Wakefield radiation generated in a dielectric-filled rectangular accelerating structure

The Vavilov-Cherenkov radiation generated by a relativistic electron bunch that propagates in the vacuum channel of a rectangular dielectric-filled wakefield accelerating structure is analytically described. The approach is based on the expansion of wake fields in terms of the eigenfunctions of operators of the transverse electric and magnetic field components. Expressions for the wake fields generated by a Gaussian electron bunch are obtained. Using the proposed formalism, parameters of a diamond-based accelerating structure admitting the generation of potential gradients above 100 MV/m have been calculated for a typical electron bunch of the Argonne Wakefield Accelerator.     

Electron bunch compression and acceleration in the laser wakefield

The compression and acceleration of an external electron bunch into the laser wakefield is studied using 3D modeling with the LAPLAC code and compared to analytical predictions. It is shown, for a laser propagating in a plasma channel, that the nonlinear laser pulse dynamics together with the finite laser spot size influence the electron bunch compression and acceleration due to the reduction of the laser pulse group velocity. The transverse bunch dynamics and loading effect determine the final bunch charge and density and restrict the compressed sizes of the trapped and accelerated electron bunch. The dynamics of the electron bunch are illustrated with a set of parameters where the accelerated bunch acquires an energy of the order of 2 GeV, and 1% energy spread with sub-micron sizes.     

An experimental study of electron acceleration with detuning of the bunch repetition frequency from that of an excited wake field

We have experimentally studied the excitation of wake fields in a dielectric structure by a train of relativistic electron bunches and the acceleration of subsequent bunches of the same train due to detuning of the bunch repetition frequency relative to that of the wake field excited in the dielectric structure at the Cherenkov resonance. Electron bunches of the first (leading) part of the train excite the wake wave, and bunches of the second (trailing) part of this train are shifted to the accelerating phase of the wake wave so as to gain additional energy. The possibility of controlling the number (repetition frequency) of bunches exciting the wake field in the dielectric structure and the number of subsequently accelerated bunches has been investigated by changing the value of detuning.     

Reflective optical system for time-resolved electron bunch measurements at PITZ

The Photo-Injector Test facility at DESY, Zeuthen site (PITZ), produces pulsed electron beams with low transverse emittance and is equipped with diagnostic devices for measuring various electron bunch properties, including the longitudinal and transverse electron phase space distributions. The longitudinal bunch structure is recorded using a streak camera located outside the accelerator tunnel, connected to the diagnostics in the beam-line stations by an optical system of about 30 m length. The temporal resolution is severely degraded by this optical system, mainly due to dispersion in the achromatic lenses. This article presents initial studies toward a system based on reflective optics which will restore the temporal resolution to a level close to the streak camera resolution of 2 ps FWHM. The study includes simulations and measurements of different mirror systems, with an emphasis on systems of parabolic mirrors. A hybrid system of lenses and mirrors, which can serve as a proof of principle, is discussed in detail.     

One GeV acceleration with laser wakefield in the linear regime

We derive an expression for the maximum energy gain of an accelerated electron, in the limit that the plasma wave created by a laser wake is linear both along the longitudinal direction and in the transverse plane, and with a maximum laser power lower than the critical power for relativistic self-focusing. With an available power of 300 TW, the energy gain is of 1 GeV.     

One GeV acceleration with laser wakefield in the linear regime

We derive an expression for the maximum energy gain of an accelerated electron, in the limit that the plasma wave created by a laser wake is linear both along the longitudinal direction and in the transverse plane, and with a maximum laser power lower than the critical power for relativistic self-focusing. With an available power of 300 TW, the energy gain is of 1 GeV.     

Simulation of instabilities in the presence of beam feedback

The effect of longitudinal and transverse instabilities in electron storage rings is simulated by tracking many superparticles for many turns through a model of a machine lattice. This lattice model is defined by a series of machine elements such as RF stations (including longitudinal and transverse wake fields), beam pick-ups, feedback kicker magnets, etc. The machine elements may be interconnected in any specified way so as to produce for example feedback on the longitudinal or transverse beam motion. Each superparticle is treated in six-dimensional phase space and the effects of quantum excitation and radiation damping are included. Insofar as possible the program has been structured to allow study of all known single-beam effects (such as synchro-betatron resonances, transverse mode coupling etc.) in the presence or the absence of some form of beam feedback. The primary goal of the program was to study the effect of a reactive beam feedback system on the threshold for transverse mode coupling.     

Optical and electron energy loss spectra of cadmium dichloride and lead dichloride crystals

Experimental electron energy loss spectra of cadmium dichloride and reflection spectra of lead dichloride are used to calculate the complete sets of fundamental optical functions of the two highly ionic materials in a wide energy range (0–30 eV). Their cumulative permittivity and volume electron energy loss curves are decomposed into transverse and longitudinal elementary components of transitions, and their main parameters are evaluated. The effect of the nature of the cation on the key features of the optical spectra is examined.     

A high-charge and short-pulse RF photocathode gun for wake-field acceleration

In this paper we present a design report on 1- cell, L-Band RF photocathode gun which is capable of generating and accelerating electron beams with peak currents> 10 kA. We address several critical issues of high-current RF photoinjectors such as longitudinal space charge effect, and transverse emittance growth. Unlike conventional short electron pulse generation, this design does not require magnetic pulse compression. Based on numerical simulations using SUPERFISH and PARMELA, this design will produce 100 nC beam at 18 MeV with r.m.s. bunch length 1.25 mm and normalized transverse emittance 108 mm mrad. Applications of this source beam for wake-field acceleration are also discussed.     

Domain configuration under rotational flux and applied stress conditions in silicon-iron

Measurements of power loss have been made on individual grains in polycrystalline specimens of commercial 3% grain-oriented silicon-iron under longitudinal and transverse ac magnetization and rotational flux conditions. The domain configurations have also been observed using a stroboscopic Kerr magnetic-optic apparatus. The effect of longitudinal stress and dc bias fields on the domain patterns and power loss under these ac magnetization conditions has also been investigated. It can be concluded that the highest power loss occurs under pure rotational flux conditions and that the application of longitudinal compressive stress increases the power loss, but to a lesser extent when a transverse ac flux is present. The application of a longitudinal dc field increases the power loss for all ac magnetizing conditions, and it has been observed that a longitudinal tensile stress has the opposite effect on the domain configuration to the application of a transverse dc field.     

Frequency control in wake field waveguide structures

The possibility to control the frequency spectrum of Vavilov-Cherenkov radiation in a waveguide accelerator structure with the aid of an external ferroelectric layer is studied. A decrease in the permittivity of a ferroelectric material under the action of an applied electric field leads to an increase in the frequency of the wake radiation field of a relativistic electron bunch. Parameters of the ferroelectric layer are calculated proceeding from the required interval of control of the wake field frequency and the tolerable power losses in the waveguide system.     

Short range wake potentials of FLASH resistive tapered collimator and European XFEL undulator intersection

The longitudinally dispersion free hybrid TE/TM numerical scheme is applied for the calculation of short range longitudinal and transverse wake potentials for the FLASH free electron laser facility resistive tapered collimator and European XFEL undulator intersection. All the investigated structures have cylindrical symmetry. The contributions of transitive resistive wakes in loss and kick factors are evaluated. The results are confirmed by the comparison with analytical approximations and other numerical codes.     

Longitudinal space charge in final-focus systems for linear colliders

In the final-focus system of a linear collider, the effect of longitudinal space charge can be significant. A diverging or converging ultra-relativistic beam experiences a longitudinal space-charge force which is independent of the beam energy. This force, though weak and often neglected, causes an energy variation across the bunch which depends on the beam size, beam-pipe radius and bunch population and this may affect the chromatic correction of a beam line. Because of the inherent large chromaticity of a typical final-focus system, a very small energy variation, induced in the final focus, can lead to an intolerable increase of the spot size at the interaction point. The space-charge force, thus, gives rise to a limit on bunch intensity beyond which the resulting spot-size increase will degrade the collider performance. In this paper, we evaluate the effect of the longitudinal space charge and derive intensity limits for the three existing or proposed final foci.     

Single particle plasmon spectroscopy of silver nanowires and gold nanorods

The excitation of surface plasmons in individual silver nanowires and gold nanorods is investigated by means of high-resolution electron energy loss spectroscopy in a transmission electron microscope. The transverse and longitudinal modes of these nanostructures are resolved, and the size variation of the plasmon peaks is studied. The effect of electromagnetic coupling between closely spaced nanoparticles is also observed. Finally, the relation between energy-loss measurements and optical spectroscopy of nanoparticle plasmon modes is discussed.     

The status and evolution of plasma Wakefield particle accelerators

The status and evolution of the electron beam-driven Plasma Wakefield Acceleration scheme is described. In particular, the effects of the radial electric field of the wake on the drive beam such as multiple envelope oscillations, hosing instability and emission of betatron radiation are described. Using ultra-short electron bunches, high-density plasmas can be produced by field ionization by the electric field of the bunch itself. Wakes excited in such plasmas have accelerated electrons in the back of the drive beam to greater that 4 G eV in just 10 cm in experiments carried out at the Stanford Linear Accelerator Centre.     

Analysis and elimination of the distortion in electro-optic measurement of low-energy subpicosecond electron bunches

Electro-optic (EO) technique is widely used to characterize longitudinal profile of electron bunches. However, electron bunches with a low energy (MeV) and a short time duration (subpicosecond) cannot be well resolved by EO measurement, which leads to distortion. The convolution theorem is proposed to analyze this distortion in EO measurement. And the factors leading to the distortion are discussed, among which the divergent Coulomb field of electrons is the main one. Distortion elimination and reconstruction of electron bunch profiles from detected EO signals is an ill-posed inverse problem. This paper proposes an iterative Tikhonov regularization method to solve this inverse problem and to reconstruct electron bunch profiles from the EO signals detected by the EO measurement. The feasibility of our proposal is tested and numerically verified based on subpicosecond electron bunches with several MeV energy.     

Effect of high energy electron irradiation on the electromechanical properties of poly (vinylidene fluoride-trifluorethylene) 50/50 and 65/35 copolymers

High energy electron irradiation with a broad range dosage was carried out on poly(vinylidene fluoride trifluorethylene) copolymer 65/35 mol% and 50/50 mol% films at different temperatures from room temperature to a temperature close to the melt temperature. The effect of irradiation on the properties of the films, such as electric field-induced strain, dielectric and polarisation behaviors, and mechanical modulus, is presented. The irradiated films can exhibit a very large electric field-induced strain, more than 4.5% longitudinal strain, and 3% transverse strain. The transverse strain of the stretched film can compare with the longitudinal strain; that of the unstretched film is much smaller than the longitudinal strain. With regard to the dielectric and polarization behaviors, we found that irradiation changes the copolymer from a typical ferroelectric to a relaxor ferroelectric in which the behavior of microregions under the electric field plays the key role. Between the two copolymers studied, we found that the 65/35 copolymer is preferred for both longitudinal and transverse strain generation. A model is proposed to explain the experimental results that the amplitude of the charge electrostrictive coefficient (Q) increases with decreasing crystallinity.     

Longitudinal self-focusing of an electron bunch under coherent emission conditions

The longitudinal motion of an electron bunch with a length much smaller than its wavelength is studied theoretically under coherent emission conditions. It is demonstrated that the bunch can be focused under the action of a longitudinal component of the ponderomotive force of the radiation fields. This mechanism is operative when a bunch moves parallel to metallic surfaces partly reducing the space-charge forces.     

光电技术在合肥光源束测量系统中的应用

介绍了光电技术在合肥同步辐射加速器束流测量系统中的应用。介绍了合肥 2 0 0 Me V电子直线加速器利用光电二极管阵列的能谱测量系统、合肥 80 0 Me V电子储存环利用 CCD技术的束流截面测量系统以及利用单光子计数法测量束团纵向精细结构。本文给出了测量结果。     

Nonlinear subpicosecond electron-bunch compressor

The minimum bunch length achievable using a magnetic bunch compression system is related to the nonlinear dispersion of the compressor and the nonlinear energy-phase correlation of the bunch's longitudinal phase space. For electron bunches with medium or high charge (on the order of 1 nC, or higher), the longitudinal wakefields in the accelerating structure cause a large nonlinearity in the initial energy-phase correlation. Specific nonlinearities in the dispersion of the bunch compressor must then be introduced in order to maximally compress the bunch length, which is done by a chicane-style compressor by increasing the bend angle. This nonlinear dispersion actually decreases the rms longitudinal-beam emittance, by compensating for the nonlinearities in the beam's energy-phase correlation. In this paper, we report on the compression theory, design, and operation of such a compressor. For this compressor, introducing nonlinear dispersion reduced the minimum compressed bunch length by about a factor of 5, resulting in measured FWHM bunch lengths of for bunch charges as large as 1 nC.