自由电子激光的预群聚

研究了自由电子激光预群聚产生的辐射,表明预群聚不仅可以提高效率而且使低增益得到增强,从而在理论上解释了由射频直线加速器驱动的自由电子激光预群聚器(prebuncher)的作用。同时还指出,预群聚有在不增加电子束能量情况下缩短激光波长和降低对电子束质量要求等作用。     

螺距和磁场强度与周期的跳变在Ka波段行波管中的应用

以弱色散特性的扇形金属-介质夹持杆螺旋线慢波结构的Ka波段行波管作为研究对象,进行了互作用特性仿真研究。采用螺距跳变和磁场跳变技术进一步提高了该行波管在工作频带的输出功率和电子效率,并解决了电子注散焦问题。设计结果表明:当工作电压为9 kV、工作电流为210 mA时,行波管在24~40 GHz整个频带内,各频点的增益在37.7~48.7 dB之间,电子效率在15.18%~19.42%之间,输出功率大于286 W。此结果较之均匀周期的设计结果,电子效率增幅在4.19%以上,输出功率增长率在4.3%以上,尤其在26~37 GHz范围内,电子效率增幅达到了11.8%以上,输出功率增长率达11.9%。     

Nonlinear time-dependent simulation of helix traveling wave tubes

A one-dimensional nonlinear time-dependent theory for helix traveling wave tubes is studied. A generalized electromagnetic field is applied to the expression of the radio frequency field. To simulate the variations of the high frequency structure, such as the pitch taper and the effect of harmonics, the spatial average over a wavelength is substituted by a time average over a wave period in the equation of the radio frequency field. Under this assumption, the space charge field of the electron beam can be treated by a space charge wave model along with the space charge coefficient. The effects of the radio frequency and the space charge fields on the electrons are presented by the equations of the electron energy and the electron phase. The time-dependent simulation is compared with the frequency-domain simulation for a helix TWT, which validates the availability of this theory.     

渐变螺距螺旋线慢波系统高频特性的仿真分析

 为实现渐变螺距螺旋线慢波结构高频参数的高精度估计，基于MAFIA仿真平台，研究了螺距变化对超宽带螺旋线慢波系统(4~18 GHz)的色散、互作用阻抗与衰减常数的影响，获得了各高频参数随螺距变化的规律。研究表明：相速几乎随螺距增大而线性变大，互作用阻抗与螺距是非线性的关系，衰减常数随螺距增大而非线性地减小；由于在维持电子注与电磁波速度同步的限制下，螺距变化的幅度不可能很大，因此可近似按线性关系来处理互作用阻抗及衰减常数与螺距之间的关系。由此提出了一种可精确计算渐变螺距螺旋线慢波结构高频参数的方法－线性插值法     

Beam dynamics studies in the driver LINAC pre-stripper section of the RIA facility

The RIA facility driver LINAC consists of about 400 superconducting (SC) independently phased rf cavities. The LINAC is designed to accelerate simultaneously several charge-state beams to generate as much as 400 kW of uranium beam power. The LINAC beam dynamics is most sensitive to the focusing and accelerating structure parameters of the pre-stripper section, where the uranium beam is accelerated from 0.17 keV/u to 9.4 MeV/u. This section is designed to accept and accelerate two charge states (28 and 29) of uranium beam from an ECR ion source. The pre-stripper section must be designed to minimize the beam emittance distortion of this two-charge-state beam. In particular, the inter-cryostat spaces must be minimized and beam parameters near transitions of the accelerating and focusing lattices must be matched carefully. Several sources of possible effective emittance growth are considered in the design of the pre-stripper section and a tolerance budget is established. Numerical beam dynamics studies include realistic electric and magnetic three-dimensional field distributions in the SC rf cavities and SC solenoids. Error effects in the longitudinal beam parameters are studied.     

Slow Wave Characteristics of Helix Structure with Elliptical Cross Section

We present a novel helix slow wave structure with an elliptical cross section shielded by an elliptical waveguide. The rf characteristics including dispersion properties, interaction impedance of zero mode in this structure have been studied in detail. The theoretical results reveal that weaker dispersion even abnormal dispersion characteristics is obtained with the increasing eccentricity of the elliptical waveguide, while the interaction impedance is enhanced by enlarging the eccentricity of elliptical helix.     

双渐变螺旋线慢波结构参数分析及优化

 双渐变螺旋线慢波结构是空间行波管中使用最广的慢波结构之一。利用Christine大信号程序对该慢波结构各部分长度和螺距对行波管效率以及增益的影响进行了分析。结果显示:增益和电子效率与慢波结构长度成正比,且输入端的长度对增益影响最大;电子效率对相速增加段的螺距的变化最为敏感。根据分析结果,通过增加输入端长度,加大相速增加段的螺距,同时减小相速减小段的螺距进行优化。优化后,电子效率由35.4%提高至36.8%,增益由54.6 dB增加至56.2 dB。     

Modelling of Secondary Electron Impact on the Self-Consistent RF Bulk Plasma Description

The impact of a secondary electron beam, generated at the electrodes and accelerated in the sheaths, on the self-consistent treatment of the electron behaviour in an rf bulk plasma has been investigated by a parametric study. Source of electrons in the plasma are collisional ionization and secondary electron injection. Electrons are lost by ambipolar diffusion to the electrodes of a parallel plate rf discharge configuration. The non-stationary Boltzmann equation is used to determine self-consistently the rf field amplitude necessary for maintaining the steady-state rf bulk plasma as well as the time resolved behaviour of the electron energy distribution function and of all contributions to the electron particle and power balance, at given source rate and energy distribution of secondary electron injection.     

A novel low-E field coil to minimize heating of biological samples in solid-state multinuclear NMR experiments

A novel coil, called Z coil, is presented. Its function is to reduce the strong thermal effects produced by rf heating at high frequencies. The results obtained at 500MHz in a 50 microl sample prove that the Z coil can cope with salt concentrations that are one order of magnitude higher than in traditional solenoidal coils. The evaluation of the rf field is performed by numerical analysis based on first principles and by carrying out rf field measurements. Reduction of rf heating is probed with a DMPC/DHPC membrane prepared in buffers of increasing salt concentrations. The intricate correlation that exists between the magnetic and electric field is presented. It is demonstrated that, in a multiply tuned traditional MAS coil, the rf electric field E(1) cannot be reduced without altering the rf magnetic field. Since the detailed distribution differs when changing the coil geometry, a comparison involving the following three distinct designs is discussed: (1) a regular coil of 5.5 turns, (2) a variable pitch coil with the same number of turns, (3) the new Z coil structure. For each of these coils loaded with samples of different salt concentrations, the nutation fields obtained at a certain power level provide a basis to discuss the impact of the dielectric and conductive losses on the rf efficiency.     

S波段相对论速调管振荡器研究

介绍利用20 GW加速器二极管产生的电子束源,开展S波段相对论速调管振荡器(RKO)的理论设计、粒子模拟和实验研究的情况.该RKO采用3个紧密耦合的圆柱腔作为振荡腔,束流经过一段漂移管的群聚后采用三轴输出腔提取微波.该振荡器具有起振时间快、结构紧凑、束波转换效率较高等优点.采用无箔空心阴极和0.9 T的恒流源磁场引出的电压1 MV、束流13kA、脉宽40 ns的环形电子束驱动RKO,单次运行输出了3.5 GW的辐射微波功率,效率27％,频率2.86 GHz,瞬时带宽2%;脉冲重复频率20 Hz运行时,输出 关键词： 相对论速调管 振荡器 三轴提取腔 高功率微波     

Abnormal heating of low-energy electrons in low-pressure capacitively coupled discharges

In low-pressure capacitively coupled plasmas, high-energy electrons are collisionlessly heated by large rf fields in the sheaths while low-energy electrons are confined in the bulk plasma by the ambipolar potential. Low-energy electrons are typically inefficiently heated due to their low collisionality and the weak rf electric field present in the bulk. It is shown, however, that as a result of the nonlinear interaction between the electron motion and the weak rf field present in the bulk, low-energy electrons can be efficiently heated. Electrons in the bulk that bounce inside the electrostatic potential well with a frequency equal to the rf excitation frequency are efficiently heated by the coherent interaction with the rf field. This resonant collisionless heating can be very efficient and manifest itself as a plateau in the electron energy probability function.     

螺旋坐标系下考虑厚度的螺旋线行波管小信号研究(英文)

采用螺旋坐标系对有限厚度的螺旋带行波管进行了注波互作用的线性理论分析,利用自洽场理论得出了此结构中引入电子注后的"热"色散方程,数值计算了其小信号增益,分析了电子注参数和螺旋线厚度对于螺旋线慢波系统的色散特性的影响。结果表明:螺旋线厚度的增加,行波管的小信号增益也会增加,且带宽也略有增加。该结论为高增益宽频带的螺旋线行波管放大器的设计提供了理论基础。     

The energy spectrum of an electron beam after interaction with an RF field in a gyrotron

The electron distribution over residual energy after electron interaction with an rf field was determined using the method of decelerating electric field. It is demonstrated that, under the maximum efficiency conditions, electrons with an energy lower than 30% of their initial energy are absent in the spectrum. Hence, the efficiency of energy-recuperation gyrotrons can substantially be increased.     

左右旋双双绕螺旋线线极化摇摆器的磁场分布及束流传输

给出了双双绕螺旋线线极化Wiggler轴线上磁场的积分表达式,以此公式数字模拟此装置端口区的磁场分布,并进行了实验测定,结果表明,端口区磁场峰值突变和对称性均比圆极化好。另一方面,用洛仑兹运动方程,考虑了电子束空间电荷效应,证明此装置对电子束传输有自聚焦能力,给出了电子运动方程,电子束流与Wiggler峰值磁场的关系,数字模拟电子束运动轨迹也表明,此装置能自聚焦传输电子束,有希望应用于线极化自由电子激光实验中。     

Efficiency enhancement in free-electron lasers by means of concurrent rf acceleration

Efficiency enhancement in free-electron lasers (FELs) using rf beam acceleration in the wiggler is described. Since the beam tube is a waveguide, there are low and high frequency resonances. Injection of low frequency power can act as an inverse-FEL accelerator concurrently with high frequency power extraction. Simulation of a FEL using this technique shows that substantial efficiency enhancements are possible without significant increases in the beam energy spread, which facilitates the use of energy recovery schemes. The technique is applicable to amplifier and oscillator configurations.     

Stimulation of high-frequency breakdown of gas in Uragan-3M torsatron by runaway electrons

In experiments on confinement and heating of plasma in the Uragan-3M torsatron, the method of high-frequency breakdown of the working gas is used. In these experiments, in conditions of a relatively stable magnetic field, the rf power supplied to the setup chamber has a frequency close to the ion-cyclotron frequency. Such a method of gas breakdown is not always sufficiently reliable. In our experiments, preliminary ionization of the working gas by the run-away electron beam is used for stabilizing the breakdown. This work contains the results of experiments on enhancement of the runaway electron beam and on the interaction of the runaway electron beam in the Uragan-3M torsatron with the HF electromagnetic pump field. This enables us to formulate a number of recommendations for using spontaneously formed beams of accelerated particles for stimulating the rf breakdown. Our results confirm the possibility of gas breakdown by runaway electrons.     

Schottky-enabled photoemission in a rf accelerator photoinjector: possible generation of ultralow transverse thermal-emittance electron beam

We present a clear signature of the Schottky effect in a rf photoinjector using photons with energy lower than the Mg cathode work function. This signature is manifested by the shift in the rf phase angle for the onset of the detection of photoelectrons via single-photon absorption and allows for a reasonable estimate of the field enhancement factor. This is a viable method to generate an electron beam with very low thermal emittance and thus, a high brightness beam.     

A Simple Closed-form Formula for Backward-Wave Start-Oscillation Condition for Millimeter-Wave Helix TWTs

An accurate and simple closed-form formula, for backward-wave start-oscillation condition for a millimeter-wave helix traveling-wave tube amplifier was developed, using an artificial neural network (ANN) algorithm. The analysis considers the effects of circuit loss and also the variation of electron beam diameter corresponding to beam filling. The formula is simple and amenable to easy computation, even using a scientific calculator, and without resorting to exhaustive numerical iterative search followed in conventional analyses and, at the same time, without sacrificing the accuracy in results. The formula was validated against published results, and excellent accuracy was observed. The analysis has been further used for inferring some physical interpretations on the effects of beam-filling factor and circuit loss on the start-oscillation condition of a typical millimeter-wave helix traveling-wave tube.     

Low frequency double electron muon resonance (Demur) in fused quartz

Double electron muon resonance was studied with comparable amplitudes of the static and rf magnetic fields. The experiments were carried out in fused quartz and employed a resonant linearly-polarized rf field and a small static field. For small amplitudes of the rf field a three-line frequency spectrum was obtained consisting of a central line at the rf frequency and two weaker lines of equal amplitude symmetrically positioned about the central line. A symmetrical three line spectrum is predicted for muonium using the rotating wave approximation (RWA). For large rf fields the amplitudes of the three lines were not symmetric and depended on whether positrons were observed parallel or perpendicular to the beam. For very large rf fields the total amplitude of all the lines was less than for lower rf fields. These results do not agree with the theory using the RWA and no satisfactory theory currently exists.     

Contraction of the space of electron drift in low-frequency bunchers

In the paper we theoretically investigate the mechanism by which the space of drift electrons is contracted in klystron-type bunchers which modulate the electron beam in ion accelerators, in particular to excite an rf field. The contraction of the drift length is effected through a decrease in the mean beam velocity in the field of an electrostatic lens or a metal tube and is based on the fact that the relative motion of velocity-modulated electrons are independent of the mean velocity of the electron beam in a weak modulation regime.