AC space-charge effects in gyroklystron amplifiers

The effects of AC space charge on multicavity gyroklystron amplifiers are studied. It is found that when AC space charge is included in the analysis of weakly relativistic multicavity gyroklystron amplifiers, the optimized nonlinear efficiency becomes a function of beam current. For a cold beam (no velocity spread), the efficiency is maximum at zero current and decreases monotonically as the current increases. The zero current limit of the optimized efficiency when AC space-charge effects are included is not the same as the optimized efficiency with no space charge; it is significantly higher. This behavior is regularized when velocity spread is taken into account; in that case, the nonlinear efficiency increases with beam current until it reaches a maximum, then falls off slowly. The increase in efficiency is attributed to enhanced bunching associated with the saturation of the space-charge instability in the drift region; the reduction in efficiency at high current occurs because space charge induces an additional velocity spread in the beam     

Space charge oscillation in electron cooling devices

Electron cooling is used for improving the parameters of ion beams. The cooling efficiency depends drastically on the space charge fluctuation intensity in the beam. The fluctuations present in the cooling region cause the stochastic heating of the ions, which adversely affects the cooling efficiency and may even annihilate the ion beam. The space charge fluctuation intensity as a function of various operating parameters of a cooler is studied experimentally. A mechanism of fluctuation generation is suggested, and the effect of fluctuations on the ion beam parameters is estimated.     

CSR电子冷却系统电子束空间电荷效应对电子束温度的影响

 通过求解电子冷却系统中冷却段漂移管区的强流电子束产生的空间电荷场,研究了电子束空间电荷效应对电子温升的影响。结合HIRFL CSR电子冷却系统的典型参数,得到了电子在自身空间电荷场和纵向螺线管磁场组成的交叉场作用下产生的漂移速度和由此引起的附加温度大小,并指出减小电子束空间电荷效应的方法。     

A Direct Convertor Based upon Space Charge Effects

A device capable of converting directly the kinetic energy of charged particles into electrical energy is considered. The device differs from earlier ones (such as Post's periodic focus electrostatic direct convertor) in that it makes use of the space charge repulsion in a high density charged particle beam. The beam is directed into a monotonic decelerating electrostatic field of a several stage planar finned structure. The collector fins coincide with vacuum equipotential surfaces. Space charge blowup of the beam directs particles onto various collector fins. The energy efficiency of a 4-stage device has been determined using a numerical simulation approach. We find that efficiencies approaching 75 percent are possible. An approximate scaling law is derived for the space charge based direct convertor and a comparison is made to the periodic focus direct convertor. We find the space charge based direct convertor to be superior in a number of ways.     

高斯涡旋光束的光束传输因子和峭度参数

基于强度二阶矩定义, 导出了高斯涡旋光束光束传输因子即M² 因子的解析表达式, 高斯涡旋光束的M² 因子唯一取决于拓扑电荷数n. 数值计算表明, 高斯涡旋光束的M² 因子随着拓扑电荷数n的增大而增大. 基于强度高阶矩, 还导出了高斯涡旋光束经傍轴ABCD光学系统传输时峭度参数的解析表达式, 高斯涡旋光束的峭度参数取决于拓扑电荷数n、参数δ、矩阵元A和矩阵元D. 在自由空间传输时, 高斯涡旋光束的峭度参数仅取决于拓扑电荷数n和参数δ. 自由空间传输时, 高斯涡旋光束峭度参数的变化规律为: 峭度参数随参数δ的增大先减小而后趋向于一最小值, 随拓扑电荷数n的增大而减小. 这一研究有助于高斯涡旋光束的实际应用.     

THE DESIGN AND STUDY OF SPACE CHARGE EFFECT FOR THE BPL 750keV BEAM TRANSPORT SYSTEM

The design of the BPL 750keV beam transport system and the study of beam space charge effect in this system are described. This system has been constructed and operated in December 1982. The beam-transfer efficiency of the quadrupole system reached 66.7%. The first buncher operated in 1983, and the beam bunching efficiency is about 60%.     

Beam Dynamics Design of a 50 mA D~+ RFQ

A new 973 project was proposed by Peking University and Institute of Modern Physics of Chinese Academy of Sciences recently.The project requires a 50 mA,162.5 MHz,cw mode radio frequency quadrupole(RFQ)to accelerate the D~+ to 1 MeV.In a high-current linear accelerator,the strong space charge effect causes the growth of envelope and emittance along with heavy beam losses.In the beam dynamics design of this RFQ,beam envelope mismatching is discussed and a matching dynamics method is proposed to minimize the envelope and emittance growth.The influence of limiting current on the beam transmission is discussed and used in the optimization of transverse and longitudinal parameters.After the optimization,the beam transmission efficiency reaches higher than 98%.     

Nonlinear analysis of wiggler taper, node competition, andspace-charge effects for a 280-GHz free electron laser

The nonlinear characteristics of a 280-GHz free electron laser (FEL) are simulated by numerical computation. The three-dimensional set of coupled nonlinear differential equations is solved for a set of TE and TM modes valid for the high gain Compton regime. The use of a nonlinear taper for efficiency enhancement, the sensitivity of gain to competing mode power levels, space-charge effects, and the effect of electron beam source distributions on gain and efficiency are examined for a 10-MeV, 3-kA beam. It is found that the nonlinear taper greatly enhances the gain and efficiency and makes the saturation power levels relatively insensitive to the competing mode power levels. The efficiency is increased to 48% by means of a nonlinear taper in which the 3-D and wiggler-averaged codes are compared and the effects of space charge are found to reduce the efficiency to a level of 32%. The effect of beam quality in terms of the four volume phase space is examined and found to have an observable effect at this wavelength     

Analysis of the triode gun and the beam dynamics of a standing waveelectron linac

The charge-density integral method is used to analyse a triode gun which will inject electrons into a standing wave electron linac. The effect of space charge on the gun output beam parameters is estimated. The beam dynamics of the entire linac system is studied. The energy spectrum, beam size and divergence at the end of the linac are evaluated neglecting space charge     

Experimental investigation of a 140 GHz gyrotron-backward wave oscillator

We report the experimental operation of a voltage tunable gyrotron backward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for application as a fast tuning source for driving high power free electron lasers or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE_1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was achieved in the TE_1,2 mode with constant magnetic field. However, this tuning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions for a cold beam. The theoretical beam velocity spread was modeled by a 3D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotron wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity spread would be needed to make the gyro-BWO operate at the desired efficiency.     

基于BEM泊松方程求解的空间电荷效应数值模拟

 为了模拟强束流在加速器及其传输线中的行为,用C++语言开发了一种包含空间电荷效应的多粒子跟踪程序(PTP-SC),它在经典的PIC方法基础上,基于边界元法(BEM)和非等距的网格求解泊松方程。束流在自由空间分布的仿真结果与解析结果保持较好的一致性。给出了一条注入线的模拟计算结果,并与ORBIT,TRACE 3-D的计算结果进行比对。结果表明：该程序与采用数值方法的ORBIT程序的计算结果有较好的一致性。该程序可用于直线加速器及回旋加速器中的空间电荷效应模拟。     

束流传输管道孔径变化引起的束团场能差

由直线加速器提供的用于自由电子激光(FEL)的电子束团,在其传输过程中常会遇到传输管道孔径变化的情况.本文在电荷密度均匀分布假定下,应用直线加速器中空间电荷束团的有限长柱模型,推导得到了束流传输管道孔径变化引起的束团场能变化及场能差公式.数值计算结果表明,它与国外以往所采用的以连续束流来代表直线加速器中电荷束团而得到的结果,有很大的差别,并且是更符合直线加速器中的电荷束团的实际情况,因而是更精确和可靠的.     

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.     

Self consistent theory for gyrotrons including effect of voltage depression

The effect of the voltage depression due to space charge on the beam parameters in a gyrotron is investigated. Although the voltage depression can be compensated to some extent by increasing the beam voltage, some loss of efficiency is to be expected, especially in the case of high currents and volume modes.     

A Particle-in-cell scheme of the RFQ in the SSC-Linac

A 52 MHz Radio Frequency Quadrupole(RFQ)linear accelerator(linac)is designed to serve as an initial structure for the SSC-Linac system(injector into Separated Sector Cyclotron).The designed injection and output energy are 3.5 keV/u and 143 keV/u,respectively.The beam dynamics in this RFQ have been studied using a three-dimensional Particle-In-Cell(PIC)code BEAMPATH.Simulation results show that this RFQ structure is characterized by stable values of beam transmission efficiency(at least 95%)for both zerocurrent mode and the space charge dominated regime.The beam accelerated in the RFQ has good quality in both transverse and longitudinal directions,and could easily be accepted by Drift Tube Linac(DTL).The effect of the vane error and that of the space charge on the beam parameters have been studied as well to define the engineering tolerance for RFQ vane machining and alignment.     

激光加速质子束空间电荷力研究

激光加速器可以输出具有独特品质的质子束,例如μm尺寸、ps脉冲长度和高峰值电流。强流粒子束的空间电荷力效应较强,对面向应用的束流传输提出了挑战。通过二维PIC模拟研究了激光加速后与质子速度接近的电子的影响。采用椭球模型估算空间电荷力的影响,比较不同电荷分布的差异。结果表明每束团质子数超过1010后空间电荷力显著影响质子束传输,甚至严重破坏束流品质。空间电荷力的影响在20 ps后显著减弱,离开靶约1.2 mm。     

导引磁场控制相对论速调管放大器电子束收集的方法

 通过改变磁场位形,利用粒子模拟方法,研究了相对论速调管放大器（RKA）中电子束收集位置对器件效率和工作稳定性的影响。合适的电子束收集位置对增加输出腔区束波作用强度、减小输出腔区强流电子束的空间电荷势能以及减少RKA中反射电子数量非常有利。对一个工作频率2.85 GHz的RKA的电子束收集方式进行了改进,在电子束参数为510 keV和8.1 kA,注入微波功率500 kW和导引磁场1.5 T时,模拟得到了1.4 GW的微波输出,效率33.7%,增益33.8 dB,改进电子束收集方式之前的模拟结果为输出功率1.1 GW,效率为26.3%。利用Surperfish设计了改进收集方式后所需的磁场位形,并导入粒子模拟程序进行了模拟,实现了对电子束收集位置的有效控制,输出效率为32%。     

同轴慢波结构相对论高功率微波产生器理论分析

 推导了同轴波导的空间电荷限制流,其值大于圆波导的空间电荷限制流。因此在阴极电势和束流相等的情况下,同轴波导中的束流具有更高的动能,同轴器件有可能获得更高的微波转换效率。理论推导出同轴慢波结构中考虑束流空间电荷影响的色散方程,利用Matlab进行了编程求解。不考虑束流空间电荷影响时,编程计算结果与Superfish模拟结果一致。由考虑束流空间电荷影响的色散方程数值计算结果,可知文献中提出的同轴慢波结构相对论高功率微波产生器工作在准TEM模的π模,频率为7.67 GHz,峰值时间增长率较高,电子束损失的能量与其初始能量之比为34%。这些结果均与文献中的数值模拟结果一致。同时理论分析说明该种器件无论在能量转换效率,还是在产生微波脉冲的上升时间上均具有优势。     

Computer Simulation of the Periodic Electrostatic Focusing Converter

A computer study was made of the various physical processes that take place in the periodic electrostatic focusing, direct energy converter. Computer simulation techniques are described and the results are presented. The effect that space charge, secondary electrons, beam size, charge exchange, and ionization have on conversion efficiency is examined. The results are applied to the design of a reactor. Finally, future plans for studies of direct converters are discussed.