低引导磁场下相对论亚纳秒电子束毫米波返波振荡器的粒子模拟

提出了一种Ka波段的相对论亚纳秒电子束毫米波慢波结构,在较低引导磁场情况下,运用粒子模拟(PIC)方法成功地模拟出器件中波束互作用的非线性物理演化过程,得到了一种超辐射状态下的微波辐射,它的产生与波相对于电子束滑移引起的电子束内电子间相互作用有关,辐射微波的峰值功率与电子束总电荷的平方成正比.粒子模拟有利于对超辐射这种束波互作用非线性物理现象的理解,并且对器件的设计有一定的参考价值.     

Generation of ultrashort microwave pulses based on cyclotronsuperradiance

We have theoretically and experimentally investigated the superradiance from a bunch of electrons rotating in a homogeneous magnetic field. A RADAN-303B modulator equipped with a subnanosecond pulse slicer has been used to generate high current subnanosecond electron bunches (250 kV, 0.1-1 kA, 0.3-0.5 ns). Transverse momentum was imparted to the electrons by a kicker. It is shown that for the experimental observation of cyclotron superradiance from high current electron bunches the optimum conditions are the conditions of group synchronism, when the translational velocity of the bunch coincides with the group velocity of the radiation propagating in the waveguide. In the 35 GHz range microwave pulses with record short duration, down to 0.4 ns, with a peak power level up to 200 kW, have been obtained     

Production of ultra-short high-power microwave pulses in Čerenkov backward-wave systems (Review)

The article presents a review of works (mainly, of experimental ones) on production of subgigawatt and gigawatt microwave pulses of extremely short duration (5–7 RF periods) using backward-wave systems fed with nanosecond and subnanosecond high-current electron beams produced by compact accelerators. Theoretical approaches to the generation process (which is essentially non-steady-state) are briefly summarized. Using the effect of spatial accumulation of energy in a short running microwave pulse allows production of pulses with peak power notably higher then the driving electron beam power. Compact microwave sources developed for operation in the Ka-band and X-band are described. Special attention is given to the issue of high pulse repetition frequency operation of the sources.     

Cyclotron superradiance of a high-current electron bunch under group synchronism conditions

We have theoretically and experimentally investigated the process of induced coherent emission (superradiance) in an electron bunch rotating in a homogeneous magnetic field. We have shown that this process makes it possible to generate ultrashort microwave pulses. In this case, the optimum conditions are found under group synchronism conditions, when the translational velocity of the bunch matches the group velocity of the radiation propagating in the waveguide circuit. For experimental investigation of the superradiance, we used a RADAN accelerator with subnanosecond electron pulse sharpener. In the 35 GHz range, we obtained microwave pulses with record short duration, down to 0.4 nsec for a peak power level up to 200 kW. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 89–97, December, 1996.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

Self-amplification of coherent spontaneous emission in a cherenkov free-electron maser

Ultrashort pulses of microwave radiation have been produced in a dielectric-lined Cherenkov free-electron maser (FEM) amplifier. An intense initial seed pulse, due to coherent spontaneous emission (CSE), arises at the leading edge of the electron pulse. There is evidence to show that 3-4 cycle spikes are produced through the amplification of these seed pulses. A strong dependence of the start-up power on the rise time of the electron pulse has been found. The experimental results are verified by a theoretical analysis. Our study shows that amplification in a FEM amplifier is always initiated by CSE arising from the edge of the electron pulse when the rise time is comparable to the electromagnetic wave period.     

Experimental observation of superradiance in the stimulated scattering of an intense microwave pump wave by a counterpropagating subnanosecond high-current relativistic electron bunch

The generation of ultrashort superradiant pulses in the stimulated scattering of an intense microwave (38 GHz) pump wave by a counterpropagating high-current relativistic electron bunch has been observed. Scattered radiation is a single ～200-ps pulse with a peak power of about 1 MW. Owing to the Doppler shift of the radiation frequency, frequencies up to 150 GHz are present in the spectrum of the scattered pulse.     

X波段重复频率GW级超辐射相对论返波管

运用超辐射机理,通过粒子模拟设计了X波段超辐射相对论返波管,并在小型Tesla脉冲源平台上开展了实验研究。通过空间功率积分和直接对辐射微波时域波形的分析得到实验结果:在束压350 kV、束流4.8 kA、脉宽3.1 ns、引导磁场2.2 T条件下,产生的微波辐射功率1.4 GW,中心频率9.36 GHz,脉宽500~700 ps,辐射模式为TE11,能在重复频率100 Hz下稳定运行。功率转换效率超过80%。实验结果与粒子模拟结果比较吻合,成功实现了在短脉冲条件下产生重复频率、亚纳秒脉宽、GW级微波辐射。     

Shortening of the radiation pulse from a plasma relativistic microwave generator in numerical calculations with plasma simulation by the particle-in-cell method

Spontaneous shortening of radiation pulses of Cherenkov microwave generators based on the interaction of a high-current relativistic electron beam with preliminarily generated plasma was studied in a numerical model. Microwave pulse shortening is caused by the appearance of a gradually expanding region near the collector, from which plasma is expelled by an electrostatic field of relativistic electrons. The absence of plasma results in a severalfold decrease in the plasma wave reflectance from the collector and violation of generator self-excitation conditions. The microwave emission duration increases with the plasma ion mass.     

The generation of superradiance pulses by high-current subnanosecond electron bunches moving in a periodic slow-wave system: Theory and experiment

Cherenkov superradiance observed when an electron bunch rectilinearly moves through a slow-wave periodic system is studied theoretically and experimentally. The simulation based on averaged equations and the direct numerical simulation using the PIC-code KARAT show that the peak power of the microwave pulses varies as the total number of the particles in a bunch squared. This finding is confirmed experimentally. Ultrashort (300 ps wide) high-power (up to 140 MW) pulses are generated at a frequency of 39 GHz. As an electron source, the high-current subnanosecond RADAN-303 accelerator is used. It injects 0.5-to 1.5-ns-wide electron bunches of current up to 2 kA and energy 200–300 keV. The simulation suggests that the power of the electromagnetic pulses can be increased further (up to 300 or 400 MW) by optimizing the accelerating voltage pulse shape.     

利用康普顿散射实现太赫兹皮秒脉冲的分析

 研究了利用微波与电子束团的康普顿散射实现太赫兹的方法、光子产额和辐射功率。推导出了单个电子产生的太赫兹的光子产额和辐射功率表达式,也推导出了电子束团产生的太赫兹的光子总产额和辐射总功率表达式。结果发现：利用微波与电子束团发生康普顿垂直散射,可以产生太赫兹皮秒脉冲;单个电子产生的太赫兹光子产额与微波功率、微波波长成正比,与微波束截面积成反比;单个电子产生的太赫兹辐射功率与微波功率、电子Lorentz因子的平方成正比,与微波束截面积成反比;电子束团产生的太赫兹光子总产额与微波功率的平方、微波波长的平方成正比,与微波束截面积的平方成反比;电子束团产生的太赫兹辐射总功率与微波功率的平方、微波波长以及电子Lorentz因子的平方成正比,与微波束截面积的平方成反比。     

X波段短脉冲相对论返波管设计与初步实验

 对基于短电子束脉冲超辐射机理的X波段相对论返波管进行了优化设计和粒子模拟,结果表明：在超辐射机理作用下,该器件能实现高峰值功率和高功率转换效率的微波辐射。在小型Tesla脉冲源基础上设计了阻抗变换段、二极管、磁场系统等装置,建立了一套小型窄脉冲电子加速器,以此为实验平台在低磁场条件下进行了器件的初步实验研究。在磁场0.73 T、束压约380 kV、束流约4.5 kA、脉宽3.1 ns条件下,实验获得的微波脉冲峰值功率约360 MW,脉宽1.10 ns,上升沿800 ps,频率9.15 GHz,功率转换效率为21%。     

L波段微波脉冲对微型计算机的辐照效应实验

 在L波段使用1.3 GHz载波频率的微波脉冲辐照微型计算机主板,通过改变脉冲宽度、重复频率和脉冲串长度等参数,实验研究了微波脉冲辐照导致微型计算机失去响应的功率阈值的变化规律,讨论了L波段微波脉冲辐射的积累效应。实验结果表明：当微波脉冲宽度增加时,微波功率阈值下降;当微波脉冲重复频率升高时,微波功率阈值呈下降趋势;在固定重复频率的条件下,微波脉冲数目的增加也会导致微波功率阈值的下降;微波脉冲功率阈值始终小于连续波微波的功率阈值。当微波脉冲间隔时间较长或者脉冲宽度较宽时,微波功率阈值由单个微波脉冲的参数确定,与脉冲重复频率没有明显关系。利用假设的微波脉冲辐射积累效应,可以定性解释和分析微波脉冲辐照微型计算机实验中功率阈值变化的趋势。     

Generation of subnanosecond microwave pulses based on the Cherenkov superradiance effect

Experimentally observed Cherenkov superradiance produced by a subnanosecond electron bunch traveling through a partially filled waveguide is reported. 400-ps microwave pulses with a peak power of 2 MW are obtained. The experimental results are in good agreement with the theoretical analysis and numerical simulations performed with the help of the PIC code KARAT.     

磁压对级联爆磁压缩脉冲发生器性能的影响

为了研究负载为mH量级的间接馈电两级级联柱-锥构型的爆磁压缩产生器的基本物理过程和能量转换机理，利用描述爆磁压缩物理过程的2维爆轰磁流体力学程序MFCG(Ⅴ)，以实验模型结构参数为基础模拟计算了一系列模型，分析了磁压对金属套筒径向膨胀速度及膨胀过程的影响。计算结果表明:套筒的径向膨胀速度取决于爆轰压与磁压的共同作用，在爆磁压缩过程的绝大部分时间里，向外膨胀的爆轰压都远大于向内压缩的磁压，因而套筒的径向膨胀速度主要是由爆轰压决定;但是在功率放大级的后半段，也就是发生器电流增长最快阶段，磁压也迅速增长，它的增长大大降低了套筒的径向膨胀速度；在功率放大级的后期，磁压已经超过爆轰压，它对系统设计的影响已经不能完全忽略。     

Polymethyl methacrylate glow under the influence of runaway electron beams generated in a gas diode

Spectral and amplitude–time characteristics of PMMA radiation under the impact of runaway electrons with subnanosecond duration are studied. The PMMA radiation spectra for a subnanosecond electron beam pulse duration are determined for the first time. The studies show that radiation of the band with a maximum at about 490 nm the intensity of which decreases toward the short-wave spectral region is recorded in the glow spectra. The glow intensity of this band varies proportionally to the number of electrons in the beam, which allows the possibility of using this radiation for determination of the number of high energy electrons in electron beams.     

Manipulating electromagnetic waves in magnetized plasmas: compression, frequency shifting, and release

A new approach to manipulating the duration and frequency of microwave pulses using magnetized plasmas is demonstrated. The plasma accomplishes two functions: (i) slowing down and spatially compressing the incident wave, and (ii) modifying the propagation properties (group velocity and frequency) of the wave in the plasma during a uniform in space adiabatic in time variation of the magnitude and/or direction of the magnetic field. The increase in the group velocity results in the shortening of the temporal pulse duration. Depending on the plasma parameters, the frequency of the outgoing compressed pulse can either change or remain unchanged. Such dynamic manipulation of radiation in plasma opens new avenues for manipulating high power microwave pulses.     

Tilted-pulse second-harmonic beam analysis for femtosecond to subnanosecond laser pulse-duration measurements

The possibility of extending the second-harmonic beam (SHB) method proposed originally for picosecond and subpicosecond pulse-duration measurements to the femtosecond region is pointed out. This can be achieved by introducing a differential time delay of the pulse wave front corresponding to a tilting of the pulse in the direction other than that applied by Wyatt and Marinero, and also by Saltiel et al., who achieved extensions towards the subnanosecond region. The solution of the wave equations for noncollinear second-harmonic generation in the case of arbitrarily tilted pulses has been carried out. Simple formulae valid from the subnanosecond to the femtosecond region are presented.     

A time-resolved study of sub-nanosecond pulse generation by the combined effects of stimulated brillouin scattering and laser-induced breakdown

The generation of subnanosecond pulses by the combined effects of stimulated Brillouin scattering (SBS) and laser-induced breakdown has been investigated by obtaining simultaneous streak photographs of the spark, the backscattered and the transmitted pulses. The results demonstrate that the output from an excimer pumped dye laser can yield subnanosecond pulses with a duration of 20ps. The pulse shortening effect results from the nonlinear reflection properties of the Brillouin mirror and the strong absorption of visible radiation by the laser-breakdown plasma.     

Promising source of high-power broadband microwave pulses with radiation frequency variable up to two octaves

Using a numerical model, we propose a source of microwave pulses, based on the interaction of a high-current relativistic electron beam with plasma. The source is an intrinsic noise amplifier with a pulse duration shorter than 3 ns, which does not permit the emergence of feedback and self-excited generation. The wave gain factor depending on the plasma concentration makes it possible to control the radiation frequency in the range 4–17 GHz, within which a spectral width of ～2 GHz, a power of ～150 MW, and an energy efficiency up to 15% are preserved. The possibility of using the available small-size source of high-voltage pulses with a high repetition rate is considered.