An Observation of Energetic Electron Beams in Low-Pressure Linear Discharges

Experimental observations of energetic axial electron beams in a linear Z pinch operating in the pseudospark mode are presented. The device is driven from a fast Marx generator and allows reproducible production of electron beams over a wide pressure range. Evidence of the importance of electrons generated in the cathode recess in the formation of the beams is presented. An electron beam of high energy which is not associated with formation of the discharge is identified. A second beam of high current density and lower energy associated with gas breakdown is also observed.     

Hollow cathode effects in charge development processes in transienthollow cathode discharges

A detailed experimental study of space charge formation and ionization growth in transient hollow cathode discharges (THCD) is presented. The experiment was performed with an applied step voltage up to 30 kV, with rise time less than 50 ns. The discharge was operated in different gases, at pressures in the range 50-750 mTorr, with cathode apertures ranging from 1 to 5 mm diameter and 5 to 20 mm long, with 10 cm electrode separation. Spatial charge formation, both in the hollow cathode region (HCR) and inter electrode space, has been studied with a capacitive probe array. Properties of high energy electron beams have been measured with a beam-target scintillator-photomultiplier arrangement. Detailed correlations of the electron beam evolution with the charge probe signals inside and outside the HCR clearly demonstrate the role of the electron beam in the initial formation and late evolution of a virtual anode and, in turn, the field enhanced ionization when the anode potential is brought close to the HCR. These results clearly identify the different regimes in which the Hollow Cathode plays a significant role in ionization growth in the inter electrode space and in the processes which eventually lead to electric breakdown     

Breakdown formation in a transient hollow cathode discharge-astatistical study

Discharge formation at low pressure is found to be greatly influenced in the presence of a suitable hollow cathode region. The formation of a moving virtual anode which extends the anode potential to within the hollow cathode region is thought to be responsible for the enhanced ionization growth which subsequently leads to gas breakdown. In this paper, the spatial evolution of the local potential in the discharge region of a pulsed hollow cathode discharge has been measured in a range of pressures with two different cathode apertures. An extensive data set has been collected and analyzed using a statistical technique. From the characteristic of the statistical distribution of the data, unique features associated with the role of hollow cathode at the different stages of discharge formation have been identified. It was found that the influence of the hollow cathode region is strongest in the start of ionization growth and in the final change over to high current breakdown     

Low-impedance plasma systems for generation of high-current low-energy electron beams

The results of experimental investigation and numerical modeling of the generation of low-energy (tens of keV) high-current (up to tens of kA) electron beams in a low-impedance system consisting of a plasma-filled diode with a long plasma anode, an auxiliary hot cathode, and an explosive emission cathode. The low-current low-voltage beam from the auxiliary cathode in an external longitudinal magnetic field is used to produce a long plasma anode, which is simultaneously the channel of beam transportation by residual gas ionization. The high-current electron beam is formed from the explosive emission cathode placed in the preliminarily formed plasma. Numerical modeling is performed using the KARAT PIC code.     

Collimation with hollow electron beams

A novel concept of controlled halo removal for intense high-energy beams in storage rings and colliders is presented. It is based on the interaction of the circulating beam with a 5-keV, magnetically confined, pulsed hollow electron beam in a 2-m-long section of the ring. The electrons enclose the circulating beam, kicking halo particles transversely and leaving the beam core unperturbed. By acting as a tunable diffusion enhancer and not as a hard aperture limitation, the hollow electron beam collimator extends conventional collimation systems beyond the intensity limits imposed by tolerable losses. The concept was tested experimentally at the Fermilab Tevatron proton-antiproton collider. The first results on the collimation of 980-GeV antiprotons are presented.     

Predicted emittance and brightness of the pseudospark electron-beam

The emittance and brightness of the electron beam generated during the hollow cathode phase of pseudospark operation are calculated using the two-dimensional hybrid fluid-particle model previously developed to study the time and space development of the plasma in a pseudospark discharge. Two distinct energy components exist in the electron beam; a high-energy component with an energy equivalent to the full discharge voltage and another, broad, low-energy component. In the 100 ns following breakdown and for the conditions of the calculations, the emittance of the high energy component decreases by an order of magnitude and the brightness of the high energy component reaches almost 10¹⁰ A/m² rad². This work demonstrates the feasibility of using the model to guide the optimization of the pseudospark electron beam properties and shows that the optimum beam properties are achieved after the plasma has filled the hollow cathode and begun to expand radially in the main gap     

Time-of-flight mass spectrometry studies of an ion beam generated by the titan source

The TITAN source generates wide-aperture beams of gas and metal ions of different materials. This is achieved because of two types of cold-cathode arc discharges, which operate simultaneously in the discharge system of the source. For metal ions to be obtained, use is made of the vacuum arc initiated between an ion-forming cathode and a hollow anode. To produce gas ions, a constricted low-pressure arc discharge is initiated with the same hollow anode. The constitution of ion beams generated by the TITAN source has been investigated using a homemade time-of-flight spectrometer. This paper describes the design of the latter and the principle of its operation, and discusses the physical peculiarities of the spectrometer operation, which affect the ion beam constitution. Institute of High Current Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 21–28, February, 2000.     

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.     

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。     

High current electron beam generation in a vacuum transient hollowcathode discharge

Experimental observations are presented of prebreakdown electron beam generation in a transient hollow cathode discharge (THCD) in a vacuum. The discharge driver consists of a 400-kV maximum voltage, 25-nF Marx operated at 450-J stored energy coupled to a 120-ns, 1.5-Ω coaxial line. Electron beams with peak currents up to tens of kA are observed when a pulsed Nd:YAG laser is used to produce a plasma at the back of the cathode surface, inside the hollow cathode region (HCR). It is found that a plasma density of a few 10¹⁸ cm^-3 in a volume of a few mm³ is required to generate intense electron beams. Optimal conditions are determined by varying the position of the laser focal spot inside the HCR and the time delay between the laser and the applied voltage. The main features of the electron beams are similar to those observed in conventional THCD at pressures in the 10-200 mtorr range     

Observation of enhanced prebreakdown electron beams in a vacuumspark with a hollow-cathode configuration

The generation of prebreakdown electron beams in a low-energy vacuum spark with a hollow-cathode configuration is observed under a range of experimental conditions. The vacuum spark studied is powered by either a 25-kV, 3.3-nF single capacitor discharge or a two-stage, 50-kV, 1.65-nF Marx. The electron beams are detected by observing the X-ray emission from the anode tip produced by electron impact. Results show that an electron beam is formed well before the onset of the electrical breakdown. This prebreakdown electron beam has an initial slow buildup phase followed by an exponential rise, leading to the breakdown of the discharge. This behavior of the electron beam evolution is in good qualitative agreement with the model simulation of the pseudospark phenomenon obtained for a transient hollow-cathode discharge     

空心阴极等离子体电子枪研究

对空心阴极等离子体电子枪的理论与机理做了较详细的分析，介绍了空心阴极放电特性，论 述了激励电极和调制电极在空心阴极内等离子体形成过程中的作用，分析了等离子体中电子 和离子的运动及主要参数，推导出空心阴极内电场与电流密度的表达式，研究了形成稳定电 子束流的基本条件.利用泊松方程、电流连续性方程和运动方程对其进行了数值模拟计算， 并给出了优化结果. 在此基础上设计出了输出束流脉宽为1μs、幅值达2kA的空心阴极等离 子体电子枪. 关键词： 空心阴极 等离子体 电子枪     

High-Efficiency Electron Source with a Hollow Cathode in Technologies of Thin Film Deposition and Surface Treatment under Forevacuum Pressures

It is shown that low-energy beams with a high efficiency in a wide range of beam currents can be obtained in electron sources with a hollow cathode in the forevacuum pressure range. By varying the geometrical parameters of the electrode system and electromagnetic optics of the electron source, we succeeded in reaching the efficiency at a level of 0.9 for an accelerating potential of 1 kV and beam currents from 100 to 300 mA. The parameters affecting most strongly the efficiency and stability of operation of the electron source with a hollow cathode have been determined.     

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.     

Formation of narrow-focused electron beams generated by a source with a plasma cathode in the forevacuum pressure range

Results are presented from experimental studies of the formation of focused electron beams produced by extracting electrons from the plasma of a steady-state discharge with a hollow cathode in the forevacuum pressure range. Based on the measurements of the energy spectrum and diameter of the electron beam, as well as of the emission parameters of the plasma produced in the course of beam-gas interaction, a conclusion is drawn about the excitation of a beam-plasma discharge that deteriorates the beam focusing conditions. The threshold beam current density for the excitation of a beam-plasma discharge is found to increase with accelerating voltage and gas pressure.     

Time evolution of hollow cathode ionization processes in the finalbreakdown phase of a transient hollow cathode discharge

The enhanced ionization processes taking place inside the hollow cathode region (HCR) of a transient hollow cathode discharge (THCD) are essential events which lead to final electrical breakdown. This ionization growth is permanently assisted by a virtual anode moving in the anode-cathode gap (A-K gap), which extends the anode potential to within the hollow cathode region. In the paper, the ionization growth inside the HCR under the enhanced field due to the close proximity of the anode potential has been studied using a statistical technique in a range of pressures, with three different cathode apertures. Statistical time distributions of an extensive experimental data set are analyzed to understand the mechanisms involved in the final stages, just before electric breakdown     

Analysis of the hollow cathode emitted electron beams in a radialmultichannel pseudospark

A radial multichannel pseudospark switch seems advantageous for switching currents exceeding 100 kA. A single common hollow cathode guarantees the same trigger conditions for all channels. The electron beams emitted out of the hollow cathode of this switch are analyzed by means of Faraday-cups by varying the circuit as well as the trigger parameters. In addition to the cup measurements, the discharge is observed using a CCD camera. It is found that each channel of the switch must receive a respective beam, before the resistance of the switch turns low, if the discharge current is to run through that channel. In addition, most of the varied parameters do not influence the electron beam     

Determination of electron density and energy influx in a hollow cathode glow discharge used for powder modification

The electron density and energy influx in an argon hollow cathode glow discharge were determined to obtain adequate parameters for subsequent surface modification of low density polyethylene (LDPE) powder to change the wettability. The electron density was studied by Langmuir probe measurement in dependence on process gas pressure and hollow cathode material. Besides the determination of the rate of increasing electron density with input power an optimal experimental pressure was determined. The energy influx was studied by thermal probe measurements in dependence on process gas pressure, bias voltage, axial position and hollow cathode material. Inside the hollow cathode the energy influx is nearly constant along the whole cathode length. With increasing pressure the energy influx decreased. At biased thermal probe the energy influx was observed to decrease up to the floating potential and beyond it increases with increasing voltage. Using different hollow cathode materials the electron density as well as the energy influx reach higher values for aluminum than for copper and stainless steel.     

Relativistic electron beams detection in a dense plasma focus

Fast electron beams into a hollow anode of a small plasma focus machine (2 kJ, 4 μF) were measured. The diagnostic method designed for this purpose is founded in a small Rogowski coil introduced into a cavity performed in the anode. By means of this, electron beam pulses of about 10 ns width generated in the plasma focus are detected. Simultaneously, hard X-ray signals obtained from a scintillator-photomultiplier system are registered. The electron beam energy was measured through the time-of-flight of the electrons between probe and anode top. The beams are found to be relativistic and its energy is into the range of hard X-rays energy. An analysis of signal intensities and relative delays for three hundred shots are here presented. Received 28 February 2002 / Received in final form 7 May 2002 Published online 24 September 2002     

猝发强流多脉冲电子束源物理设计

给出一台脉冲间隔100～1 000 ns、脉冲数2~5个、二极管电压3 MV、引出束流强度2.5 kA的猝发多脉冲电子束源的物理设计及初步调试结果。在设计中,采用感应叠加和阻抗匹配方案获得二极管高电压脉冲;试验中分别采用天鹅绒和大发射面储备式热阴极获得猝发多脉冲电子束。调试结果表明:采用大发射面热阴极可避免阴极等离子体产生,确保二极管在猝发多脉冲状态下稳定运行。初步调试获得大于2.7 MV猝发三脉冲二极管高压,并获得1.6 kA的三脉冲电子束流。