The role of multielectrode geometry in the generation of pulsedintense electron beams in preionization-controlled open-endedhollow-cathode transient discharges

High-voltage hollow-cathode glow discharges are used more and more to generate intense, pulsed electron beams. Such intense electron beams can be produced with high efficiency in preionization-controlled open-ended hollow-cathode transient discharges (PCOHC). This novel discharge is initiated by a low-current dc preionization discharge. The beam parameters are similar to those of the electron beam generated in pseudospark discharges. In this work, we present some measurements of the parameters for the electron beam generated by using a multielectrode (multigap) system instead of the single-gap device in this PCOHC configuration. This kind of multielectrode device was already used in pseudosparks to improve the intensity and collimation of the extracted beam. By using the multigap instead of the single gap, the total beam current (100-120 A) and the energetic part of the beam current (peak current 60-90 A and electron energies higher than approximately 3 keV) were substantially increased. However, the energy spectrum of the fast component has a large fraction of electrons at lower energies (4-10 keV for 26 kV breakdown voltage) when a multigap device is used instead of the single-gap configuration. A comparison between the single-gap and multigap PCOHC-produced pulsed intense electron beam is made too. The differences between the high-power pulsed electron beams produced in single-gap and multigap PCOHC configurations seem to be due to different developments of beam generation phases     

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.     

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     

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。     

不同入射角度下强流脉冲电子束能量沉积剖面和束流传输系数模拟计算

 强流脉冲电子束在材料中的能量沉积剖面、能量沉积系数和束流传输系数受其入射角的影响很大，理论计算了0.5~2.0MeV的电子束以不同的入射角在Al材料中的能量沉积剖面和能量沉积系数，并且还计算了0.4~1.4MeV电子束以不同入射角穿透不同厚度C靶的束流传输系数。计算结果表明，随着入射角的增大，靶材表面层单位质量中沉积的能量增大，电子在靶材料中穿透深度减小，能量沉积系数减小，相应的束流传输系数也减小；能量为0.5~2.0MeV的电子束当入射角在60°~70°时在材料表面层单位质量中沉积的能量较大。     

Pseudospark produced pulsed electron beam for material processing

An intense pulsed electron beam produced by a pseudospark discharge is used for material processing. The electron beam propagates in a self-focused manner in the background gas. Hardly 12 ns after the beginning of the discharge the fraction of space charge neutralization is about 96%. To sustain the neutralization effect high energy electrons (E <500 keV) are accelerated in radial direction at the beam head, due to strong electric field gradients. At current maximum the beam pinches due to its own magnetic field. At peak current of 400 A and charging voltage up to 16 kV power density reaches 10⁹ W/cm ² on the target surface. Some results of copper thin films are presented. Due to the high expansion velocity of 10⁴ m/s of the ablated target material a copper-matrix has been masked     

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     

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.     

High-Power Modulated Intense Relativistic Electron Sources with Applications to RF Generation and Controlled Thermonuclear Fusion

Recent advances in the physics and technology of the modulated intense relativistic electron beams (IREB's) are reviewed in this paper. Bunched dense electron beams can be used to construct high-power RF sources, which may critically affect future progress in fusion technology. In this paper a system is described in which electrical energy can be converted from a single pulse of relatively long duration into a series of subpulses of short duration (nanosecond and subnanosecond) and of high power (~1010 W). This electrical system consists of an IREB propagating through passive structures. The mutual interaction between the electron beam and one passive structure modifies the IREB so that power compression and beam modulation occur. When the modified IREB interacts with the next passive structure, the kinetic energy of the electrons is converted into electrical energy or RF energy. The beam current modulation depends on the injected IREB and the structure parameters. A 100-percent modulation of the current has been achieved. A single-beam source may be used for exciting radiation in a frequency range of 60 MHz to 10 GHz. In the frequency range of 60-750 MHz a modulated beam with power ~1010 W has already been achieved. IREB modulation at a frequency of ~3 GHz was performed and RF energy was extracted from the bunched beam with power output of 5 × 108 W.     

Space-time-dependent development of the plasma in a pulsedhollow-cathode discharge

The fast (nanosecond) breakdown of hollow-cathode discharges is studied. Streak camera investigations are presented on the space-time-dependent development of pulsed discharges, starting from low-current preionization discharges. The discharges start near the entrance to the cathode, then move further back into the cathode, and then spread over a wider range along the axis of the cathode. The depth range of the intense pulsed hollow-cathode plasma was found to be two to eight times the cathode diameter     

强流脉冲电子束与固体介质相互作用的研究

采用强流脉冲电子束轰击固体介质样品,研究了强流脉冲电子束与固体介质的相互作用。对晶体中出现的开裂和有机聚合物中出现的树枝状径迹进行了讨论,认为电子束的能量及电荷沉积导致了上述现象。     

Measurements on electron beams in pulsed hollow-cathode discharges

Two different regimes of electron beams have been reported in the pulsed hollow cathode discharge-a low-current, high-energy beam, and a relatively high-current beam of low energy. The high-energy beam is related to the hollow cathode geometry and is found to be present even in the absence of subsequent gas breakdown, while the low-energy beam is always associated with voltage breakdown. Detailed measurements of the spatial and temporal distribution of the electron beam transported beyond a semitransparent anode associated with gas breakdown are reported. In particular, a high-energy component is observed after electrical breakdown. Low-energy electron beams are observed to be transported beyond the anode throughout the main discharge period     

天鹅绒与碳纳米管阴极强流脉冲发射特性

在2 MeV直线感应加速器注入器平台上开展了天鹅绒阴极与碳纳米管阴极的强流脉冲发射特性综合实验。研究结果表明:天鹅绒阴极与碳纳米管阴极均具有强流脉冲发射性能,在1.61 MV的二极管电压下,天鹅绒阴极与碳纳米管的发射电流密度分别为84,108 A/cm2;启动时间分别为21,40 ns;放气量分别为0.29,0.91 PaL;放出气体分子数目与发射电子数目之比分别为64,225;两种冷阴极强流脉冲发射时的放气质谱相似。     

Beam electron energy distribution at a volume nanosecond discharge in atmospheric-pressure air

The energy distributions of beam electrons and x-ray photons in a volume nanosecond discharge on atmospheric-pressure air are studied. Several groups of elevated-energy electrons are found. It is shown that electrons with an energy from several tens to several hundreds of kiloelectronvolts (which is lower than a maximal voltage across the gap) make a major contribution to the beam current measured behind thin foils. It is corroborated that fast electrons (with an energy from several kiloelectronvolts to several tens of kiloelectron-volts) arise 100–150 ps before the basic peak of the beam current, elongating the current pulse and significantly increasing its amplitude. The contribution from electrons with an anomalously high energy (exceeding a maximal voltage across the gap) to the beam current is shown to be insignificant (less than 5%). The x-ray spectra in gas-filled diodes of different design are studied. Techniques of measuring the subnanosecond electron beam current and mechanisms generating fast and runaway electrons in volume high-pressure gas discharges are analyzed.     

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     

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。     

Hollow-cathode magnetron discharge as a generator of charged particles

A magnetron discharge with a cold hollow cathode and an uncooled rod cathode is studied. It is shown that such a discharge can be efficiently used to generate a plasma emitting charged particles. For a discharge current of 2 A and an accelerating voltage of 10 kV, ion and electron emission currents of 0.1–0.15 and 1 A, respectively, are achieved. The energy cost of ion extraction is 1–2 W/mA, which is two to five times less than for typical ion sources, and the energy efficiency is 15 mA/W, which is a factor of five or six higher than for electron emitters based on a hollow-cathode reflex discharge.     

Intense ion beams for inertial confinement fusion

Intense beams of light and heavy ions are being studied as inertial confinement fusion (ICF) drivers for high yield and energy. Heavy and light ions have common interests in beam transport, targets, and alternative accelerators. Self-pinched transport is being jointly studied. This article reviews the development of intense ion beams for ICF. Light-ion drivers are highlighted because they are compact, modular, efficient and low cost. Issues facing light ions are: (1) decreasing beam divergence; (2) increasing beam brightness; and (3) demonstrating self-pinched transport. Applied-B ion diodes are favored because of efficiency, beam brightness, perceived scalability, achievable focal intensity, and multistage capability. A light-ion concept addressing these issues uses: (1) an injector divergence of ⩽24 mrad at 9 MeV; (2) two-stage acceleration to reduce divergence to ⩽12 mrad at 35 MeV; and (3) self-pinched transport accepting divergences up to 12 mrad. Substantial progress in ion-driven target physics and repetitive ion diode technology is also presented. Z-pinch drivers are being pursued as the shortest pulsed power path to target physics experiments and high-yield fusion. However, light ions remain the pulsed power ICF driver of choice for high-yield fusion energy applications that require driver standoff and repetitive operation     

Nuclear fragmentation with 100 MeV α-particles

On the basis of low pressure spark discharges a new type of a particle accelerator as been developed. It produces pulsed ion as well as electron beams of high intensities in a gas atmosphere at pressures of the order of 1 mbar. The method was used to produce in hydrogene at particle energies of about 70 keV an electron beam with a current density of more than 10⁶ A/cm² at 140 A total current. It is shown that magnetic confinement by the pinch effect takes place in the discharge. Discharge times smaller than 5 ns and spark frequencies up to 2 MHz can be obtained.     

S波段速调管双间隙输出腔的设计和实验研究

 介绍了S波段强流相对论速调管放大器(RKA)双间隙输出腔高频系统的设计,并利用3维粒子模拟程序模拟和优化了短脉冲强流相对论调制电子束经过双间隙输出腔后的微波提取。在束压640 kV、束流6 kA、基波调制深度80%的条件下,模拟得到功率为1.1 GW的微波,频率约为2.85 GHz,效率28%。在高频分析和粒子模拟的基础上进行了实验研究,实验中采用束压640 kV、束流6 kA的环行电子束,经过优化调节RKA参数,在中间腔后得到了约4.6 kA的基波调制电流,加上双间隙提取腔后从该RKA获得了频率为2.9 GHz、功率为1 GW、脉宽22 ns的输出微波,束波转换效率26%。