Space-charge waves of periodic-electrostatically focused electron beams

Space-charge wave propagation for periodic-electrostatically focused electron beams with nonuniform radial beam density distributions is the subject of this paper. The dispersion equation is obtained by solving a differential equation for the ac electric potential inside the electron beam and by matching the solution at the boundaries. An approximate analytic solution and a digital computer treatment are described. The reduced plasma frequency obtained is expressed in terms of the focusing parameters and is compared with that of a Brillouin beam. A study of the differential equation involved suggests that there are certain ranges of system parameters ("forbidden" zones) for which no wave propagation is possible, that is, no space-charge mode can be supported there.     

Azimuthally periodic electrostatically focused electron ribbon beams

Methods of enhancing transmission of ribbon-shaped electron beams undergoing circular motion in azimuthally periodic cylindrical electrostatic lenses are treated. Lens forces add to the naturally occurring curvature focusing to achieve this increase in beam transmission. Three basic configurations, differing only in the method of applying lens voltages, are treated: symmetrical focusing, deflection focusing, and asymmetric focusing (including inner and outer electrode segmentation). Expressions for beam perveance and rippling are derived from the paraxial-ray equation for thin electron ribbons advancing along a two-dimensional curved optical axis. The stiffness of azimuthally periodic electrostatically focused electron ribbons (i.e., the immunity of the beam to transverse disturbing forces) also receives attention. Mathematical extensions of the analysis to ribbon beams undergoing helical motion are discussed, and numerical results applicable to selected values of lens parameters are presented. Experimental results for an azimuthally periodic lens, constructed to test the analysis, are discussed in light of the theory developed.     

Measurement of noise along electrostatically focused electron beams

Noise along periodic-electrostatically focused electron beams have been measured. The noise variation is periodic with the wavelength of the reduced plasma frequency. It is concluded that electrostatic focusing exhibits similar noise properties as magnetic focusing.     

Aperture lens formula corrected for space charge in the electron stream

The aperture lens formula of Davisson and Calbick is rederived to include the effect of space charge forces. The correction is applied to a parallel flow Pierce gun, leading to the focal length reduction as a function of perveance, e.g., 30 per cent reduction at perveance 3 × 10^-6.     

Influence of space charge on lux-ampere characteristics of high-resistivity CdTe

The concentrations of defects forming at near-midgap level in high-resistivity CdTe were estimated on the basis of room-temperature lux-ampere characteristics. A simple model explaining their sublinear dependence based on the presence of discrete levels near the midgap is presented. It is shown that accumulation of space charge in the sample can explain the observed experimental data. Theoretical calculations show that a maximum concentration of the midgap level leading to the observed slope α of lux-ampere characteristic, I_ph=C×I^α, is less than 10¹³ cm⁻³ for a wide range of capture cross sections of electrons and holes. This result supports models that assume formation of a high-resisitivity state with a minimum deep-level doping.     

The effect of space charge on shot noise in crossed-field electron guns

The two-dimensional electronic behavior of the crossed-field potential minimum is analyzed by means of a feedback network which provides a vehicle for understanding the complex phenomena, while allowing quantitatively accurate numerical calculations. The solutions, limited to the low-frequency range where transit time may be neglected, Show shot-noise smoothing as a function of magnetic field and cathode length. For low magnetic fields and short cathodes, the smoothing approaches the results of the North theory for zero magnetic field. It is also shown that if the cathode length is greater than about 11 normalized (Kino) units or 0.55 of the cycloid length for the existing field conditions, an initial perturbation of the emission current will lead to growing fluctuations of the beam current. This instability is considered to be responsible for the observation, in some crossed-field tubes, that the output noise increases when the cathode is heated sufficiently to form a potential minimum in front of the cathode. This criterion for the existence of excess noise is applied to all the experiments for which sufficient data have been published and no disagreement is found.     

Linearly focused acoustic beams for acoustic microscopy

A linearly focused acoustic beam is investigated to be introduced into acoustic microscopy for characterising materials in the nonscanning version. The new acoustic beam enables us to detect acoustic anisotropies of materials to be measured successfully. For forming the acoustic beam, an acoustic sapphire lens with a cylindrical concave surface of 1.0 mm in radius is made and the acoustic field distributions are investigated at 200 MHz.     

The effects of initial electron velocities and space charge in secondary emission

This paper treats a secondary emitter as a fixed-temperature thermionic emitter with an equivalent work function which depends for its value upon the current density of the incident primary electrons. This permits Langmuir's treatment of the parallel-plane thermionic diode to be applied to secondary emission. The resulting equations account quantitatively for observed secondary-emission effects caused by space charge and initial electron velocities. The paper concludes with a discussion of three specific electron devices in which secondary-emission effects due to space-charge and initial electron velocity are important.     

Orthogonal waves on electron beams

The propagation of slow electrokinetic waves on finite electron beams which fill a conducting tunnel is discussed for modes of axial symmetry. In addition to the familiar space-charge modes, a pair of modes exists which are related to the vortex frequency (omega_{upsilon} = omega_{c} - 2theta_{0}). The finite magnetic field introduces a coupling between these modes. A set of four orthogonal modes can be derived by the use of matrix transformations, thus eliminating the necessity of solving a complicated characteristic equation. For infinitely high magnetic fields, the two additional modes disappear and the four modes reduce to the fast and slow space-charge waves.     

异常涡旋光束的拓扑荷的测量

提出了一种测量异常涡旋光束的拓扑荷的方法。在接收孔径之前放置一个特殊样式的掩膜板,通过接收平面的光强图来分辨异常涡旋光束的拓扑荷值。首先采用理论推导的方法得出接收平面光强分布的解析式,然后通过数值仿真得出光强图。研究表明,接收平面上的强度分布图为多环结构,环数与异常涡旋光束的拓扑荷值有关,而与光束阶数无关。研究成果对于促进异常涡旋光束的应用具有重要意义。     

Use of the BFCPIC and BFCRAY codes to describe space charge limited emission in electron guns for gyrotrons

Numerical modelling of an electron gun in the space charge limited regime requires determining the current density distribution as well as the electric fields and electron trajectories. This is a rather complicated self-consistent problem, since the space charge influences the electric field, which in turn influences the electron trajectories. Previous simulations of magnetron electron guns using the BFCPIC and BFCRAY codes used a simple emission model (constant current density) that is approximately valid for thermionic emission. The code has been modified to include space charge limited emission. Several different ways of doing this are considered. One of the models considered uses Gauss’s law to force the electric field on the emitter to vanish; it was used in the original version of BFCPIC for the simulation of ion diodes. A second is based on the use of Child’s law (locally), which may be more appropriate for extension to fully electromagnetic particle-in-cell (PIC) codes. Calculations were performed with both models, and the results compared with each other and with experiments performed at FZK.     

半绝缘GaAs光电导开关限累模式实验研究

Experiments with the limited space-charge accumulation(LSA) mode of oscillation in a large gap semiinsulating (SI) GaAs photoconductive semiconductor switch(PCSS) are discussed.It has been observed that growth and drift of a photo-activated charge domain(PACD) are quenched only when the bias voltage is more than twice the threshold voltage.The original negative resistance characteristics are directly utilized in the LSA mode;during LSA operation the spatial average of the electric field varies over a large portion of the negative differential mobility region of the velocity-electric field characteristic.The work efficiency of an SI GaAs PCSS is remarkably enhanced by electric field excursions into the positive resistance region when the total electric field is only below the threshold part of the time.The LSA mode can only operate in the certain conditions that satisfy the quenching of the accumulation layer and the smaller initial domain voltage.     

Triode electron injection systems for hollow beams

Novel electron guns, in which a conical hollow electron beam is projected at a large angle to the axis into a coaxial deflection region, were tested. The guns have a triode structure so that the perveance can be varied easily. The strong deflection increases the effective perveance of the beam and makes the trajectories insensitive to current variations. In the form of a device with the gun at a large radius and projecting the beam inward, the electron paths are sensitive to scattering in the gun. The inverted gun, projecting the beam outward, is relatively free from this difficulty. The systems generally behave as expected, and should be quite useful for initiating variable-current hollow electron beams in various available focusing arrangements.     

Microwave deflection of electron beams

When an electron beam interacts with a time-varying electric field, it is usual to ignore the effect of the induced magnetic field. It is shown that this may cause errors at microwave frequencies even for slow electrons. The particular case of electrons traversing a microwave cavity is examined in detail.     

基于相对论超短电子束的THz源

THz波的产生有多种途径,清华大学加速器实验室正致力于发展基于相对论电子束的高峰值功率的THz辐射源。本文分别从高峰值流强的单束团与基于激光整形的多脉冲束团序列出发,介绍实验室在宽谱高电场梯度和准单频可调谐THz辐射等方面的研究进展。高亮度电子束团由新一代光阴极电子枪产生,经3 m行波加速管加速,能量可达~50 MeV,采用速度压缩机制可使束团长度达到百飞秒量级。该超短相对论束团可通过多种机制产生高峰值功率的THz辐射,目前,实验室已通过相干渡越辐射(CTR)获得单脉冲能量18μJ的THz辐射输出。同时,提出了一种结合欠压缩工作模式和漂移压缩方法的新束团压缩方案,可进一步将束团压缩至几十飞秒,有望获得频率范围为0.1 THz~30 THz的超宽谱THz辐射。本文同时介绍了多脉冲超短电子束序列的束团测量实验结果,通过模拟研究验证了基于该束团序列CTR产生高功率、可调谐准单频THz辐射的可行性,并进行了相应的实验方案设计。     

双束聚焦强红外激光引起可见荧光的光-声-光相互作用的研究

在强红外激光脉冲激发下引起BCl_3分子可见荧光,同时产生脉冲声,当这种声再进入振动激发态分子气体时,引起分子的再激发并再度引起分子分解,并发出可见荧光,即光-声-光现象。由于这种光-声-光现象对于研究激光化学及激光分离同位素的能量转移过程以及激光引起的气体动力学过程是极为重要的,所以,在本工作中使用二个同时的不重迭的强CO_2脉冲聚焦光束对此现象作了进一步的研究,得到了令人鼓舞的结果。将一束激光能量密度降至不足以引起光-声-光致荧光,然后单独加入另一束不重迭聚焦光束,在第一束激光辐照区域亦未     

Air core coils for deflecting micron-size electron beams

Bobbin-type air core coils have been designed and experimentally evaluated for astigmatic properties in line scan operation over deflection angles of ± 20°. The approximate design technique is successful in specifying the proper operating focal distance when this is comparable to the coil length. Line fields in excess of 20 000 spot diameters have been measured using a 10 kV beam having a 5 × 10-³radian convergence half-angle.