利用电压异步调节束流能散度

对直线感应加速器加速电压的同步性对束流能散度的影响进行了分析,结果表明:考虑束流负载效应后,束流的不同部分将得到不同的能量增益,前后沿部分相对于平顶部分得到更多的能量增益,从而在一定程度上增加整个束流的能散度,限制了加速器光源尺寸的进一步减小。因此,提出利用调节加速电压异步的方法来调节束流不同部分得到的能量增益,从而在一定程度上降低整个束流脉宽期间的能散度。初步的试验结果也证明,加速电压的异步调节可以明显改变束流整体的的能散度。     

Adiabatic plasma buncher

In this paper, we present a new scheme of injection into a plasma accelerator, aimed at producing a high-quality beam while relaxing the demands on the bunch length of the injected beam. The beam dynamics in the injector, consisting of a high-voltage pulsed photodiode, is analyzed and optimized to produce a λ_p/20 long electron bunch at 2.5 MeV. This bunch is injected into a plasma wave in which it compresses down to λ_p/100, while accelerating up to 250 MeV. This simultaneous bunching and acceleration of a high-quality beam requires a proper combination of injection energy and injection phase. Preliminary results from simulations are shown to assess the potentials of the scheme     

Bunch length measurement at Tsinghua Thomson scattering X-ray source

The length of electron beam from a photocathode RF gun is determined by a spectrometer, according to the relative energy spread induced by the bunch length during the acceleration in a linac. For a photocathode RF gun, different laser injected phase and beam charge are studied. The compression is changed for the different laser phases, as from 10° to 30°, and the bunch length is lengthened due to the strong longitudinal space charge force, caused by the increased charge.     

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。     

Acceleration of nonmonoenergetic electron bunches injected into a wake wave

The trapping and acceleration of nonmonoenergetic electron bunches in a wake field wave excited by a laser pulse in a plasma channel is studied. Electrons are injected into the region of the wake wave potential maximum at a velocity lower than the phase velocity of the wave. The paper analyzes the grouping of bunch electrons in the energy space emerging in the course of acceleration under certain conditions of their injection into the wake wave and minimizing the energy spread for such electrons. The factors determining the minimal energy spread between bunch electrons are analyzed. The possibility of monoenergetic acceleration of electron bunches generated by modern injectors in a wake wave is analyzed.     

Laser wakefield acceleration of short electron bunches

The theory of electron acceleration in a plasma wake wave is developed, and the dependence of the main characteristics of accelerated electron bunches on the wakefield parameters is investigated, It is shown that using a prebunching stage, under proper conditions, the final electron density of a compressed and accelerated bunch can exceed the initial electron beam density by orders of magnitude and that longitudinal bunch compression provides quasi-monoenergetic acceleration to high energies, It is demonstrated that, for an initial electron beam radius smaller than the optimal one for efficient beam trapping, the energy spread of the compressed and accelerated electron bunch and its length can be evaluated by using the simple analytical predictions of a one-dimensional (1-D) theory. The obtained analytical results are confirmed by three-dimensional (3-D) numerical modeling     

Effect of the ponderomotive scattering and injection position on electron-bunch injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turns out that the injection of electron bunches longer than the plasma wavelength can also generate accelerated femtosecond bunches with a relatively low energy spread. This is of great interest because such injecting bunches can be provided, e.g., by photo cathode rf linacs. Here we show that when an e-bunch is injected into the wakefield, it is important to take into account the interaction of the injected bunch with the laser pulse in the vacuum region located in front of the plasma. We show that at low energies of the injected bunch, this leads to ponderomotive scattering of the bunch and results in a significant drop of the collection efficiency. For certain injection energies the ponderomotive scattering may result in a smaller energy spread in the accelerated bunch. It is found that the injection position in the laser wakefield plays an important role. Higher collection efficiency can be obtained for certain injection energies, when the bunch is injected in plasma at some distance from the laser pulse; the energy spread, however, is typically larger in this case. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch. PACS 52.38.Kd; 41.75.Jv; 41.85.Ar     

合肥光源单束团下束团长度和能散的测量

 根据同步光与储存环中的束流信号具有相同的时间结构的原理,测量同步光脉冲的半高全宽值可以计算出束团的长度。根据合肥光源的特点和实际需要,选择快速光电接收器搭配高速高带宽示波器作为在线测量束团长度和纵向分布等的主要手段。对单束团模式下束团长度随流强和高频腔腔压的变化趋势进行了测量。测量结果表明：束团长度与腔压的0.3次方成反比,比理论值0.5小;而束团长度随流强的增长率为2.0 ps/mA。通过测量纵向量子寿命进行了能散随流强变化的间接测量,结果表明,束团的拉伸是能散变化和势阱效应共同作用的结果。     

Cohesive acceleration and focusing of relativistic electrons in overdense plasma

We describe our studies of the generation of plasma wake fields by a relativistic electron bunch and of phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma's density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35 MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.     

Optimization of the beam quality in ionization injection by a tailoring gas profile

A new scheme is proposed to improve the electron beam quality of ionization-induced injection by tailoring gas profile in laser wakefield acceleration. Two-dimensional particle-in-cell simulations show that the ionization-induced injection mainly occurs in high-density stage and automatically truncates in low-density stage due to the decrease of the wakefield potential difference. The beam loading can be compensated by the elongated beam resulting from the density transition stage. The beam quality can be improved by shorter injection distance and beam loading effect. A quasi-monoenergetic electron beam with a central energy of 258 Me V and an energy spread of 5.1% is obtained under certain laser–plasma conditions.     

Velocity bunching in travelling wave accelerator with low acceleration gradient

We present the analytical and simulated results of our study of the influence of the acceleration gradient in the velocity bunching process, which is a bunch compression scheme that uses a travelling wave accelerating structure as a compressor. Our study shows that the bunch compression application with low acceleration gradient is more tolerant to phase jitter and more successful in obtaining a compressed electron beam with symmetrical longitudinal distribution and low energy spread. We also present a transverse emittance compensation scheme to compensate the emittance growth caused by the increase of the space charge force in the compressing process, which is easy to adjust for different compression factors.     

Forward acceleration and generation of femtosecond megaelectronvolt electron beams by an ultrafast intense laser pulse

We present a new mechanism of energy gain of electrons accelerated by a laser pulse.It is shown thatwhen the intensity of an ultrafast intense laser pulse decreases rapidly along the direction of propagation,electrons leaving the pulse experience an action of ponderomotivc deceleration at the descending part ofa lower-intensity laser field than acceleration at the ascending part of a high-intensity field, thus gain netenergy from the pulse and move directly forward. By means of such a mechanism, a megaelectronvoltelectron beam with a bunch length shorter than 100 fs could be realized with an ultrafast(≤30 fs),intense (>10~(19)W/cm~2)laser pulse.     

L波段强流单束团注入器的束流动力学计算

自由电子激光器要求高亮度、低能散度的电子束注入波荡器(Undulator)。本文叙述提供高亮度电子束的高频电子直线加速器中的注入器部分的设计计算。注入器由L波段(1300MHz)的十二分频和三分频两个谐振腔预聚束器和一个基波频率的变相速聚束器组成。粒子运动方程中考虑了空间电荷效应和束流负载效应。电子枪的注入参数:脉冲宽度T=4ns;电流I=5A;电子的初始动能E_0=100keV;电子束分布为高斯型。参数优化设计结果:单束团宽度小于25ps,峰值电流达400A以上,电子的平均归一化能量>4,束团内的能量差小于200keV。     

电子在紧聚焦激光束中加速的最佳入射

通过数值模拟研究了电子在真空紧聚焦激光束中的动力学行为和能量增益。研究发现对应于两种不同的轨迹，电子可以得到纵向和横向两种电场的加速。阐明了电子的能量增益与电子的入射角和入射动量间的关系。对于给定的激光参数，给出了电子的最佳入射参数。     

Bunch compression at the SPring-8 linac and successive generation of THz pulse train in the isochronous ring

We have demonstrated the idea of circulating a short, intense linac bunch for some tens of turns in an isochronous ring. We compressed a bunch from the SPring-8 linac to a few picoseconds rms by means of an energy compression system and a beam transport line from the linac to NewSUBARU. The NewSUBARU storage ring was set to a quasi-isochronous condition and the bunch circulated for about 50 turns after injection while maintaining the short bunch length. At the same time, a pulse train of strong coherent synchrotron oscillation from the short bunch was observed.     

On the theory of photocathode rf guns

In this paper we give a set of analytical formulae to describe the characteristics of photocathode rf guns at any rf frequencies, such as energy, energy spread, bunch length, out going current, and emittance etc. as functions of the laser injection phase, which are useful in the design and practical operation of rf guns.     

On the theory of photocathode rf guns

In this paper we give a set of analytical formulae to describe the characteristics of photocathode rf guns at any rf frequencies, such as energy, energy spread, bunch length, out going current, and emittance etc.as functions of the laser injection phase, which are useful in the design and practical operation of rf guns.     

Analysis of electron capture acceleration channel in an Airy beam

Using numerical simulation, we have studied in detail vacuum electron acceleration induced by an Airy beam. The phase of the field varies slowly, and the intensity of the field is independent of the decaying parameter of the beam in the asymmetric field channel [(AFC) Opt. Express 18, 7300 (2010)] formed by the Airy beam. Results show that an electron entering into the AFC may be captured and gain high energy. Meanwhile, the decaying parameter, injection energy, and injection angle of the electron play important roles in the electron energy gain.     

粒子束团状态对测量束流发射度及能量的影响

为了验证国产质子注入器的参数是否满足需求,注入器团队设计了束流测量系统用于测量国产质子直线注入器束流的流强、发射度、能量以及能散等关键指标。此测量系统包含了采用变聚焦法测量发散度、采用分析磁铁测量束流能量和能散的主要功能。利用束流输运线设计软件Tracewin(版本2.11.4.1)进行了系统束线的物理设计,对束测系统测量质子束流的发散度和能量的精度进行了模拟计算。由于经过RFQ-(APF)DTL加速后的粒子束团为“拖尾”的非理想粒子束团,需要针对非理想束团对束测系统测量发射度和能量产生的影响进行分析。通过对模拟计算结果的分析,发现相对于测量理想粒子束团的结果非理想粒子束团对束测系统测量发射度精度影响较大;非理想粒子束团对束测系统测量能量精度影响较小。     

Design studies on a 500 kV DC gun photo-injector for the BXERL test facility

BXERL is a proposal for a test facility(Beijing X-ray Energy Recovery Linac),which requires its injector to provide an electron beam of 5 MeV,77 pC/ bunch at a repetition rate of 130 MHz(average current of 10 mA).In this paper,we present the design of the injector,which consists of a 500 kV photocathodeDC gun equipped with a GaAs cathode preparation device,a 1.3 GHz normal conducting RF buncher,two solenoids,and one cryomodule containing two 1.3 GHz 2-cell superconducting RF cavities as the energy booster.The detailed beam dynamics show that the injector can generate electron bunches with a RMS normalized emittance of 1.49 7rmm-mrad,a bunch length of 0.67 mm,a beam energy of 5 MeV and an energy spread of 0.72%.