Electromagnetic dispersion characteristics of a high energy electron beam guided with an ion channel

Taking self-fields into consideration,dispersion properties of two types of electromagnetic modes for a high energy electron beam guided with an ion channel are investigated by using the linear perturbation theory.The dependences of the dispersion frequencies of electromagnetic waves on the electron beam radius,betatron frequency and boundary current are revealed.It is found that the electron beam radius and betatron frequency have different influences on the electromagnetic waves dispersion behavior by compared with the previous works.As the boundary current is taken into account,the TM modes will have two branches and a lowfrequency branch emerged as the new branch in strong ion channel case.This new branch has similar dispersion behavior to the betatron modes.For TE modes,there are two branches and they have different dispersion behaviors in strong ion channel case.However,in weak ion channel case,the dispersion behaviors for both of the low frequency and high frequency branches are similar.     

Induced ion-channel instability

In this paper, a new mechanism of electromagnetic instability, the induced ion-channel instability, is studied. It is based on the transverse driven betatron oscillation of relativistic electron beam induced by an additional magnetic undulator with a period close to the betatron wavelength in an ion channel. As its amplitude is sensitive to the electron beam energy, the driven betatron oscillation may determine electron beam grouping in the ponderomotive potential by selecting the undulator strength and period, and it provides a new mechanism of electron bunching, resulting in electromagnetic instability. Under proper condition, a new free-electron laser based on this mechanism may be realized.     

Multi-layer structure formation of relativistic electron beams in plasmas

A two-dimensional electromagnetic particle-in-cell simulation model is proposed to study the density evolution and collective stopping of electron beams in background plasmas. We show here the formation of the multi-layer structure of the relativistic electron beam in the plasma due to the different betatron frequency from the beam front to the beam tail. Meanwhile, the nonuniformity of the longitudinal wakefield is the essential reason for the multi-layer structure formation in beam phase space. The influences of beam parameters (beam radius and transverse density profile) on the formation of the multi-layer structure and collective stopping in background plasmas are also considered.     

Influence of the Beam Self-Fields on the Dispersion Characteristics of EM Waves in a Dielectric WaveguideFilled with Plasma简

Considering the self-fields of relativistic electron beam （REB）, the electron beam injected in plasma will form an ion-channel, which can stably guide REB transmitted in plasma. Based on the effect of ion-channel, the EM dispersion characteristics in a dielectric waveguide filled with plasmas is studied by using linear electromagnetic （EM） hydrodynamics perturbation theory. Both the TM mode dispersion equation and the dispersion relation are derived. After that, a discussion on the conditions of Cherenkov radiation in this system is given. It was found that the betatron oscillation of the REB is one of the important factors for the slow-wave EM instability in this system. The dispersion relation is of the form of electromagnetic-electrostatic （EM-ES） hybrid mode, which is different from the case where the effect of the ion channel is neglected.     

Focusing characteristics of the relativistic electron beam transmitting in ion channel

Based on the beam–plasma system model established in this paper, the trajectory of the electron beam in the ion channel is studied quantitatively through the envelope equation. Under different initial system parameters, the focusing transmission conditions of the beam in the ion channel are discussed. Then, a series of particle-in-cell simulations are performed, which generally versifies the theoretical results and shows some further details of the focusing behavior of the beam. It is found that the deceleration of some electrons around the focusing point or the beam–plasma interaction at the ion channel boundary will result in the generation of the residual electrons,which forms the electron return current that leads to the new instabilities influencing the focusing characteristics of the beam.     

Single Particle and Collective Effects Observed in the Electron Beam of the Maryland ERA Experiment

The formation of rotating, relativistic electron beams with properties suitable for collective ion acceleration has been studied under a variety of experimental conditions. A straight, cylindrical, relativistic electron beam (typical energy 2-3 MeV, typical current 2-10 kA) is passed through a narrow magnetic cusp, and the resulting rotating downstream beam has been studied using a number of diagnostic techniques. Two current regimes have been investigated: one in which the self-fields of the downstream electron beam are small compared to the applied fields, and one in which the self-fields are comparable to the applied fields. The beam characteristics in both regimes have been compared to single particle expectations. Experiments have also been conducted in which the effect of an inner and outer conducting boundary on the time-resolved beam cross section has been measured. Results will be discussed in the context of collective ion acceleration experiments now in preparation.     

Measurement of channeling radiation using a 45 MeV betatron

An experimental setup for the measurement of channeling radiation from relativistic electrons in single crystals is described. A betatron with a maximum beam energy of 45 MeV is used as electron source. The beam divergence is tuned to less than 0.3 mrad by means of a beam optic system. Results of the measurement of channeling radiation in silicon and beryllium crystals are reported and compared with calculations.     

Investigation of the confinement of high energy non-neutral proton beam in a bent magnetic mirror

By using the particle-in-cell (PIC) simulation method, we studied how the proton beam is confined in a bent magnetic mirror. It is found that the loss rate of the charged particles in a bent mirror is less than that in the axi-symmetric mirror. For a special bent mirror with the deflection angle of the coils α = 45°, it is found that the loss rate reaches maximum value at certain ion number density where the ion electrostatic oscillation frequency is equal to the ion cyclotron frequency. In addition, the loss rate is irrelevant to the direction of the proton beam. Our results may be helpful to devise a mirror. In order to obtain the least loss rate, we may choose an appropriate deflection angle, and have to avoid a certain ion number density at which the ion electrostatic oscillation frequency is equal to the ion cyclotron frequency.     

RFQ冷却聚束器束流冷却后的性质模拟

介绍了RFQ冷却聚束器的基本工作原理和束流在其中被约束和冷却的过程。采用真实相互作用势（RIP）模型进行蒙特卡罗模拟,得出束流在RFQ冷却聚束器中完全冷却后的性质。结果表明：束流冷却后的束斑和能量分散随时间一直保持平衡;束流冷却后束斑和能量分散不随缓冲气体气压、入射离子能量的变化而变化;冷却后束斑半径与射频（RF）四极电场频率呈反比,而冷却后束流的能量分散仅与温度有关。     

The electromagnetic instability in electron flow with ion background

Electromagnetic (EM) behavior and instability resulting from the interaction between EM wave and plasma wave are analyzed based on linear perturbation theory. It is shown that the instability is caused by the the coupling between high frequency electromagnetic field and electron transverse oscillation derived from the deflection of electron longitudinal oscillation due to self-produced magnetic field. The influences of the self-produced magnetic field and plasma density on the instability are studied. In addition characteristics of EM wave propagation at different angles are investigated. The present results are of significance to new type plasma radiation source, ion accelerator and plasma diagnostic techniques.     

Effects of Magnetic Field and Ion Velocity on SPT Plasma Sheath Characteristics

The distribution of magnetic field in Hall thruster channel has significant effect on its discharge process and wall plasma sheath characteristics. By creating physical models for the wall sheath region and adopting two-dimensional particle in cell simulation method, this work aims to investigate the effects of magnitude and direction of magnetic field and ion velocity on the plasma sheath characteristics. The simulation results show that magnetic field magnitudes have small impact on the sheath potential and the secondary electron emission coefficient, magnetic azimuth between the magnetic field direction and the channel radial direction is proportional to the absolute value of the sheath potential, but inversely proportional to the secondary electron emission coefficient. With the increase of the ion incident velocity, secondary electron emission coefficient is enhanced, however, electron density number, sheath potential and radial electric field are decreased. When the boundary condition is determined, with an increase of the sinmlation area radial scale, the sheath potential oscillation is aggravated, and the stability of the sheath is reduced.     

Study of the O-mode in a relativistic degenerate electron plasma

Using the linearized relativistic Vlasov-Maxwell equations,a generalized expression for the plasma conductivity tensor is derived.The dispersion relation for the O-mode in a relativistic degenerate electron plasma is investigated by employing the Fermi-Dirac distribution function.The propagation characteristics of the O-mode (cut offs,resonances,propagation regimes,harmonic structure) are examined by using specific values of the density and the magnetic field that correspond to different relativistic dense environments.Further,it is observed that due to the relativistic effects the cut off and the resonance points are shifted to low frequency values,as a result the propagation regime is reduced.The dispersion relations for the non-relativistic and the ultra-relativistic limits are also presented.     

Investigations on the time evolution of the plasma density in argon electron-beam plasma at intermediate pressure

The time evolution of the argon electron-beam plasma at intermediate pressure and low electron beam intensity was presented.By applying the amplitude modulation with the frequency of 20 Hz on the stable beam current,the plasma evolution was studied.A Faraday cup was used for the measurement of the electron beam current and a single electrostatic probe was used for the measurement of the ion current.Experimental results indicated that the ion current was in phase with the electron beam current in the pressure range from 200 Pa to 3000 Pa and in the beam current range lower than 20 mA,the residual density increased approximately linearly with the maximum density in the log-log plot and the fitting coefficient was irrelative to the pressure.And then three kinds of kinetic models were developed and the simulated results given by the kinetic model,without the consideration of the excited atoms,mostly approached to the experimental results.This indicated that the effect of the excited atoms on the plasma density can be ignored at intermediate pressure and low electron beam current intensity,which can greatly simplify the kinetic model.In the end,the decrease of the plasma density when the beam current was suddenly off was studied based on the simplified model and it was found that the decease characteristic at intermediate pressure was approximate to the one at high pressure at low electron beam intensity,which was in good accordance with the experimental results.     

K-shell excitation dielectronic recombination resonance strengths of highly charged He-like to O-like Xe ions

Dielectronic recombination is an important process in high temperature plasmas.In the present work,the KLn (n =L,M,N and O) DR resonance strengths of He-like to O-like xenon ions are measured at the Shanghai electron beam ion trap using a fast electron beam energy scanning method.The experiment uncertainty reaches about 6％ with significant improvement of statistics.A relativistic configuration interaction calculation is also made.Theoretical results agree with the experiment results within 15％ in most cases.     

Mechanism analysis of radiation generated by the beam-plasma interaction in a vacuum diode

When we were studying the vacuum switch,we found that the vacuum diode can radiate a broadband microwave.The vacuum diode is comprised of a cathode with a trigger device and planar anode,there is not a metallic bellows waveguide structure in this device,so the radiation mechanism of the vacuum diode is different from the plasma filled microwave device.It is hard to completely imitate the theory of the plasma filled microwave device.This paper analyzes the breakdown process of the vacuum diode,establishes the mathematical model of the radiating microwave from the vacuum diode.Based on the analysis of the dispersion relation in the form of a refractive index,the electromagnetic waves generated in the vacuum diode will resonate.The included angle between the direction of the electromagnetic radiation and the initial motion direction of electron beam is 45 degrees.The paper isolates the electrostatic effect from the beam-plasma interaction when the electromagnetic radiation occurs.According to above analyses,the dispersion relations of radiation are obtained by solving the wave equation.The dispersion curves are also obtained based on the theoretical dispersion relations.The theoretical dispersion curves are consistent with the actual measurement time-frequency maps of the radiation.Theoretical deduction and experiments indicate that the reason for microwave radiating from the vacuum diode can be well explained by the interaction of the electron beam and magnetized plasma.     

电子储存环注入过程中补注电荷的三维位置信息提取

电子储存环的注入过程是特殊的瞬态过程，研究补注电荷和储存电荷的三维位置融合过程可在线评估注入器与储存环的匹配度，也可为下一代光源诊断系统的搭建奠定技术基础。目前，上海同步辐射光源束流检测组研发了一套基于纽扣电极拾取信号和高速采集板卡的三维逐束团诊断系统，可精确测量逐束团三维位置。注入过程中补注电荷的三维位置信息可通过电荷加权平均法和比例系数法分别提取。通过对补注电荷横向betatron振荡和纵向同步振荡的分析，可原位提取betatron振荡振幅、同步振荡振幅、初始到达时间、同步振荡阻尼时间等多个动态参数，不需提供特别的机器研究时间，为储存环动力学研究提供了有力工具。     

3D Position Information Extraction of Refilled Charge during Injection Process of Electron Storage Ring

The injection process of electron storage ring is a special transient process. The mismatch between the injector and the storage ring can be evaluated online by studying the 3D merging of refilled charge and stored charge. For the next-generation light source, it can lay a technical foundation for building a diagnostic system. The beam instrumentation group of the Shanghai Synchrotron Radiation Facility developed a 3D bunch-by-bunch diagnostic system based on button pick-up signal and high-speed acquisition boards to measure the beam transverse position and longitudinal phase. Extracting the 3D position information of the refilled charge during the injection process is the critical issue in this system. The charge-weighted average method was proposed to extract the transverse position and the proportional coefficient method was proposed to extract the longitudinal phase. The transverse betatron oscillation and longitudinal synchrotron oscillation of the refilled charge were analyzed. 3D position information of the refilled charge and several dynamic parameters of ring, such as longitudinal maximum oscillation amplitude, initial arrival time and synchrotron damping time, can be extracted during the user operation mode, which provides a strong toolkit for accelerator physics.     

Linear hybrid simulations of low-frequency fishbone instability driven by energetic passing particles in tokamak plasmas

A linear simulation study of energetic passing particle-driven low-frequency fishbone instability in tokamak plasmas has been carried out using the global kinetic-MHD (magnetohydrodynamics) hybrid code M3D-K. This work is focused on the interaction of energetic passing beam ions andn=1 mode with a monotonic safety factorqprofile andSpecifically, the stability and mode frequency as well as mode structure of then=1 mode are calculated for scans of parameter values of beam ion beta, beam ion injection energy, beam ion orbit width, beam ion beta profile, as well as background plasma beta. The excited modes are identified as a low-frequency fishbone with the corresponding resonance ofwhereis the beam ion toroidal transit frequency andis the beam ion poloidal transit frequency. The simulated mode frequency is approximately proportional to the beam ion injection energy and beam ion orbit width. The mode structure is similar to that of internal kink mode. These simulation results are similar to the analytic theory of Yuet al.     

Measurements of Beam Behavior in the Karlsruhe Cyclotron

From the measurements made during the first operation period of the Karlsruhe cyclotron those of radial beam density, phase width and axial beam distribution are mentioned. Because of the satisfactory separation of the single orbits in a large radial range it has been easy to obtain information on the energy gain per turn, the amplitudes of the coherent radial oscillation and the radial dependence of the radial and axial betatron frequencies.     

Bunch-by-bunch beam size measurement during injection at Shanghai Synchrotron Radiation Facility

Fast beam profile measurement is important in fast beam dynamic behavior investigations. A bunch-bybunch beam size measurement system, which is presently used to measure horizontal profile, has been developed at the Shanghai Synchrotron Radiation Facility(SSRF) and is capable of measuring bunches within a separation of 2 ns.The system is based on a direct-imaging optical system and high-speed photomultiplier array detector. A high-bandwidth linear signal amplifier and acquisition module have also been designed to process bunch-by-bunch multichannel signals from the detector. The software resampling technique and principal component analysis method were developed to obtain the synchronized data and enhance the signal-to-noise ratio. The fast injection of transients was successfully captured and analyzed. Moreover, the bunchby-bunch positions and sizes exhibited strong oscillation after the injection at the horizontal betatron oscillation frequency of the SSRF storage ring, and this demonstrated the bunch-by-bunch measurement capability of our system.