Electro-optic technique with improved time resolution for real-time, nondestructive, single-shot measurements of femtosecond electron bunch profiles

Electro-optic detection of the Coulomb field of a relativistic electron bunch combined with single-shot cross correlation of optical pulses is used to enable single-shot measurements of the shape and length of femtosecond electron bunches. This method overcomes a fundamental time-resolution limit of previous single-shot electro-optic measurements, which arises from the inseparability of time and frequency properties of the probing optical pulse. Using this new technique we have made real-time measurements of a 50 MeV electron bunch, observing the profile of 650 fs FWHM ( approximately 275 fs rms) long bunches.     

Temporal characterization of femtosecond laser-plasma-accelerated electron bunches using terahertz radiation

The temporal profile of relativistic laser-plasma-accelerated electron bunches has been characterized. Coherent transition radiation at THz frequencies, emitted at the plasma-vacuum boundary, was measured through electro-optic sampling. Frequencies up to the crystal detection limit of 4 THz were observed. Comparison between data and theory indicates that THz radiation from bunches with structure shorter than approximately = 50 fs (root-mean-square) is emitted. The measurement demonstrates both shot-to-shot stability of the laser-plasma accelerator and femtosecond synchronization between bunch and probe beam.     

THz Coherent Vavilov-Cherenkov Radiation in a Special Three-Mirror Cavity

In the modern science and technology a compact and having enough output power terahertz radiation source working in room temperature have earned great attention. This paper is devoted to utilize electron bunches stimulate Vavilov-Cherenkov Radiation (VCR) in a special three-mirror quasi-optical cavity to generate coherent THz waves. This novel three-mirror quasi-optical resonant cavity has the coaxial field pattern which enables field establish in this cavity effectively. The analytical theory of the radiation exited by a train of electron bunches in the special kind of three-mirror cavity has been carried out and the coherent VCR has been achieved by the computer simulation. All those shows that this method can be used to establish useful THz radiation source by the normal electron gun and the commonly used microwave devices.     

Measurements of harmonic wake fields excited by rough surfaces

An experiment has been carried out at the TESLA Test Facility linac to investigate the wake fields generated by picosecond electron bunches in narrow beam pipes with an artificially roughened inner surface. The energy structure imposed on the bunches by the wake fields has been analyzed with a magnetic spectrometer. Strong harmonic-wake-field effects are observed as expected from simulations in which the rough surface is modeled by a dielectric layer.     

Generation of electron nanobunches and short-wavelength radiation upon reflection of a relativistic-intensity laser pulse from a finite-size target

We have proposed an efficient scheme of generation of short dense electron bunches during the interaction at large angles of incidence of a laser pulse with a thin transversally semibounded laser target. Streams of bunches can be used to simultaneously and independently generate pulsed X-ray radiation as fast electrons hit secondary targets. Dependences of bunch parameters (the number of particles in the bunch and the bunch energy and thickness) on the angle of incidence and laser intensity have been obtained. It has been shown that, upon reflection from the target, the relativistic-intensity laser wave is efficiently converted (the energy-conversion factor reaches ～20%) into a sequence of electromagnetic tens-of-nanometer-long atto pulses, which follow one after another in the period of the initial laser wave. We have investigated how the parameters of the atto pulse depend on the angle of incidence and the laser intensity. We have shown that atto pulses are generated most efficiently at large angles of incidence (≥50°) of the laser pulse on the target.     

Higher harmonics in the output spectrum of a generator with turbulent electron beam

Formation of electron bunches in turbulent electron beams is numerically simulated and experimentally studied. A prototype of a laboratory generator is proposed, and control parameters (in particular, electron spread with respect to longitudinal velocities and additional deceleration of electron beam by electrostatic field of collector) that affect the characteristics of electron bunches and provide an increase in the number and amplitude of higher harmonics in the output spectrum are determined.     

Saturation of a One-Sided Multipactor in Crossed Fields

We analyze the saturation mechanism of a vacuum resonance microwave discharge (multipactor) for systems in which a magnetostatic field returns electrons to the metal surface with secondary-emission multiplying. The steady-state model suggested earlier is improved essentially. In particular, we take into account the near-surface interaction of saturated electron bunches during the limited time of discharge-electron-surface impact. It is found that the Coulomb defocusing of electron bunches prevails over their microwave focusing (toward the resonance phase of the field), making it possible to extract superfluous secondary electrons when the electron bunches are reproduced on the discharge surface. Basic characteristics of the discharge, such as its existence conditions and proper fields and phase disposition of the resonance electron bunches, are determined as functions of the magnetostatic field and the secondary-emission coefficient. The results are discussed in comparison with the data of known experiments. An explanation of some differences related to both the observation conditions and the used analytical model is given.     

Experiment of X-ray Generations Using Laser-Compton Scattering at LINAC of SINAP

Laser Compton scattering(LCS) can generate X-rays or y-rays with high brightness and easy controlled polarization by applying high-peak-power laser pulses to relativistic electron bunches.One of the most promising approaches to short pulsed X-ray sources is the laser synchrotron source.It is based on LCS between picoseconds relativistic electron bunches and picoseconds laser pulses.A project of Shanghai laser electron gamma source with LCS method has been proposed on Shanghai synchrotron radiation facility.Before that,a prototype has been developed in the beamline of the linear accelerator at the Shanghai Institute of Applied Physics,Chinese Academy of Sciences.The LCS experiment was carried out by using the 107 MeV,5 Hz,1 ns,0.1 nC electron bunches from the linear accelerator and the 18 ns,10 MW peak power,Nd:YAG laser pulses.In this communication,we describe the details and report the first results of this experiment.     

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.     

Generation of high-power ultrashort electromagnetic pulses on the basis of effects of superradiance of electron bunches

One of the promising methods for generation of ultrashort electromagnetic pulses (with duration of about ten periods of high-frequency oscillations) is radiation from spatially localized electron ensembles (bunches), which can be considered a classical analog of Dicke superradiance known in quantum electronics. In classical electronics, superradiance can be related to various mechanisms of stimulated radiation. Until now, cyclotron, undulator, and ?erenkov (in the case of interaction with both copropagating and counterpropagating waves) superradiance of electron bunches as well as superradiance during stimulated scattering of a pump wave have been studied theoretically and experimentally. As a result of these studies based on high-current RADAN and SINUS accelerators and their modifications, a new class of oscillators producing pulsed electromagnetic radiation has been created. They have such unique characteristics as pulses of high peak power (up to 1 GW and 3 GW in the millimeter-and centimeter-wave ranges, respectively) and ultrashort duration (from 300 ps to 1 ns, respectively). In this case, regimes with a peak radiation power exceeding the electron-beam power are experimentally realized. Regimes with high (kilohertz) pulse repetition rate and high average power (up to 2.5 kW) are obtained.     

Generation of ultrashort microwave pulses based on cyclotronsuperradiance

We have theoretically and experimentally investigated the superradiance from a bunch of electrons rotating in a homogeneous magnetic field. A RADAN-303B modulator equipped with a subnanosecond pulse slicer has been used to generate high current subnanosecond electron bunches (250 kV, 0.1-1 kA, 0.3-0.5 ns). Transverse momentum was imparted to the electrons by a kicker. It is shown that for the experimental observation of cyclotron superradiance from high current electron bunches the optimum conditions are the conditions of group synchronism, when the translational velocity of the bunch coincides with the group velocity of the radiation propagating in the waveguide. In the 35 GHz range microwave pulses with record short duration, down to 0.4 ns, with a peak power level up to 200 kW, have been obtained     

Transverse dynamics and bunch-to-bunch energy exchange in a dielectric-filled accelerating structure

The self-consistent transverse dynamics of high-current relativistic electron bunches used for generating wakefields in multiple-bunch schemes of wakefield acceleration, which are applied in dielectric-filled structures, is studied. The flight range of the electron bunch under no-focusing conditions and the energy transferred to the bunch being accelerated in schemes with profiled (nonuniform) and uniform distributions of the charge over a sequence of generator bunches are determined. Requirements are formulated for a focusing system in which the profiled distribution offers an advantage over the uniform distribution for the efficiency of energy transfer from generator bunches to that being accelerated.     

The Potential-Well Distortion Effect and Coherent Instabilities of Electron Bunches in Storage Rings

The effect of electromagnetic interaction between electron bunches and the vacuum chamber of a storage ring on the longitudinal motion of bunches is studied. Specifically, the potential-well distortion effect and the so-called coherent instabilities of coupled bunches are considered. An approximate analytical solution for the frequencies of incoherent oscillations of bunches distributed arbitrarily within the ring is obtained for a distorted potential well. A new approach to determining frequencies of coherent oscillations and an approximate analytical relation for estimating the stability of a system of bunches as a function of their distribution in the accelerator orbit are presented.     

Diagnostics of FEL and ILC ultrashort electron bunches based on undulator and synchrotron radiation

The diagnostics of ultrashort electron bunches developed basing on undulator and synchrotron radiation in the framework of the JINR-DESY collaboration is designated for the International Linear Collider (ILC) project, as well as for Free Electron Lasers (FELs) such as the FLASH and X ray XFEL laser. All these accelerator complexes require diagnostics of ultrashort electron bunches with lengths of 20–300 μm.     

Intense gamma-ray source in the giant-dipole-resonance range driven by 10-TW laser pulses

A gamma-ray source with an intense component around the giant dipole resonance for photonuclear absorption has been obtained via bremsstrahlung of electron bunches driven by a 10-TW tabletop laser. 3D particle-in-cell simulation proves the achievement of a nonlinear regime leading to efficient acceleration of several sequential electron bunches per each laser pulse. The rate of the gamma-ray yield in the giant dipole resonance region (8相似文献   

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.     

Energy-gain measurements from a microwave inverse free-electron-laser accelerator

Experiments are reported on inverse free-electron-laser acceleration, including for the first time observations of the energy change as a function of relative injection phase of the electron bunches. The microwave accelerating structure consists of a uniform circular waveguide with a helical wiggler and an axial magnetic field. Acceleration of the entire beam by 6% is seen for 6 MeV electron bunches at optimum relative phase. Experimental results compare favorably, for accelerating phases, with predictions of a three-dimensional simulation that includes large-orbit effects.     

Preparation and characterization of oriented silica nanowires

Large-scale of oriented closely packed silica nanowire bunches have been synthesized by using large size (1-10 μm in diameter), low melting point tin droplets as catalyst on silicon wafers at 980 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses show that the amorphous silica nanowires have lengths of 50-100 μm and diameters of 100-200 nm. Unlike any previous observed results using high melting point metal (such as gold and iron) as catalyst, the Sn catalyst growth exhibits many interesting phenomena. Each Sn ball can simultaneously catalyze the growth of many silica nanowires, which is quite different from the conventional vapor-liquid-solid process.     

Numerical analysis of space charge effects in electron bunches at laser-driven plasma accelerators

Laser-driven Plasma Accelerators (LPA) have successfully generated high energy, high charge electron bunches which can reach many kA peak current, over short distances. Space charge issues, even in transport lines as simple as a drift section, have to be carefully taken into account since they can degrade the beam quality, preventing any further application of such electron beams. We analyse the space charge effects within an electron bunch with numerical simulations in order to assess their effect on the beam. We use LPA beam parameters published in previous experimental studies. These studies can give an indication of the working point where space charge can dominate the beam dynamics and has to be taken into account in the application of such beams.     

Evidence of ultrashort electron bunches in laser-plasma interactions at relativistic intensities

The second harmonic of the laser light (2omega(0)) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at 2omega(0). The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.