BTCF上离子引起的束流不稳定性研究

对所设计的北京τ-粲工厂(BTCF)高亮度对撞模式下电子环中的离子引起的束流不稳定性做了研究.计算了不同束团填充空隙下的离子俘获比,然后用自编的模拟程序对BTCF上快束流–离子不稳定性做了模拟跟踪,得到的快束流–离子不稳定性的增长时间和理论解析计算值基本一致.     

BEPCⅡ横向束流反馈系统样机在BEPC上的实验

为了抑制BEPCⅡ产生的耦合束团不稳定性, 提高储存环中的流强, 研制了一套逐束团横向束流反馈系统的样机并在BEPC上做了实验, 得到了理想的结果. 当束团不稳定时, 频谱仪上可以观察到横向边带, 此时将反馈系统闭环工作于阻尼状态时, 横向边带立即消失. 本文介绍了样机系统的组成、各部分的功能和实验结果.     

合肥光源逐束团测量和横向束流反馈系统设计

 介绍了在合肥光源开展逐束团测量（横向和纵向）和横向束流反馈系统研究和研制的重要性,同时还介绍了设计思想。合肥光源高频频率为204 MHz,因此,系统至少需要100 MHz的带宽。还较详细地介绍了宽带部件和系统参数的选择原则。该系统不仅可用于研究由于高频腔中的高阶模和真空室的阻抗壁效应所引起的耦合束团不稳定性,而且还能抑制耦合束团不稳定性振荡、快速束流离子不稳定性和注入大幅度振荡等,从而将提高机器的运行性能。     

HIAF-BRing束团合并过程中的束流负载效应模拟

强流重离子加速器HIAF-BRing在加速完成后进行束团合并,为研究BRing中的束流负载效应对束团合并的影响,对238U35+束流进行了粒子跟踪模拟。模拟结果显示,在束团合并过程中,束流负载效应引起束团长度和束团中心位置的振荡,导致束流动量分散和束团长度的增长。束团合并过程中尾场电压以及不同束团间尾场的耦合导致的势阱畸变,是引起束团长度和束团中心振荡及束流发射度增长的原因。为了降低束流负载效应的影响,采用多谐波前馈系统进行补偿,达到了补偿束团合并过程中的束流负载效应的目的,从而确保了BRing中引出束流的品质,同时根据模拟结果确定了前馈系统需要覆盖的频率范围和需要补偿的最大尾场电压。     

合肥同步辐射光源上的束腔不稳定性研究

 束腔相互作用激发的束流不稳定性是目前高能电子加速器上最重要的不稳定性之一。本文从粒子纵向运动基本方程出发,详尽分析了单束团和多束团情况下束腔纵向偶极不稳定性,并给出了我国合肥同步辐射光源上的计算和实验结果。     

HLS储存环高频调制实验

在合肥光源(HLS)储存环上进行了高频调制实验, 结果表明采用调制频率接近同步振荡频率f_s的高频幅度或相位调制, 可以提高束流寿命. 而通常采用的是2f_s的调制方式. 实验测量了最佳调制频率和合适的调制度, 以及不同流强下调制引起寿命增长的程度. 同时观察到束流频谱中高次分量的降低, 这有利于抑制多束团耦合不稳定性.     

上海软X射线自由电子激光装置束流分配系统设计（英文）

束流分配系统是自由电子激光装置中至关重要的一部分,它可以将直线加速器产生的电子束团分配至不同的波荡器中。提出了一种基于上海软X射线自由电子激光装置的束流分配系统设计方案。针对该方案,详细介绍了三维从头至尾的束团跟踪模拟以及在传输过程中的束流动力学分析,模拟结果表明,该束流分配系统设计可以保证束流发射度增长小于8%,同时可以保证峰值电流、能散以及束团长度在经过该分配系统时未受到破坏。此外,针对束团在直线加速器中的微束团不稳定性和抖动也进行了分析。     

上海软X射线自由电子激光装置束流分配系统设计

束流分配系统是自由电子激光装置中至关重要的一部分,它可以将直线加速器产生的电子束团分配至不同的波荡器中。提出了一种基于上海软X射线自由电子激光装置的束流分配系统设计方案。针对该方案,详细介绍了三维从头至尾的束团跟踪模拟以及在传输过程中的束流动力学分析,模拟结果表明,该束流分配系统设计可以保证束流发射度增长小于8%,同时可以保证峰值电流、能散以及束团长度在经过该分配系统时未受到破坏。此外,针对束团在直线加速器中的微束团不稳定性和抖动也进行了分析。     

双束流激发的纵向尾场与纵向不稳定性的研究

 用解析的方法研究了同向和反向运动束流激发的纵向尾场,并且给出了线性近似下束团沿环不均匀分布情形下的双束流纵向多束团不稳定性的计算公式,得出了双束流不稳定性与高频腔(或其它类腔结构)在环上位置及模的对称性有关的结论。     

双束流负载效应

分析研究了双束流储存环上的束流负载效应.束团纵向尾场的作用分别为:能量损失、同步频移和耦合束不稳定性,这里只研究前两个方面.以来团连续分布的束团串情形和均匀分布情形为例,给出了同步相移、频移与高次模参数及高频腔分布的关系.研究结果有助于优化高频腔的工作模式(通过调节调谐杆的位置来改变高次模的频率)和环上束团的分布,以减小束流负载效应.当高频腔关于对撞点对称分布时,正负电子的束流负载效应相同,可以用补偿单束流负载效应的方法来补偿正负电子束团的相位.     

Transverse oscillations of a long-pulse electron beam on alaser-formed channel

An intense relativistic electron beam may be transported in low-pressure gas using an ion channel which focuses and guides the beam. The beam can be unstable to the growth of transverse oscillations caused by the electric force between the beam and channel-the ion hose instability. Beam propagation on channels created by photoionization of gas with an excimer laser is discussed. Ion hose oscillations have been recorded which have a betatron wavelength of approximately 1.5 m. The growth rate of the ion hose instability in the linear regime was measured as 1.67±0.45. At this level of growth, the amplitude of beam oscillations equals the channel radius after a period of one-third of an ion oscillation time     

Stochastic heating of ions in a modulated electron beam-plasma system

A modulation of an electron beam density at frequencies lower than an ion cyclotron frequency enhanced the low frequency instability of broad band and heated plasma ions in a beam-plasma system. The heating of ions is explained by the stochastic process of a weakly turbulent plasma.     

Upconversion of whistler waves by gyrating ion beams in a plasma

A gyrating ion beam, with a ring-shaped distribution in velocity, supports negative energy beam modes near the harmonics of beam gyro-frequency. An investigation of the non-linear interaction of high-frequency whistler waves with the negative energy beam cyclotron mode is made. A non-linear dispersion relation is derived for the coupled modes. It is shown that a gyrating ion-beam frequency upconverts the whistler waves separated by harmonics of beam gyro-frequency. The expression for the growth rate of whistler mode waves has been derived. In Case 1, a high-amplitude whistler wave decays into two lower frequency waves, called a low-frequency mode and a side band of frequency lower than that of pump wave. In Case 2 a high-amplitude whistler wave decays into two lower frequency daughter waves, called the low-frequency mode and whistler waves. Generation mechanism of these waves has application in space and laboratory plasmas.     

High-frequency emissions during the propagation of an electron beamin high-density plasma

A relativistic annular electron beam passing through a high-density plasma excites Langmuir waves via Cerenkov interaction. The Langmuir waves are backscattered off ions via nonlinear ion Landau damping. At moderately high amplitudes these waves are parametrically up-converted by the beam into high-frequency electromagnetic radiation, as observed in some recent experiments. A nonlocal theory of this process is developed in a cylindrical geometry. It is seen that the growth rate of the Langmuir wave scales as one-third the power of beam density. The growth rate of parametric instability scales as one-fourth the power of beam density and the square root of beam thickness     

Counterpropagating pattern dynamics: From narrow to broad beams

The development of patterns in counterpropagating (CP) beams in saturable Kerr-like media is investigated in a numerical study, by varying the width of beams. A model of interacting mutually incoherent beams is adopted, with slow local changes in the index of refraction, and a numerical simulation of propagating beams is performed for different values of the driving parameters. Linear stability analysis (LSA) of CP beams is carried out, and a modulational instability threshold for the appearance of side beams is determined. The results of LSA, concerning the instability of plane waves, are compared with the numerical results concerning broad hyper-Gaussian beams whose width is varied. Qualitative agreement is found, due to the similarity between plane waves and flat-top hyper-Gaussian beam profiles. Owing to very slow quasi-steady-state changes in the system during time evolution, the transverse characteristics of beam distributions, determined by LSA, are found to persist for long in the transient evolution of CP beams.     

Filamentation of laser radiation in DC biased GaAs

In this paper, we have examined the filamentation instability of laser radiation in a GaAs sample in the negative differential resistivity region. A dc electric field is applied to assist the laser beam transferring electrons in the conduction band from the lower valley to upper satellite valleys, i.e. in increasing the effective mass of electrons. The instability causes the space charge perturbations in the microwave range of frequencies. The nonlinearity arises through the energy dependence of the effective mass and the ponderomotive force on the electrons. The instability has finite frequency and possesses large growth rate which is greatly influenced by the dc electric field.     

Decay instability in a plasma channel

Near the critical layer, a large-amplitude laser beam creates a transverse plasma channel due to the ponderomotive force. The axis of the channel is normal to the ambient density gradient. Inside the channel, the laser beam decays into counter-propagating ion acoustic and Langmuir waves. A nonlocal theory of the process is developed for a parabolic density profile. The growth rate of the instability is significantly reduced due to the nonlocal effects     

Effect of self-fields on the electron cyclotron maser instability in a dielectric loaded waveguide

The influence of self-fields on the cyclotron maser instability in a hollow electron beam propagating parallel to a uniform axial magnetic field B ₀ ê _z in a dielectric loaded waveguide is investigated. The theoretical analysis is carried out within the framework of linearized Vlasov-Maxwell equations. It is assumed that the beam is thin with the radial thickness much smaller than the beam radius. A new dispersion relation for azimuthally symmetric electromagnetic perturbation is derived and analyzed numerically. The influence of self-fields on the cyclotron maser instability in a dielectric loaded waveguide for different dielectric medium is studied. It is found that unlike the hollow waveguide the growth rate is increased by increasing self-fields. The instability band width decreases due to the increasing self-fields. The maximum growth rate increases gradually as self-fields increase as regards a different dielectric medium.     

束—等离子体放电中低频等离子体波的非稳定性研究

本文采用非中性冷等离子体模型,研究了沿磁场入射的电子束和等离子体相互作用引起的低频等离子体波的非稳定性。计算表明,在束-等离子体放电情况下,振动频率和增长率都随电子束密度增加而增加,当束电流一定时,它们随l增加而略有增加。l=1时其频率数量级与离子迴旋频率相同。l≥3时,振动频率几乎不随k_z变化,其振动频率和增长率数量级与低混杂振荡频率相同,实质是低混杂漂移非稳定性。理论结果与实验一致。     

黑腔中空间束匀滑光束成丝不稳定性的数值模拟北大核心CSCD

为深入研究黑腔内激光成丝不稳定性产生和发展,建立了空间束匀滑光束的传播模型,利用三维激光等离子体相互作用程序LAP3D模拟了黑腔物理条件下的大尺度成丝现象。模拟结果可正确描述成丝后光强的束发散现象及谱空间光强分布特征。从激光能量参数角度分析了导致成丝不稳定性发展的物理因素,并讨论了应用空间束匀滑技术抑制成丝不稳定性的局限性。研究表明:空间束匀滑光束发展成丝的能力取决于光斑内小焦斑的成丝表现,当具备成丝能力的小焦斑份额达到一定比例后,就会导致整个空间束匀滑光束成丝,因此对于光斑平均功率密度较高的光束,须考虑结合其他束匀滑手段来抑制成丝发展。