BEPCII直线注入器的尾场效应

BEPCII直线注入器中的强流、短束团的尾场效应将损害束流的性能.用分析解和数值模拟计算的方法,系统地研究了尾场对纵向和径向束流动力学的影响,包括单束团的短程尾场和多束团的长程尾场对束流能量、能散、发射度、轨道和初级电子束在正电子产生靶上束斑尺寸的影响等.研究了有效抑制这些尾场效应的措施     

Developments in laser wakefield accelerators:From single-stage to two-stage

Laser wakefield accelerators(LWFAs)are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators.It is attributed to their high acceleration gradient which is about 3 orders of magnitude larger than the traditional ones.The past decade has witnessed the major breakthroughs and progress in developing the laser wakfield accelerators.To achieve the LWFAs suitable for applications,more and more attention has been paid to optimize the LWFAs for high-quality electron beams.A single-staged LWFA does not favor generating controllable electron beams beyond 1 Ge V since electron injection and acceleration are coupled and cannot be independently controlled.Staged LWFAs provide a promising route to overcome this disadvantage by decoupling injection from acceleration and thus the electron-beam quality as well as the stability can be greatly improved.This paper provides an overview of the physical conceptions of the LWFA,as well as the major breakthroughs and progress in developing LWFAs from single-stage to two-stage LWFAs.     

Plasma Photocathodes

Plasma wakefield accelerators offer accelerating and focusing electric fields three to four orders of magnitude larger than state-of-the-art radiofrequency cavity-based accelerators. Plasma photocathodes can release ultracold electron populations within such plasma waves and thus open a path toward tunable production of well-defined, compact electron beams with normalized emittance and brightness many orders of magnitude better than state-of-the-art. Such beams will have far-reaching impact for applications such as light sources, but also open up new vistas on high energy and high field physics. This paper reviews the innovation of plasma photocathodes, and reports on the experimental progress, challenges, and future prospects of the approach. Details of the proof-of-concept demonstration of a plasma photocathode in 90° geometry at SLAC FACET within the E-210: Trojan Horse program are described. Using this experience, alongside theoretical and simulation-supported advances, an outlook is given on future realizations of plasma photocathodes such as the upcoming E-310: Trojan Horse-II program at FACET-II with prospects toward excellent witness beam parameter quality, tunability, and stability. Future installations of plasma photocathodes also at compact, hybrid plasma wakefield accelerators, will then boost capacities and open up novel capabilities for experiments at the forefront of interaction of high brightness electron and photon beams.     

尾波场与相对论效应对激光脉冲自相位调制及频移影响的比较研究

对共振和非共振情形下尾波场效应与相对论效应对强激光脉冲在稀薄等离子体中传输的影响 作了详细比较.尾波场效应导致脉冲自相位调制的不对称性，其大小与初始脉冲形状和脉宽 大小有关.在长脉冲极限下，尾波场效应远小于相对论效应，从而可以忽略；在短脉冲极限 下，尾波场效应可以抵消相对论效应.在共振情况下，尾波场效应导致脉冲内大部分光子减 速. 关键词： 脉冲传输 等离子体 相对论效应 尾波场效应 自相位调制     

Simulation of a chirped femtosecond relativistic laser pulse interaction with underdense plasma by using a hydrodynamic approach

A chirped laser pulse indicates that the laser frequency changes over the duration of the pulse: a positively (negatively) chirped pulse implies that the laser frequency increases (decreases) with time. In this paper, we use a simplified, fully relativistic hydrodynamic approach to simulate the influence of chirp on the propagation of a femtosecond relativistic laser pulse in underdense plasma. Based on this simplified cold‐fluid model, the influence of chirp on the main dynamics of the laser pulse, such as self‐steepening, red‐shift in the leading edge, variation of the frequency chirp, and the generated wakefields can be studied self‐consistently. The simulation results show that a pulse with a positive chirp results in a larger increment in the intensity parameter a₀ when propagating a certain distance into an underdense plasma compared with an un‐chirped and a negatively chirped pulse, which is largely because of a much greater forward shift of the peak amplitude and more severe pulse self‐steepening effect due to the frequency red‐shift at the leading edge when exciting a plasma wave. The ponderomotive force, which relates to the first‐order differential of the laser pulse intensity envelope, is expected to be stronger for a positively chirped pulse because of its steeper leading edge and larger intensity parameter a₀. As a result, the wakefield driven by the positively chirped laser pulse is more intense than that driven by an un‐chirped and a negatively chirped laser pulse, which is confirmed by our self‐consistent hydrodynamic simulation.     

Effects of Higher-Order Relativistic Nonlinearity and Wakefield During a Moderately Intense Laser Pulse Propagation in a Plasma Channel

Using a variational approach, the propagation of a moderately intense laser pulse in a parabolic preformed plasma channel is investigated. The effects of higher-order relativistic nonlinearity (HRN) and wakefield are included. The effect of HRN serves as an additional defocusing mechanism and has the same order of magnitude in the spot size as that of the transverse wakefield (TWF). The effect of longitudinal wakefield is much larger than those of HRN and TWF for an intense laser pulse with the pulse length equaling the plasma wavelength. The catastrophic focusing of the laser spot size would be prevented in the present of HRN and then it varies with periodic focusing oscillations.     

激光高能X射线成像中探测器表征与电子影响研究

基于激光尾场加速电子的高能X射线源具有高光子能量与小源尺寸的特点,在高空间分辨无损检测方面发挥着十分重要的作用.在X光机上测量了CsI针状闪烁屏、锗酸铋(BGO)闪烁阵列与DRZ闪烁屏的本征空间分辨率,并模拟了三类探测器对高能X射线的能量沉积响应,其中CsI针状闪烁屏的空间分辨率高达8.7 lp/mm.采用Ta转换靶产生的高能X射线开展透视照相,能够分辨最高面密度33.0 g/cm~2的两层客体结构.开展了X射线照相、X射线与电子混合照相以及电子照相三种情况的比对实验,在X射线产额不足或探测效率不够情况下采用X射线与电子混合透视照相的方案,以牺牲对比度为代价,能较大程度地提高图像信号强度.     

The wakefield and energy loss study of micro-bunch trains passing through plasmas

Particle-in-cell simulations are performed to study the wakefield and the energy loss of a train of micro-bunches passing through background plasmas. The wakefield and the energy loss are found to be directly connected to the interval of each micro-bunch. If the interval is once the length of the wakefield wavelength, the wakefield excited by each micro-bunch will be superimposed. The energy loss of the micro-bunch increases along the bunch train. On the other hand, if the interval is 1.5 times the length of the wakefield wavelength, the wakefield almost disappears. Micro-bunches on the odd position of the train will transfer their energies to the plasma, and then, these energies are absorbed by the subsequent even micro-bunches. Thus, the total energy loss of the train is greatly reduced.     

Laser-driven electron beam and radiation sources for basic,medical and industrial sciences

To date active research on laser-driven plasma-based accelerators have achieved great progress on production of high-energy, high-quality electron and photon beams in a compact scale. Such laser plasma accelerators have been envisaged bringing a wide range of applications in basic, medical and industrial sciences. Here inheriting the groundbreaker’s review article on “Laser Acceleration and its future” [Toshiki Tajima, (2010)],¹⁾ we would like to review recent progress of producing such electron beams due to relativistic laser-plasma interactions followed by laser wakefield acceleration and lead to the scaling formulas that are useful to design laser plasma accelerators with controllability of beam energy and charge. Lastly specific examples of such laser-driven electron/photon beam sources are illustrated.     

超短脉冲激光与稀薄等离子体相互作用的准静态粒子模拟研究

具体讨论准静态近似在粒子模拟技术中的实现方法，主要包括：简化物理模型的建立，数值模拟方法的实现，程序结构的建立.最后，给出了三个成功的数值模拟结果，短脉冲激光的自聚焦现象、激光尾流场的激发和自生磁场现象，并进行了物理分析. 关键词： 准静态 粒子模拟 激光尾流场 自聚焦