree with the experimental measurements. In addition, the dynamics of volume charged species generation, discharge current waveform, current density distribution at an electrode, charge density, electron temperature, and electric field variations are investigated in detail based on the model. The results indicate that the model can contribute valuable insights into the physics of an air plasma discharge.     

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.     

Damage of beryllium surface under irradiation with a high-current electron beam

Radiation damage of a beryllium surface by a high-current electron beam at the GSEP-3 accelerator is considered. The degree of damage of beryllium samples has been determined. The temperature fields inside the sample and the distributions of thermal stresses have been calculated. The reasons for beryllium surface cracking formation have been found. The concentrations of point defects have been calculated. The possible reasons for an increase in microhardness of the irradiated beryllium surface layer are discussed.     

Precisely balanced ionizer using atmospheric pressure glow discharge in air

Ion balance of an ionizer previously developed has been improved, using an atmospheric pressure glow discharge in air as a bipolar ion source. Although the glow ionizer itself has had good performance in ion balance, further we have investigated the possibility of precise control of the ion balance by controlling the discharge itself or the ion flow from the discharge to the object to be neutralized. To control the discharge, we have adjusted the voltage waveform to produce the glow discharge: a dc bias voltage superimposed on a sinusoidal waveform or widths of positive and negative rectangular pulses. Also an adjustable potential of a grid electrode additionally arranged has been employed to control the ion flow. By controlling any of them, we have succeeded in precisely controlling the ion balance of the glow ionizer.     

Sliding discharge in air at atmospheric pressure: electrical properties

Several studies have shown that a surface non-thermal plasma may be used as an electrofluidodynamic actuator for airflow control. For few years, we has been working on this subject, especially in the case of DC corona discharges and AC barrier discharges established at the wall of profiles. The present paper deals with a new type of surface plasma using a sliding discharge. This discharge, excited here by a negative AC voltage with a positive DC component, is created in a three-electrode geometry: one DC positive electrode and two negative AC electrodes at the same voltage. Then a barrier discharge is established between the positive electrode and the first negative one when a surface corona discharge or sliding discharge is generated between the positive electrode and the second negative one. In this preliminary study, the goal is to obtain a stable sliding discharge. Then the electrical properties of this discharge are observed and briefly discussed.     

Numerical simulation and experimental validation of a direct current air corona discharge under atmospheric pressure

Air corona discharge is one of the critical problems associated with high-voltage equipment. Investigating the corona mechanism plays a key role in enhancing the electrical insulation performance. An improved self-consistent multi-component two-dimensional plasma hybrid model is presented for the simulation of a direct current atmospheric pressure corona discharge in air. The model is based on plasma hydrodynamic and chemical models, and includes 12 species and 26 reactions. In addition, the photoionization effect is introduced into the model. The simulation on a bar-plate electrode configuration with an inter-electrode gap of 5.0 mm is carried out. The discharge voltage-current characteristics and the current density distribution predicted by the hybrid model agree with the experimental measurements. In addition, the dynamics of volume charged species generation, discharge current waveform, current density distribution at an electrode, charge density, electron temperature, and electric field variations are investigated in detail based on the model. The results indicate that the model can contribute valuable insights into the physics of an air plasma discharge.     

Dynamic fracture of copper under the action of a relativistic high-current electron beam

This paper presents the results of investigations of the dynamic spall fracture of bulk (2–6 mm) copper targets under the action of a relativistic high-current electron beam (1.3 MeV electron energy, 50 ns pulse duration, ∼10¹⁰ W/cm² power density) generated by the SINUS-7 accelerator. By numerical simulation with the use of the BETAIN1 software package it has been found that the amplitude of the stress wave formed is 6 GPa and the deformation rate is 5·10⁵ s⁻¹. As established experimentally, there is a practically linear relationship between the thickness of the target and the thickness of the spalled layer. Comparison of the experimental data and the simulation results has shown that the spall strength of copper under the given conditions is 1.3 GPa, which is in good agreement with the data available in the literature. Analysis performed with the use of fractographic teqniques has revealed that in the case of recrystallized copper the size of the spall pits formed inside the grains is four times greater than the size of the pits formed on the grain boundaries. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 69–74, July, 2006.     

大气压介质阻挡放电中的自组织斑图结构

采用水作为电极,对大气压介质阻挡放电中的自组织斑图结构进行了研究,观测到了规则的斑图结构(包括准六边形斑图和条带状斑图)和不规则的斑图结构.实验发现斑图中放电丝的密度随着电介质层厚度的增加及放电气隙宽度变大而减小,而随外加电压及频率增大而增大.采用空间相关函数的方法分析了准六边形斑图结构中的放电丝分布与驱动电源电压的关系 关键词： 介质阻挡放电 自组织斑图 放电丝密度 空间相关函数     

较大体积的大气压空气介质阻挡放电特性研究

利用三电极介质阻挡放电装置, 在主放电区产生了较大体积的大气压空气均匀放电. 利用光学与电学方法, 对主放电特性进行了研究, 发现随驱动功率的不同, 主放电存在等离子体羽和等离子体柱两种模式, 等离子体羽的击穿电压随外加电压峰值的增加而减小. 利用光电倍增管对两种放电模式进行了空间分辨测量, 发现等离子体羽是以发光光层的形式传播, 而等离子体柱是连续放电. 通过采集两种放电的发射光谱, 对其振动温度和转动温度进行了测量. 发现两种放电模式的振转温度均随着U_p的增大而降低. 关键词： 介质阻挡放电 等离子体羽 等离子体柱 发射光谱     

Spall fracture of coarse-grained and ultrafine-grained aluminum under nanosecond relativistic high-current electron beam

The results of investigations of fracture behavior in coarse-grained and ultrafine-grained aluminum under the action of a nanosecond relativistic high-current electron beam in a SINUS-7 accelerator and under conditions of quasi-static tensile loading are reported. It is shown that for both types of deformation, irrespective of the grain size, the fracture is ductile both in deformation and structural features. Based on the examination of the fracture surface, it is found out that under quasi-static loading decohesion of coarse-grained and ultrafine-grained aluminum occurs through shear, and under spalling condition by rupture. It is shown for both grain structures that the thickness of the separated layer increases with the irradiated specimen thickness. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 32–38, December, 2007.