Methods of computer processing of experimental data on the intensity of bunches in synchrotrons

Methods of computer processing of experimental data on the intensity of bunches in synchrotrons for the purpose of receiving functional dependences from time during the accelerating cycle for a number of circulating particles and the mean-square length of a bunch are discussed. Examples of such dependences for the beam at the nuclotron (JINR) and PSB (CERN) are presented.     

Charged particles bunch–accelerator extraction window matching

The criterion for matching the charged particle bunch with the accelerator beam extraction window, which consists in minimizing the emittance of the accelerated beam extracted through a metal foil, is derived using the distribution function for moving particles scattered on foil nuclei. A technique for constructing the phase portrait of the scattered bunch and the criterion for minimizing its area (emittance) are presented. The formula relating the emittances of incident and scattered bunches and the ellipse equation describing the phase portrait of the matched bunch are derived.     

Tunable subpicosecond electron-bunch-train generation using a transverse-to-longitudinal phase-space exchange technique

We report on the experimental generation of a train of subpicosecond electron bunches. The bunch train generation is accomplished using a beam line capable of exchanging the coordinates between the horizontal and longitudinal degrees of freedom. An initial beam consisting of a set of horizontally separated beamlets is converted into a train of bunches temporally separated with tunable bunch duration and separation. The experiment reported in this Letter unambiguously demonstrates the conversion process and its versatility.     

Luminosity of a Collider with Asymmetric Beams

A formula for the collider luminosity is derived for a head-on collision of two beams with different parameters. The formula is valid for colliding and partially separated (“merging”) beams. Three particular cases are presented: the collision of two identical axially symmetric bunches, the collision of a bunch with a disbanded beam, and the collision of two disbanded bunches. The colliding beams have coinciding longitudinal axes. The formula is valid for colliding both counter propagating and co-propagating beams.     

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     

Subpicosecond electro-optic measurement of relativistic electron pulses

Time-resolved measurements of the transverse electric field associated with relativistic electron bunches are presented. Using an ultrafast electro-optic sensor close to the electron beam, the longitudinal profile of the electric field was measured with subpicosecond time resolution and without time-reversal ambiguity. Results are shown for two cases: inside the vacuum beam line in the presence of wake fields, and in air behind a beryllium window, effectively probing the near-field transition radiation. Especially in the latter case, reconstruction of the longitudinal electron bunch shape is straightforward.     

Emittance-dominated long bunches in dual harmonic RF system

The storage of long bunches for long time intervals needs flattened stationary buckets with a large bucket height. The longitudinal motion of the initially mismatched beam has been studied for both the single and dual harmonic RF systems. The RF amplitude is determined to be r.m.s wise matched. The bucket height of the single harmonic system is too small even for shorter bunch with only 20% increased energy spread. The Halo formation and even debunching can be seen after a few synchrotron periods for single particles with large amplitude. In the case of small energy spread for a cooled beam, Coulomb interaction cannot be ignored. The external voltage has to be increased to keep the r.m.s bunch length unchanged. The new voltage ratio R(N) simplifies physics for the emittance-dominated bunches with modest particle number N. For the single harmonic system, substantial amount of debunching occurs without increasing the external voltage, but very little if the RF amplitude is doubled. Results from the ORBIT tracking code are presented for the 1 GeV bunch in the HESR synchrotron, part of the GSI FAIR project.     

双谐波高频系统的同步加速器中发射度主导的长束团研究

在质子储存环中,长束团的长期储存要求采用比较平的 bucket 以增加纵向接收度。对于经过相空间冷却的低能散的束流,束流内部的库仑散射作用不能被忽略。本文讨论了对于处在临界能量以下的发射度主导的长束团双谐波高频系统的特性。为了保持束团长度不变,高频电压应作适当的提高以补偿空间电荷效应的作用。初始失配的束流的纵向运动也分别对于单谐波或双谐波系统进行了研究,对于前者失配度为20%时,纵向接收度太小导致经过几个同步周期后就出现明显的束流纤维化和散束。本文还引入了一种空间电荷作用因子和纵向失配度的概念来研究和分析发射度主导的长束团的纵向运动。采用ORBIT程序对FAIR-HESR加速器中的束流纵向运动进行了模拟跟踪计算,并与理论分析进行了比较。     

相干衍射辐射在超短电子束团长度测量中的应用

利用相干衍射辐射(CDR)光学自相关技术在线无阻拦频域测量超短电子束团的长度是当前国际束测领域的研究热点. 文中分析和数值计算了利用上海应用物理研究所(SINAP)飞秒电子束装置提供的超短电子束团产生的宽带连续强CDR，介绍了超短电子束团长度测量的实验原理和装置示意，并研究了分束器对束团长度测量的影响. 结果表明，该束团可直接用于产生覆盖远红外至毫米波段的宽带连续强CDR；辐射能量主要集中在轴线附近，宏脉冲辐射能量可达毫焦耳量级；利用光学自相关技术研制的远红外Michelson干涉仪和Golay探测仪组成束团长度测量系统，通过实验测量CDR干涉图FWHM可近似求得超短束团长度；干涉图籍助傅立叶变换光谱法，可推算求得束团电子密度分布的信息.     

A new method to generate relativistic comb bunches with tunable subpicosecond spacing

We propose and analyze a scheme to produce comb bunches, i.e. a bunch consisting of micro-bunch trains, with tunable subpicosecond spacing. In the scheme, the electron beam is first deflected by a deflecting cavity which introduces a longitudinal-dependent linear transverse kick to the particles. After passing through a drift space, the transverse beam size is linearly coupled to the longitudinal position of the particle inside the beam, and a mask is placed there to tailor the beam, then the mask distribution is imprinted on the beam's longitudinal distribution. A quadrupole magnet and another deflecting cavity are used in the beam line to compensate the transverse angle due to the first deflecting cavity. Analysis shows that the number, length, and spacing of the trains can be controlled through the parameters of the deflecting cavity and the mask. Such electron bunch trains can be applied to an infrared free electron laser, a plasma-wakefield accelerator and a supper-radiance THz source.     

Generation and characterization of electron bunches with ramped current profiles in a dual-frequency superconducting linear accelerator

We report on the successful experimental generation of electron bunches with ramped current profiles. The technique relies on impressing nonlinear correlations in the longitudinal phase space using a superconducing radio frequency linear accelerator operating at two frequencies and a current-enhancing dispersive section. The produced ~700-MeV bunches have peak currents of the order of a kilo-Ampère. Data taken for various accelerator settings demonstrate the versatility of the method and, in particular, its ability to produce current profiles that have a quasilinear dependency on the longitudinal (temporal) coordinate. The measured bunch parameters are shown, via numerical simulations, to produce gigavolt-per-meter peak accelerating electric fields with transformer ratios larger than 2 in dielectric-lined waveguides.     

基于束流位置探测器的束团长度测量

分析了驱动电子束团的频域特性,研究了基于该特性进行长度测量的理论基础;使用三维模拟软件对束流位置探测器(BPM)进行建模,用模拟的方法对传输阻抗进行了数值计算;对不同长度的束团进行了测量和计算,并且分析了束团位置在真空管道中偏移对束团长度测量的影响。由测量结果可见,电子束团长度在10~100 ps(3~30 mm)时,测量误差均小于2%,满足中国工程物理研究院高平均功率自由电子激光太赫兹实验测量的使用要求。     

Computerized Tomography of Ion Bunches at the Nuclotron

Methods for processing a digital signal proportional to the longitudinal intensity of charged particles bunches during their acceleration in a synchrotron are discussed. Computerized tomography is used to reconstruct a longitudinal two-dimensional distribution function of particles in the bunch from its profiles depending on the synchrotron motion of particles. Examples of tomographic studies of ion bunches at the Nuclotron are given.     

Emittance Dominated Long Bunches in Dual Harmonic RF Systems

The storage of long bunches for long time intervals needs flattened stationary buckets with large bucket height. In the case of small energy spread with a cooled beam, the Coulomb interaction cannot be ignored. The properties of the dual harmonic RF systems are discussed for the emittance dominated long bunches below transition. The external voltage has to be increased to compensate the Coulomb interaction to keep bunch length unchanged. The longitudinal motion of the initially mismatched beam has been studied for both single and dual harmonic RF system. Bucket height of the single harmonic system is too small even for 20 % mismatch. Halo formation and debunching can be seen after a few synchrotron periods for single particles with large amplitude. The simulation results by using ORBIT code for the FAIR-HESR synchrotron has been given together with theoretical analysis.     

Tracking simulations for the HLS-Ⅱ with a passive harmonic cavity in the symmetricand asymmetric fill patterns

A simulation code that executes the tracking of longitudinal oscillations of the bunches for the double rf system of the Hefei Light Source Ⅱ Project (HLS-Ⅱ) is presented to estimate the mean beam lifetime and the Robinson instabilities. The tracking results show that the mean beam lifetime is in agreement with the analytical results and the system is stable when we tune the harmonic cavity in the optimum lengthening conditions. Moreover, the simulated results of the asymmetric fill pattern show that some bunches are compressed only with a 7% gap (3 gaps), which will lead to the reduction in the mean bunch lengthening and potential beam lifetime. It is demonstrated that HLS-Ⅱ with a passive higher harmonic cavity is not suitable for operating in an asymmetric fill pattern.     

BEPCⅡ零模束流反馈系统研制

 为了抑制BEPCⅡ多束团(93团)、大流强（910 mA）时的纵向振荡（零模）,引入了BEPCⅡ零模束流反馈系统。该系统首次应用于BEPCⅡ储存环上,它包括了束流信号采集、滤波、下变频、鉴相、移相等几个部分。使用该系统后,束流的纵向边带被抑制度大于10 dB,这表明,系统能够抑制束流的纵向振荡。     

Generation of wakefields in a dielectric structure filled with plasma

The effect of plasma on the amplitude of the wakefield excited in a dielectric structure by a relativistic electron bunch train is studied. The structure under study is a dielectric cylindrical waveguide with an axial drift channel filled with plasma. The dependences of the amplitude of a longitudinal electric field on the plasma density are obtained for the following three cases: the parameters of the dielectric structure and bunches are fixed; the inside or outside radius of the dielectric tube changes according a change in the plasma frequency, and the bunch repetition frequency is adjusted to the plasma frequency and the frequency of the first radial mode of a dielectric wave. It is shown that, when the eigenwave frequencies are adjusted to the bunch repetition frequency via a change in the structure radii, the maximum of the accelerating field is determined by a plasma wave, and there is a plasma density range where a dielectric wave significantly contributes to the total field amplitude. In the case of changing the outside radius, this range is substantially wider.     

CSR纵向束团压缩腔研究

针对兰州重离子加速器冷却储存环的发展目标,为了满足高能量密度（涉及重离子驱动惯性约束核聚变新能源）等物理研究的需要, 使用三维电磁场计算程序MAFIA研究了一种新型的适用于CSR的纵向束团压缩腔。 此纵向束团压缩腔采用高磁导率软磁合金材料进行加载,相比于铁氧体加载的高频腔, 可以得到高的电场梯度。以250 MeV/u的238U72+离子为例进行了模拟计算, 得出了此纵向束团压缩腔的工作频率为1.15 MHz, 峰值工作电压为80 kV, 由两个1/4波长同轴谐振腔组成, 每个谐振腔峰值工作电压为40 kV,能够满足在CSR上进行纵向束团压缩的要求。 The scheme of longitudinal bunch compression cavity for the Cooling Storage Ring(CSR) is an important issue. Plasma physics experiments require high density heavy ion beam and short pulsed bunch, which can be produced by non adiabatic compression of bunch implemented by a fast compression with 90° rotation in the longitudinal phase space. The phase space rotation in fast compression is initiated by a fast jump of the RF voltage amplitude. For this purpose, the CSR longitudinal bunch compression cavity, loaded with FINEMET FT 1M is studied and simulated with MAFIA code. In this paper, the CSR longitudinal bunch compression cavity is simulated and the initial bunch length of 238U72+ with 250 MeV/u will be compressed from 200 ns to 50 ns. The construction and RF properties of the CSR longitudinal bunch compression cavity are simulated and calculated also with MAFIA code. The operation frequency of the cavity is 1.15 MHz with peak voltage of 80kV, and the cavity can be used to compress heavy ions in the CSR.     

离子俘获的线性理论及其应用

从线性理论出发,研究了电子束团在储存环里均匀分布和束团串两种情况下离子俘获的条件;在离子系统里引入Twiss参量,推导出电子束团串俘获离子的阈值流强的公式;并以此讨论北京正负电子对撞机(BEPC)在同步辐射专用运行时观察到的电子束流现象,提出采用束团串运行来克服BEPC中的束流寿命下降.     

Micro-bunch production with radio frequency photoinjectors

In this paper we discuss the generation of ultra-short electron bunches using laser-driven RF guns. The designs are tailored for future plasma accelerators. Second generation plasma accelerators are expected to be very demanding in terms of bunch length, since the accelerated beam is expected to be short with respect to the wavelength of the excited Langmuir space-charge plasma wave. Since the anticipated wavelength ranges from 100 to 300 μm, 10-50 μm-long bunches are required with a bunch population of the order of 10⁸ particles. The laser-driven RF gun is a promising candidate to attain such beams. The rationale for this choice as well as the main limitations in terms of minimum bunch length will be analyzed and discussed in the following. Two possible configurations are evaluated: the direct production at the photocathode surface of ultra-short electron bunches by illumination of the cathode with 160-fs-long laser pulses and the acceleration of a 1-ps electron bunch with further magnetic compression in a wiggler