Features of the formation of electron bunches upon the development of generation in multiwave Čerenkov devices

The dynamics of electron bunch formation are analyzed using data obtained by means of numerical simulation. It is shown that upon the development of microwave generation, electron bunches start to form in a relativistic flow in the first section of the electrodynamic structure. At the entrance to the second section, a complex structure of electron distribution over a longitudinal pulse is observed inside the bunch in the stationary mode of generation. It is concluded that the duration of a microwave radiation pulse can be increased by optimizing the length of the first section of the generator.     

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     

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.     

双束平行入射电子束引导的自注入电子加速效果的研究

在粒子束引导的等离子尾波场加速机制中,为了加速电子获得最大能量,大量研究集中于改变单束牵引粒子束的线度、形状、电荷性质等参数. 综合考虑已有的实验结果,本文提出了一种相比于单束电子牵引更为有效的加速方式,利用双束平行电子束来加速自注入的电子. 通过2.5维粒子程序模拟,发现在牵引电子束具有相同能量、电量、尺寸的条件下,通过双束平行电子束加速得到的电子具有长程加速、高能和准单能性的特性. 同时在空泡内形成了一束独特的回流电子,进一步使得自注入电子具有更好的准直性. 关键词： 电子束尾波场加速 双束平行电子束 粒子模拟     

Benchmarking of electro-optic monitors for femtosecond electron bunches

The longitudinal profiles of ultrashort relativistic electron bunches at the soft x-ray free-electron laser FLASH have been investigated using two single-shot detection schemes: an electro-optic (EO) detector measuring the Coulomb field of the bunch and a radio-frequency structure transforming the charge distribution into a transverse streak. A comparison permits an absolute calibration of the EO technique. EO signals as short as 60 fs (rms) have been observed, which is a new record in the EO detection of single electron bunches and close to the limit given by the EO material properties.     

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     

Simulation study of electron injection into plasma wake fields by colliding laser pulses using OOPIC

An electron injector concept for a laser-plasma accelerator has been developed which relies on the use of counter propagating ultrashort laser pulses. In this paper, we use OOPIC the fully self-consistent, twodimensional, particle-in-cell code to make a parameter study to determine the bunches that can be obtained through collisions of two collinear laser pulses in uniform plasma. A series of simulations show that one can obtain a short (<10fs) bunch with its charge of about 15pC, and energy spread of about 15%. We also discussed the variation of the transverse spot size of the electron bunch and found the bunch would undergo the betatron oscillations.     

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.     

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.     

Synchronization of wakefield modes in the dielectric resonator

Multimode excitation of a wakefield in a planar dielectric resonator by a regular sequence of relativistic electron bunches is studied analytically and numerically. It is shown that, if the wakefield being excited consists of a large number of radial modes, the fields from the regular sequence of the bunches add together. Conditions under which this sequence provides synchronization of the dielectric resonator modes are found. Acceleration of a test bunch in the wakefield of the bunch sequence is studied.     

Diagnoctics development at JINR for ILC and FEL ultrashort electron bunches

Different methods for diagnostics of ultrashort electron bunches are developed at JINR-DESY collaboration within the framework of the FLASH and XFEL projects and JINR participation in the ILC project. The main peculiarity of these accelerator complexes is related to formation of ultrashort electron bunches with r.m.s. length 20–300 μm. Novel diagnostics is required to provide femtoscaie time resolution in the modem FEL like FLASH and future XFEL and ILC projects. Photon diagnostics developed at JINR-DESY collaboration for ultrashort bunches is based on calorimetric measurements and detection of undulator radiation. The MCP-based radiation detectors are effectively used at FLASH for pulse energy measurements. The infrared undulator constructed at JINR and installed at FLASH is used for longitudinal bunch shape measurements and for two-color lasing provided by the FIR and VUV undulators. Two-color lasing in pump-probe experiments permits one to investigate dynamics of atomic and molecular systems with time resolution of 100–500 fs. A special magnetic spectrometer is planning to be used at ILC for measurements of average electron energy in each bunch. The first test spectrometer measurements were performed within the JINR-DESY-SLAC collaboration. A special synchrotron radiation detector applied for measurement of bunch average electron energy was constructed at JINR.     

Simulation study of electron injection into plasma wake fields by colliding laser pulses using OOPIC

An electron injector concept for a laser-plasma accelerator has been developed which relies on the use of counter propagating ultrashort laser pulses. In this paper, we use OOPIC the fully self-consistent, two-dimensional, particle-in-cell code to make a parameter study to determine the bunches that can be obtained through collisions of two collinear laser pulses in uniform plasma. A series of simulations show that one can obtain a short (<10fs) bunch with its charge of about 15pC, and energy spread of about 15%. We also discussed the variation of the transverse spot size of the electron bunch and found the bunch would undergo the betatron oscillations.     

Characteristics of terahertz coherent transition radiation generated from picosecond ultrashort electron bunches

This paper presents a method of generating terahertz (THz) coherent transition radiation (CTR) from picosecond ultrashort electron bunches including single and train bunches, which are produced by a photocathode radio frequency gun. The radiation characteristics of THz CTR including formation factor and energy spectrum are analysed in detail. With the help of a 2-dimensional particle-in-cell simulation, the radiation characteristics including power, energy and magnetic field are analysed. The results show that the radiation frequency can be adjusted by tuning the repetition frequency of the train bunch and the energy can be enhanced with the train bunches.     

Ultra-low emittance X-band photocathode RF gun

In this paper, we present the simulation results of a 1.6 cell X-band photocathode RF gun for ultra-low emittance electron beams. It will work at 9.3 GHz. The emittance, bunch length, electron energy and energy spread at the gun exit are optimized at bunch charge of 1pC using PARMELA. Electron bunches with emittance about 0.1 mm·mrad and bunch length less than 100 fs can be obtained from this gun. A PITZ type coupler is adopted in this gun and an initial simulation by MAFIA is also given in this paper.     

Observation of frequency-locked coherent terahertz Smith-Purcell radiation

We report the observation of enhanced coherent Smith-Purcell radiation (SPR) at terahertz (THz) frequencies from a train of picosecond bunches of 15 MeV electrons passing above a grating. SPR is more intense than other sources, such as transition radiation, by a factor of Ng, the number of grating periods. For electron bunches that are short compared with the radiation wavelength, coherent emission occurs, enhanced by a factor of Ne, the number of electrons in the bunch. The electron beam consists of a train of Nb bunches, giving an energy density spectrum restricted to harmonics of the 17 GHz bunch train frequency, with an increased energy density at these frequencies by a factor of Nb. We report the first observation of SPR displaying all three of these enhancements, NgNeNb. This powerful SPR THz radiation can be detected with a high signal to noise ratio by a heterodyne receiver.     

Excitation of a wake field by a relativistic electron bunch in a semi-infinite dielectric waveguide

A wake field excited by a relativistic electron bunch in a semi-infinite metal waveguide filled with a dielectric consists of the Vavilov-Cherenkov radiation, the “quenching”-wave field, and transient radiation, which interfere with each other. An exact analytic expression for the transient component of the field of a thin relativistic annular bunch is derived for the first time. The evolution of the space distribution of a field excited by a finite-size electron bunch is numerically calculated. The excitation of the wake field by a periodic train of electron bunches in a finite-length waveguide is studied.     

Wakefield radiation generated by an electron bunch in a rectangular dielectric waveguide

Vavilov-Cerenkov radiation generated by a relativistic electron bunch in a rectangular waveguide with a transverse-inhomogeneous dielectric filling is analyzed. A method is proposed for constructing an orthogonal basis of the transverse operator, which can be subsequently used for determining the wakefield of the relativistic bunch moving parallel to the waveguide axis. The dispersion equation for the structure is derived and the expressions for the wake field produced by such a bunch are obtained. The formalism described here forms the basis for calculating parameters of the accelerating structure for generator bunches of the Argonne Wakefield Accelerator (AWA) at the Argonne National Laboratory and the FACET complex of the SLAC accelerator. It is shown that using such structures, accelerating field gradients higher than 150 MV/m can be generated at frequencies 20?C35 GHz and exceeding 1 GeV/m in the frequency range ??1 THz.     

High-gradient plasma-wakefield acceleration with two subpicosecond electron bunches

A plasma-wakefield experiment is presented where two 60 MeV subpicosecond electron bunches are sent into a plasma produced by a capillary discharge. Both bunches are shorter than the plasma wavelength, and the phase of the second bunch relative to the plasma wave is adjusted by tuning the plasma density. It is shown that the second bunch experiences a 150 MeV/m loaded accelerating gradient in the wakefield driven by the first bunch. This is the first experiment to directly demonstrate high-gradient, controlled acceleration of a short-pulse trailing electron bunch in a high-density plasma.     

Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma

The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations.