Two-pulse generation of high-current relativistic electron beams and their transportation in the gaseous medium of a plasmochemical reactor

Experimental results on two-pulse generation and transportation of high-current relativistic electron beams (REBs) through the gaseous medium of a plasmochemical reactor (PR) are presented. The generation of two consecutive high-current REB pulses with a duration of 60 ns was achieved at the Tonus accelerator with modified schemes of high-voltage pulse formation. The first version of the formation scheme enabled pulse powers of 2 and 4.0–9.6 GW with a time interval between the pulses of 500 ns. The second version enabled one to generate pulses with powers of 1.8 and 16 GW and time interval between the pulses of 160 μs. The transportation parameters of an REB injected into a 1.4-m-long PR filled with an N₂: O₂ gas mixture are studied. The conductance of the plasma produced under the action of the electron beams is measured. It is shown that the schemes proposed provide more efficient (by 35–45%) transportation of the REBs in the reactor volume as compared to single-pulse high-current REBs of the same power and pulse duration.     

Control of gigawatt REB pulse duration using gas injection

A technique for controlling the voltage and current pulse duration of a gigawatt relativistic electron beam (REB) was investigated. The pulse duration of a beam of 250 keV and 10 kA was controlled from 60 ns to 10 ns by injecting gases (air,, argon, and hydrogen) into the diode gap at a pressure ranging from 10^-5 torr to 10^-1. The observed dependence of the pulse duration on the nature and pressure of the gas is explained in terms of volume ionization of the gas by beam electrons. It is concluded that the pulse duration is governed by the time at which the plasma density created by the beam reaches a critical value of the order of beam electron number density     

Study of the influence of the injection section on the stability of a high-current relativistic electron beam propagating through the gaseous medium of a plasmochemical reactor

Results are presented from experimental study of the influence of the injection section on the stability of a high-current relativistic electron beam (REB) propagating through the gaseous medium of a plasmochemical reactor (PR). An REB with the electron energy E _e =1 MeV, beam current I _b =10–22 kA, and pulse duration t=60 ns was generated by the Tonus accelerator and, then, injected into a 0.1-m-diameter PR filled with air. The PR consisted of two sections with lengths L ₁= 0.3–1.0 m and L ₂=1.1–2.5 m; the total length of the system was no longer than 3.5 m. The first section was filled with air at a pressure of P ₁=0.8–1.5 torr, and the pressure in the second section was varied within the range P ₂=0.1–760 torr. The current I _b of an REB passed through both sections of the PR was measured with the help of a sectioned vacuum Faraday cup. The transportation efficiency of the beam was determined as the ratio I _b /I _inj, where I _inj is the beam current measured at the point of injection into the PR. It is shown that, for the optimal dimensions of the first PR section, it has a stabilizing action on the REB with a current density of up to 3 kA/cm², which makes it possible to increase the effective length of the second (working) PR section, which is filled with a gas at various pressures, to L ₂=(25–35)L _bet, where L _bet is the beam betatron length.     

Ion-focused propagation of a relativistic electron beam in the self-generated plasma in atmosphere

It is known that ion-focused regime (IFR) can effectively suppress expansion of a relativistic electron beam (REB). Using the particle-in-cell Monte Carlo collision (PIC-MCC) method, we numerically investigate the propagation of an REB in neutral gas. The results demonstrate that the beam body is charge neutralization and a stable IFR can be established. As a result, the beam transverse dimensions and longitudinal velocities keep close to the initial parameters. We also calculate the charge and current neutralization factors of the REB. Combined with envelope equations, we obtain the variations of beam envelopes, which agree well with the PIC simulations. However, both the energy loss and instabilities of the REB may lead to a low transport efficiency during long-range propagation. It is proved that decreasing the initial pulse length of the REB can avoid the influence of electron avalanche. Using parts of REB pulses to build a long-distance IFR in advance can improve the beam quality of subsequent pulses. Further, a long-distance IFR may contribute to the implementation of long-range propagation of the REB in space environment.     

Angular velocity spread of relativistic electron beam generated by high current diode

Theoretical analysis and numerical simulations of the Relativistic Electron Beam (REB) generation in a high current diode immersed in an external magnetic field has been done. The calculations have confirmed that the generated beam is homogeneous and monoenergetic in a broad central region. In the case of cylindrical diode the mixing of electron trajectories has been observed only in a narrow periphery beam region. The angle between particle trajectories and external longitudinal magnetic field varies chaotically from 0° to –25°. This phenomenon suppresses the excitation of two stream instability excited by the REB in the plasma column.     

Mode excitation in a generator of diffraction radiation with a multistage interaction space

We present the results of studying theoretically and experimentally the peculiarities of mode excitation in a generator of diffraction radiation with the two- and three-cascade interaction space in one open resonator. To describe the multistage interaction mechanism, the theoretical model used considers the excitation of the TEM_0nq modes, which have n in-phase field “spots,” i.e., cascades, along the direction of the electron beam motion. It is shown that even the use of the two-cascade interaction space with the TEM_01q modes allows one to reduce the starting current of the generator by 1.91 times. The results of studying experimentally the two-cascade generator in the 5 and 1.5-mm wavelength ranges are presented. The in-phase property of the electron beam with the resonance field was achieved by disrupting the periodicity of the lattice slots at the center of the interaction space. The output power of the two-stage generator of 5-mm diffraction radiation in the TEM013 mode ranged from 3 to 5 W. We also observed a 4–5-fold increase in the starting current for the regime of surface waves.     

Relativistic electron beam generation in a plasma-filled diode withfoilless injection into a dense plasma

In the experiments an relativistic electron beam (REB)-plasma interaction, the foilless injection of REB from a magnetized diode is of special interest due to the low spread angle of the beam and high repetition rate of the shots. In the experiments presented, the problem of diode shortening in the presence of a dense plasma from the interaction chamber has been solved using a special drift pipe as an anode of the foilless diode. The electron beam (U_d~0.7 MeV, t _b~100-200 ns) produced by an axially symmetric magnetically insulated diode has been injected into a magnetized hydrogen plasma column with density ranging from 1·10¹⁵-3·10 ¹⁶ cm^-3. It has been found that the anode pipe substantially reduces the plasma flow into the diode gap, but does not stop it completely, thus the REB generation in a plasma-filled diode has taken place. In some regimes of the beam generation it becomes possible to increase significantly the injected current and total energy deposition of the beam in comparison with the case of a vacuum magnetized diode of the same geometry. The experiments have shown effective dense plasma heating under the foilless injection     

电子束泵浦百焦耳级准分子激光实验研究

利用“闪光二号”相对论电子束加速器研制了百焦耳级ＸｅＣｌ和ＫｒＦ准分子激光器，激光器激活体积２４ｌ，输出口径２０×２０ｃｍ２，泵浦功率密度１．５～２．０ＭＷ／ｃｍ３。ＸｅＣｌ激光器最大能量１３６Ｊ，波长３０８ｎｍ，脉宽５０～１００ｎｓ，峰值功率１．５ＧＷ，采用虚共焦非稳腔技术，束散角达１．３ｍｒａｄ；ＫｒＦ激光最大能量１５７Ｊ，波长２４８ｎｍ，脉宽８０～１００ｎｓ，峰值功率２ＧＷ。     

The influence of an electronegative gas admixture on the intenseE-beam transport and induced plasma currents in argon

The propagation of an electron beam, with electron energy ~1.2 MeV, beam current ~8 kA, and an induced plasma current is investigated using a metal drift tube ~0.5 m long filled with argon at atmospheric pressure. The behavior of electron concentration and electron temperature was calculated for several concentrations of the SF₆ admixture. The comparison of the results of the experiments and numerical calculations shows that the dropping of plasma current when electronegative gas is added is due to the electron attachment to the SF ₆ molecules. The results of the experiment and the calculations provide the evidence of at least three mechanisms responsible for the increase of the REB transport efficiency when electronegative gas is present     

Collisional effect on the Weibel instability in the limit of high plasma temperature

The Weibel instability (WI) of relativistic electron beam (REB) penetrating an infinite collisional plasma was studied in the following models: (i) REB model, where the total equilibrium distribution function f ₀($ \vec p $ \vec p ) is approximated by nonrelativistic background electron and REB distribution functions and (ii) relativistic monoenergetic beam (RMB) model, where f ₀($ \vec p $ \vec p ) is approximated by nonrelativistic background electrons and RMB distribution functions.     

MC55低阻抗强流脉冲电子加速器研制

介绍了MC55低阻抗强流脉冲电子加速器的结构及主要技术参数,给出了加速器的初步调试结果。该加速器由Marx发生器、10Ω水介质单筒形成线、高压自击穿气体开关、水介质传输线和真空二极管组成。经过初步调试,该加速器可以产生电压500 kV,电流50 kA,脉冲宽度50 ns的强流电子束。目前MC55加速器已应用于同轴虚阴极振荡器高功率微波源的实验研究。     

Temporal extension of stable glow discharges in fluorine-based excimer laser gas mixtures by the addition of xenon

The effect of addition of xenon on the long term homogeneity of discharges in F₂and ArF excimer laser gas mixtures was investigated in a small-volume discharge chamber. The gas mixture in the discharge chamber was preionized by X-rays. A special electrical excitation circuit containing a pulse forming line provided a long, square-shaped current pumping pulse of a predetermined duration to the discharge electrodes. The initiation and the development of the discharge was monitored via its fluorescence signal with an intensified CCD camera. We found that adding Xe up to partial pressures of 0.53 mbar extended the homogeneous phase of the discharge from 80 ns to approximately 200 ns in He/F₂as well as in He/Ar/F₂and Ne/Ar/F₂excimer laser gas mixtures. Monitoring of the ArF and XeF spontaneous emission signals showed that the formation of ArF excimers remained unaffected by the addition of xenon (up to 1.3 mbar) to the laser gas mixture.     

Interaction of a relativistic electron beam with a spiral waveguide

The nonlinear period in the interaction of a relativistic electron beam (REB) with an electromagnetic wave in a spiral waveguide at the stage of saturation is investigated for a Cherenkov interaction mechanism. On the basis of the kinetic equation and laws of energy and momentum conservation, analytical relations are obtained, allowing the efficiency of excitation and amplification of the electromagnetic wave to be studied using REB in a retarding structure. The results of the analysis are in good agreement with the data of actual and numerical experiments.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 31–36, May, 1988.It remains to thank É. A. Perel'shtein, Yu. I. Aleksakhin, and their colleagues for discussion of the results.     

X-ray source for irradiation of large-area objects

The results of experiments with a three-ring large-area diode that were conducted on an MIG pulse generator are reported. The MIG generator makes it possible to produce in a matched load electrical pulses up to 2 TW in power with an FWHM of 50–60 ns (1.2–1.4 TW and 80–90 ns in our experiments). In the operating mode of the generator, the current amplitude through the load is 2 MA (the current of a relativistic electron beam) at a diode voltage of ≈ 500 kV. As a load, a large-area vacuum diode with three ring-shaped cathodes is used. It is shown that about 20% of the energy stored in the capacitor bank can be converted to the energy of a relativistic electron beam by matching the output resistance of the MIG generator to the load resistance. When the beam slows down on a condensed foil target, the parameters of the resulting source are the following: the mean energy of X-ray quanta is ≈ 70 keV; irradiated area, 500 cm²; pulse FWHM, 65 ns; energy flux in the spectrum, 2 J/cm²; and percentage of X-ray radiation (10–100 keV) in the flux, ≈ 50%.     

Modeling of terahertz radiation emission from a free‐electron laser

In this article, we report the generation of terahertz (THz) radiation using the interaction of a laser‐modulated relativistic electron beam (REB) with a surface plasma wave. Two laser beams propagating through the modulator interact with the REB, leading to velocity modulation of the beam. This results in pre‐bunching of the REB. The pre‐bunched beam travels through the drift space, where the velocity modulation translates into density modulation. The density‐modulated beam, on interacting with the surface plasma pump wave, acquires an oscillatory velocity that couples with the modulated beam density to give rise to a nonlinear current density which acts as an antenna to give THz radiation. By optimizing the parameters of the beam and the wiggler, we obtain power of the order of 10⁻⁴ using the current scheme.     

Spectroscopic Investigation of a Double Discharge Pulsed Electron Beam Generator

ABSTRACT

The wide range of applications of the plasma-based electron beam generator make it necessary to diagnose the device with a noninterfering method. The results of experimental and modeling studies of neutral helium and hydrogen beta spectral lines emitted from the double discharge pulsed electron beam generator are presented in this paper. Neutral helium lines emitted from the plasma in the pressure range 0.1–0.4 torr are studied and compared with results of the collisional radiative model. The duration of the electron beam is shorter than 100 ns, and the peak current intensity is of order amperes. The full width at half maximum of the H _β spectral line is used for the determination of the plasma electron density, found as 3.16 × 10²¹ m^?3 at 0.3 torr, and good agreement is obtained by comparing with the full computer simulation method.     

Diamagnetic and Paramagnetic Effects in Free-Electron Lasers

The effect of high-current relativistic electron beams (REB's) on the undulator field amplitude in free-electron lasers (FEL's) is investigated. Two mechanisms of excitation of periodic magnetostatic self-fields by REB are considered: 1) a static mechanism that is realized at stationary motion of REB in the undulator field; and 2) a dynamic mechanism that is realized at signal wave amplification. The static mechanism in the absence of an axial magnetic field leads to a decrease of the total undulator field amplitude (a diamagnetic effect). The dynamic mechanism for low-density beams leads to an increase of the total undulator field amplitude (a paramagnetic effect), with a subsequent increase of electron efficiency. For high-density beams, the effect of the phase shift of the total undulator field is most essential, due to which the growth of the signal wave amplitude is limited by nonlinear mismatch of synchronism.     

Electrical characterization of dielectric barrier discharge driven by repetitive nanosecond pulses in atmospheric air

Dielectric barrier discharge (DBD) is an important method to produce non-thermal plasma, which has been widely used in many fields. In the paper, a repetitive nanosecond-pulse generator is used for the excitation of DBD. Output positive pulse of the generator has a rise time of about 15 ns and a full width at half maximum of 30–40 ns, and pulse repetition frequency varies from single shot to 2 kHz. The purpose of this paper is to experiment the electrical characteristics of DBD driven by repetitive nanosecond pulses. The variables affecting discharge conditions, including air gap spacing, dielectric thickness, barrier fashion, and applied pulse repetition frequency, are investigated. The relationship between electric field, discharge current, instantaneous discharge power across air gap, and estimated electron density with the length of air gap, dielectric thickness, barrier fashion, and pulse repetition frequency is obtained respectively, and the experimental results are also discussed. In addition, two typical images exhibiting homogeneous and filamentary discharges are given with different experimental conditions.     

基于连续谱的氩气纳秒脉冲放电中电子温度的研究

采用了一种针对针的放电结构,将其放置在一个高纯氩气的密闭腔室中,通过施加正极性的过电压产生可重复的大气压纳秒脉冲放电,并提出建立大气压放电的连续辐射模型来诊断氩气纳秒脉冲放电中的电子温度。实验利用电压和电流探头分别获取放电过程中的电压和电流波形图,其放电脉宽约为20 ns。通过消色差透镜、单色仪和ICCD等光学系统的组合来测量放电正柱区在不同时刻（0<t<20 ns）的时间分辨发射光谱。结果表明,放电中连续谱的强度随时间先增加（0<t<10 ns）后减小（10 ns<t<20 ns）,但是氩原子的谱线强度则随时间的增加而一直增大。研究表明连续谱强度与电子密度成正相关,因而电子密度随着时间也是先增加而后减小,这与放电电流的变化规律是完全一致的。根据连续谱模型拟合得到放电过程中（0<t<10 ns）的电子温度为(1.4±0.2) eV。随着驱动电压的下降（10 ns<t<20 ns）,电子温度逐步减小至0.9 eV。在0<t<10 ns中,激发态氩原子主要是由电子碰撞激发产生的,因而谱线强度随着电子密度的增加而增大。然后,随着电子温度的减小,Ar+₂复合反应速率激增,导致电子与离子的复合过程主导产生激发态氩原子,即谱线强度继续增大。通过加入0.5%的水蒸气以获取OH的振转光谱。实验发现,OH(A)的产生机制使其偏离玻尔兹曼平衡分布,本文采用了双温的OH(A-X)光谱模型来考察气体温度。在放电过程中,气体温度保持不变,大约为400 K。此外,水蒸气的加入使得短波长的连续谱发生显著增强。光谱分析认为H₂O在放电中能够解离产生H₂,继而与氩原子的亚稳态发生能量转移生成激发态H₂(a3Σ+_g)。H₂(a3Σ+_g)将会自发辐射跃迁到排除态H₂(b3Σ+_u),同时发射短波长的连续谱。由于短波长的连续谱对电子温度（T_e>1 eV）的响应较为灵敏,所以载气中少量的水蒸气将会对连续谱诊断电子温度带来较大的影响。     

充中性气体相对论返波振荡器的粒子模拟研究

 用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制，成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程，在电子束传输的路径上形成离子通道，有效中和电子束径向空间电荷力，有利于电子束的传输及束波相互作用产生微波。增加中性气体密度，返波管的输出频率明显上移，其辐射的功率和效率比相同的真空器件也有明显的提高。