虚阴极振荡器中微波频率的研究

 在虚阴极振荡器的实验中，同时得到了几百兆瓦5.17GHz和几兆瓦7.74GHz的微波输出。根据虚阴极振荡器的单电荷层模型可知，主频5.17GHz的微波由虚阴极自身振荡产生；7.74GHz 的微波由虚阴极自身振荡与虚阴极振荡或电子往返振荡产生的调制束流相互作用产生。     

离子通道中的虚阴极辐射

提出了一种新型的基于等离子体的多级虚阴极振荡器物理模型并展开了研究.研究表明:当电子束通过稠密等离子体背景时,由于离子背景和焦点处虚阴极的共同作用,得以形成多级虚阴极;多级虚阴极对电子的作用,使电子在各级之间振荡,从而产生高功率微波辐射.这是与离子通道横向的betatron振荡完全不同的一种辐射机理.通过质点网格(particle in cell)法模拟验证了多级虚阴极的形成,模拟中发现,所选参数TM024模被激发起来.最后,在所建模型的基础之上对辐射特性做了详细分析.     

虚阴极的单电荷层模型

 通过引入虚阴极振荡的单电荷层模型，得到了虚阴极振荡产生微波的线性色散关系。同时，分析了入射、反射、透射束流中振荡成份与虚阴极振荡的两本征模式相互作用，得到非线性色散关系，并给出了虚阴极振荡产生微波的机制。     

同轴式反射三极管高功率微波源

 运用两维半、全电磁模型的MAGIC程序对同轴式反射三极管中的虚阴极振荡现象进行了粒子模拟研究。模拟结果表明,同轴式反射三极管的辐射主要由在实虚阴极之间来 回振荡的反射电子提供,得到的微波瞬时峰值输出功率可达兆瓦级,瞬时峰值效率在1%左右,提高效率的方法是在腔中央加一个收集极。     

空心束虚阴极激励微波的主模式的研究

把虚阴极振荡激励微波的过程看成是一个时空振荡电流元激励微波的过程,在薄电荷层近似下,求得空心束虚阴极激励微波的主模式关系,并讨论了束几何位形与微波输出功率之间的关系。我们所确定的主模式与实验和粒子模拟的结果相符。     

同轴虚阴极微波输出模式分析

 对不同透过率的阳极网进行实验研究，得到了主频为3.4 GHz和3.7 GHz的辐射微波，对同轴虚阴极内的电子束分布进行了分析。对同轴虚阴极内的本征模式进行了计算，并与实验结果进行比较。实验结果表明：网内电子束的分布对微波输出模式有重要影响；同轴虚阴极微波频率是阴阳极间距及谐振腔共同作用的结果；阳极网透过率对频率有显著影响，透过率越高，则微波频率相应提高。     

平板型虚阴极振荡器频率特性实验研究

 介绍了在“闪光二号”加速器上开展平板型虚阴极高功率微波振荡器频率特性实验研究的结果及分析。结果表明：平板型虚阴极振荡器表现出多频特性,且在微波脉宽内频率随时间变化;其频率分布在C波段和X波段,主频在C波段;主频由虚阴极自身振荡产生并且等于局部电子束的相对论等离子体频率,主频与二极管间隙电压有弱的依赖关系,与二极管间距成反比,而与阴极尺寸和阳极材料关系不大。     

虚阴极振荡产生高功率微波的粒子模拟

独立研制了一个柱对称、非周期边界12/2维相对论性电磁模粒子模拟程序。用它研究了强相对论电子束射入一端封闭、一端开口的波导管所产生的虚阴极振荡和激发的高功率微波。得到了清晰、合理的物理图象,虚阴极振荡频率符合(1～2π~(1/2)ω_(peb)的经验规律。激发微波的效率可高达10％。此外,通过模拟计算,研究了辐射频率、辐射效率与外加导引磁场、电子束流强度、电子束能量和电子束半径的依赖关系。得到了一些合理而有价值的规律。     

碳纤维阴极对输出脉宽及能量的影响

 在反射三极管实验平台上，通过浸渍碘化铯(CsI)碳纤维阴极与不锈钢阴极实验对比，研究了碳纤维阴极对虚阴极振荡器输出微波脉宽及能量的影响。实验得出碳纤维阴极虚阴极振荡器输出微波脉宽约200 ns，较不锈钢阴极虚阴极振荡器增加了30%；对输出微波脉宽面积近似积分，得出碳纤维阴极虚阴极振荡器与不锈钢阴极虚阴极振荡器的输出微波脉宽面积比值约为2.273，表明碳纤维阴极虚阴极振荡器输出微波能量是不锈钢阴极虚阴极振荡器的2倍左右。分析认为碳纤维阴极的材料特性及其综合发射机制导致阴阳极间等离子体膨胀速度及温度降低，延缓了阴阳极间隙的闭合，从而增加了输出微波脉宽，提高了微波输出能量。     

轴向反馈式虚阴极振荡器的频率特性

 采用粒子模拟和实验相结合的方法对轴向反馈式虚阴极振荡器进行了研究。结果表明，轴向反馈式虚阴极振荡器能够有效地锁定微波频率，提高输出功率。在二极管电压为450kV，束流为22kA的条件下，当阴阳极间距大于1.3cm时，辐射微波主频约为3.5GHz，此时与腔的耦合较好，微波辐射功率较高；当阴阳极间距小于1.3cm时，辐射微波主频约为4.8GHz。实验和模拟符合得较好。     

虚阴极振荡激励高功率微波的数值模拟

模拟了轴对称虚阴极振荡器产生微波的过程。根据实验装置的特点,建立了理想的物理模型。从物理学的基本规律出发,用CIC方法编制了二维空间、三个动量分量的2 1/2维、全电磁、相对论性的数值模拟程序。选择柱状电子束进行模拟,获得了正确的计算结果和清晰的物理图象,并对计算结果作了分析。     

A Radially and Axially Extracted Virtual-Cathode Oscillator (Vircator)

Experiments are described in which a 5-? 1-MeV pulser drives virtual-cathode systems optimized to emit microwave radiation in the X band. Radiation is extracted both axially in TM modes and radially in TE modes. The threshold for microwave emission is found to be several times the space-charge limited current. The radiating frequency is approximately proportional to the electron beam plasma frequency with a proportional constant of (27?)1/2. Anode/cathode spacing is varied to demonstrate the frequency tunability of the virtual cathode as a microwave source. The radiation is broad band and frequency chirping is observed. Radiation patterns of the axially extracted radiation are described. The instantaneous efficiency of energy conversion from electrical to microwaves is ~2 percent. External excitation from an external source does not alter microwave generation.     

Dynamics of an electron beam with a virtual cathode in a vircator without a magnetic field and with drift space filled by neutral gas

The physical processes in a vircator without an external magnetic field and with a drift space filled by neutral gas are studied. Three typical dynamic regimes are found: (1) the suppression of virtual cathode oscillations, (2) pulse generation, in which the suppression and resumption of virtual cathode oscillations are observed, and (3) continuous virtual cathode generation. The frequency dynamics of the vircator’s output radiation is investigated for the last case, and it is shown that the generation frequency of the vircator can rise when the gas pressure exceeds a certain critical value.     

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     

Excitation of parametric oscillations in a triode with a virtual cathode with an external electromagnetic wave

The electromagnetic radiation of an electron flow in a triode with a virtual cathode is studied from the viewpoint of a parametric resonance. It is shown that the phase modulation of the electrons and the radiation saturation take place due to the oscillations of the virtual cathode as a result of large space charge. An expression for the coherent radiation intensity of the electron flow in a resonator is derived. It is shown that the phase modulation depth and the radiation efficiency increase with an external electromagnetic wave. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 76–81, February, 2000.     

Power of microwave generation in an ultrarelativistic electron beam in the regime of virtual cathode formation in an externally applied magnetic field

A numerical study of the output power characteristics of microwave radiation from a relativistic electron beam (REB) with a virtual cathode in the presence of externally applied longitudinal magnetic field is performed. Typical dependences of the output microwave power of the relativistic vircator system on the external magnetic field strength are obtained, showing a number of local maxima. It is found that the characteristic behavior of the radiation power is determined by the conditions and mechanisms of virtual cathode formation in the presence of external longitudinal magnetic field and a self-magnetic REB field.     

无箔二极管强流电子束空间密度分布初步研究

通过在强磁场条件下,利用环形刀口石墨阴极(刀口尺寸38~39mm)开展电子束轰击收集极内表面铜箔和垂直轰击金属靶片实验,对无箔二极管中电子束的空间密度分布进行了初步研究,并对其产生原因进行了分析。研究结果表明,电子束径向分布在37.2~40.2mm,存在密度较高区域(38.8~39.4mm)和密度最大值点(39.2mm),且均偏向于阴极外侧。无箔二极管环形阴极爆炸发射产生电子束的径向密度分布可用偏态分布近似。     

Numerical simulations of the reditron

The reflected-electrons discrimination microwave generator (reditron) is a high-power, narrow-band, and single-mode microwave generator that makes exclusive use of the oscillatory character of the virtual-cathode of a relativistic electron beam. The complex, nonlinear character of the virtual-cathode device necessitates particle-in-cell plasma simulation techniques. Investigations indicate two sources of the radiation: (1) the trapped electrons reflexing between the real and virtual cathodes, and (2) the oscillation of the virtual cathode. In the conventional design, the two mechanisms coexist and interfere with each other destructively, causing degradation of the efficiency of microwave generation. The authors have investigated a configuration with a slotted, thick anode and an external magnetic field, which effectively eliminates the reflexing electrons. Two-dimensional particle-in-cell simulations showed that such a configuration exploits the oscillation of the virtual cathode exclusively, and it generates single-mode, narrow bandwidth, and high-power microwave radiation with a potential efficiency over 10%. It was found that further optimization could be achieved by the use of a density (current) modulated electron beam at appropriate frequencies