Relativistic Electron Bunches Research Articles

X-Ray Amplification from a Raman Free-Electron Laser

We demonstrate that a mm-scale free-electron laser can operate in the x-ray range, in the interaction between a moderately relativistic electron bunch, and a transverse high intensity optical lattice. The corrugated light-induced ponderomotive potential acts simultaneously as a guide and as a low-frequency wiggler, triggering stimulated Raman scattering. The gain law in the small signal regime is derived in a fluid approach, and confirmed from particle-in-cell simulations. We describe the nature of bunching, and discuss the saturation properties. The resulting all-optical Raman x-ray laser opens perspectives for ultracompact coherent light sources up to the hard x-ray range.

Tunable few-cycle coherent terahertz radiation with watt-level power from relativistic femtosecond electron beam

We experimentally demonstrated the generation of watt-level, few-cycle coherent terahertz radiation with tunable frequency from 0.3THz to 0.8THz. The radiation is produced by an ultra-short relativistic electron bunch when passing through a magnetic undulator. The electron bunch is emitted from a thermionic gun and compressed to approximately 250fs by an alpha magnet. With an average power up to 1.2W, the easy-to-implement scheme enables various new applications.

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 20–35 GHz and exceeding 1 GeV/m in the frequency range ∼1 THz.

All-optical Compton gamma-ray source

One of the major goals of research for laser-plasma accelerators1 is the realization of compact sources of femtosecond X-rays2,3,4. In particular, using the modest electron energies obtained with existing laser systems, Compton scattering a photon beam off a relativistic electron bunch has been proposed as a source of high-energy and high-brightness photons. However, laser-plasma based approaches to Compton scattering have not, to date, produced X-rays above 1 keV. Here, we present a simple and compact scheme for a Compton source based on the combination of a laser-plasma accelerator and a plasma mirror. This approach is used to produce a broadband spectrum of X-rays extending up to hundreds of keV and with a 10,000-fold increase in brightness over Compton X-ray sources based on conventional accelerators5,6. We anticipate that this technique will lead to compact, high-repetition-rate sources of ultrafast (femtosecond), tunable (X- through gamma-ray) and low-divergence (∼1°) photons from source sizes on the order of a micrometre.

Experimental demonstration of wakefield effects in a THz planar diamond accelerating structure

We have directly measured THz wakefields induced by a subpicosecond, intense relativistic electron bunch in a diamond loaded accelerating structure via the wakefield acceleration method. We present here the beam test results from the diamond based structure. Diamond has been chosen for its high breakdown threshold and unique thermoconductive properties. Fields produced by a leading (drive) beam were used to accelerate a trailing (witness) electron bunch, which followed the drive bunch at a variable distance. The energy gain of a witness bunch as a function of its separation from the drive bunch describes the time structure of the generated wakefield.

Terahertz radiation from a pipe with small corrugations

We have studied through analytical and numerical methods the use of a relativistic electron bunch to drive a metallic beam pipe with small corrugations for the purpose of generating terahertz radiation. For the case of a pipe with dimensions that do not change along its length, we have shown that—with reasonable parameters—one can generate a narrow-band radiation pulse with frequency ∼1THz, and total energy of a few milli-Joules. The pulse length tends to be on the order of tens of picoseconds. We have also shown that, if the pipe radius is tapered along its length, the generated pulse will end up with a frequency chirp; if the pulse is then made to pass through a compressor, its final length can be reduced to a few picoseconds and its peak power increased to ∼1GW. We have also shown that wall losses tend to be significant and need to be included in the structure design.

Experimental study of coherent transition radiation from relativistic electron bunches entering a dihedral angle

The angular distributions of the intensity of transition radiation from a bunch of relativistic electrons entering a dihedral angle between two conducting planes have been measured in a millimeter wavelength range. A microtron with a particle energy of 7.4 MeV is used as a source of electrons. The effect of the particle injection direction and the magnitude of the dihedral angle on the angular distribution of the radiation intensity has been analyzed. The measurements show that the character of the distribution of radiation from a charge entering the dihedral angle significantly differs from that for a charge escaping the angle. A comparatively small change in the magnitude of the dihedral angle can lead to qualitative changes in the angular distribution of radiation from a charge entering the dihedral angle.

Coherent terahertz synthesizer

Photonic manipulation of the spatial distribution of charge in relativistic electron bunches provides a promising way to generate intense coherent terahertz radiation.

Soft x-ray femtosecond coherent undulator radiation in a storage ring

We propose to produce femtosecond pulses of soft x-ray coherent undulator radiation in a storage ring for user pump–probe experiments using two energy exchanges between a picosecond relativistic electron bunch and two external ultra-short laser pulses. The coherent emission is generated thanks to the two laser–electron interactions that modulate the longitudinal charge distribution of the electron bunch at a harmonic of the laser wavelength, such as in the echo-enabled harmonic generation in free-electron lasers. Application to the SOLEIL storage ring in the soft x-ray range leads to coherent radiation and improvement of the flux of the photons by several orders in magnitude compared to the conventional slicing scheme. This is also accompanied by a significant enhancement of the signal-to-noise ratio.

Electron bunch profile reconstruction in the few fs regime using coherent Smith-Purcell radiation

Advanced accelerators for fourth generation light sources based on high brightness linacs or laser-driven wakefield accelerators will operate with intense, highly relativistic electron bunches that are only a few fs long. Diagnostic techniques for the determination of temporal profile of such bunches are required to be non invasive, single shot, economic and with the required resolution in the fs regime. The use of a radiative process such as coherent Smith-Purcell radiation (SPR), is particularly promising with this respect. In this technique the beam is made to radiate a small amount of electromagnetic radiation and the temporal profile is reconstructed from the measured spectral distribution of the radiation. We summarise the advantages of SPR and present the design parameters and preliminary results of the experiments at the FACET facility at SLAC. We also discuss a new approach to the problem of the recovery of the `missing phase', which is essential for the accurate reconstruction of the temporal bunch profile.

High power THz source based on coherent radiation of picosecond relativistic electron bunch train

Tunable and compact high power terahertz (THz) radiation based on coherent radiation (CR) of the picosecond relativistic electron bunch train is under development at the Tsinghua accelerator lab. Coherent synchronization radiation (CSR) and coherent transition radiation (CTR) are researched based on an S-band compact electron linac, a bending magnet or a thin foil. The bunch train’s form factors, which are the key factor of THz radiation, are analyzed by the PARMELA simulation. The effects of electron bunch trains under different conditions, such as the bunch number, bunch charges, micro-pulses inter-distance, and accelerating gradient of the gun are investigated separately in this paper. The optimal radiated THz power and spectra should take these factors as a whole into account.

Characterizing the Temporal Structure of a Relativistic Electron Bunch

Using proper beam energy chirp and the undulator detuning effect, we propose a modified optical replica synthesizer scheme to characterize the temporal structure of a relativistic electron bunch, which predicts a 100-fs temporal resolution in numerical simulation. The proof of principle experiment demonstrates a peak current of 9 A and a slice energy spread of about 0.5 keV for the uncompressed electron beam of the Shanghai Deep UV Free Electron Laser Facility.

Collective properties of a relativistic electron beam injected into a high intensity optical lattice

Behaviour of a relativistic electron bunch, injected and trapped in a high intensity optical lattice resulting from the interference of two laser beams is studied. The optical lattice modifies the phase space distribution of the electron bunch due to the trapping and compression of the electrons by a ponderomotive force. High-frequency longitudinal beam eigenmodes of the trapped electron bunch are described in the framework of fluid and kinetic models. Such beam oscillations are expected to play a pivotal role in a stimulated Raman scattering of laser beams on the electrons.

Progress on Reflective Terahertz Imaging for Identification of Water in Flow Channels of PEM Fuel Cells

This work reports an application of reflective terahertz (THz) imaging for identification of water distribution in the proton exchange membrane (PEM) fuel cell. The THz radiation generated from relativistic femtosecond electron bunches is employed as a high intensity source. The PEM fuel cell is designed specifically for the measurement allowing THz radiation to access the flow field region. The THz image is constructed from reflected radiation revealing absorptive area of water presence. The technique is proved to be a promising tool for studying water management in the PEM fuel cell. Detailed experimental setup and results will be described.

Single-cycle terahertz pulses with >0.2 V/Å field amplitudes via coherent transition radiation

We demonstrate terahertz pulses with field amplitudes exceeding 0.2 V/Å generated by coherent transition radiation. Femtosecond, relativistic electron bunches generated at the Linac Coherent Light Source are passed through a beryllium foil, and the emitted radiation is characterized as a function of the bunch duration and charge. Broadband pulses centered at a frequency of 10 THz with energies of 140 μJ are measured. These far-below-bandgap pulses drive a nonlinear optical response in a silicon photodiode, with which we perform nonlinear autocorrelations that yield information regarding the terahertz temporal profile. Simulations of the spatiotemporal profile agree well with experimental results.

Observation of coherently enhanced tunable narrow-band terahertz transition radiation from a relativistic sub-picosecond electron bunch train

We experimentally demonstrate the production of narrow-band (δf/f≈20% at f≈0.5 THz) transition radiation with tunable frequency over [0.37, 0.86] THz. The radiation is produced as a train of sub-picosecond relativistic electron bunches transits at the vacuum-aluminum interface of an aluminum converter screen. The bunch train is generated via a transverse-to-longitudinal phase space exchange technique. We also show a possible application of modulated beams to extend the dynamical range of a popular bunch length diagnostic technique based on the spectral analysis of coherent radiation.

Generation of a few femtosecond keV x-ray pulse via interaction of a tightly focused laser copropagating with a relativistic electron bunch

It is demonstrated in a numerical simulation that an intense fs keV x-ray pulse can be generated by the interaction of a tightly focused femtosecond laser copropagating with an electron bunch. In general, the interaction of a loosely focused (focal spot $\ensuremath{\sim}100\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in diameter) laser with a copropagating electron is rather weak so that the radiation is not only weak but also produced in the vicinity of laser wavelength. However, in the case of tight focus (focal spot on the order of wavelength), the radiation characteristics turn out to be drastically different so that a keV x-ray pulse can be produced at high flux. This is due to the nonparaxial fields induced in a tight-focus regime. The radiation characteristics are discussed for different beam waists and electron beam energies. This simulation suggests that the interaction of a tightly focused laser with a copropagating electron bunch can be a unique source for an x-ray pulse of the photon energy from 10 to 100 keV that lasts a few femtoseconds.

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.

Vacuum ultraviolet circularly polarized coherent femtosecond pulses from laser seeded relativistic electrons

We have demonstrated the generation of circularly polarized coherent light pulses at 66 nm wavelength by combining laser seeding at 263 nm of a 375 MeV relativistic electron bunch with subsequent coherent harmonic generation from an elliptical undulator of APPLE-II type. Coherent pulses at higher harmonics in linear polarization have been produced and recorded up to the sixth order (44 nm). The duration of the generated pulses depends on the temporal overlap of the initial seed laser pulse and the electron bunch and was on the order of 200 fs. Currently, this setup is the only source worldwide producing coherent fs-light pulses with variable polarization in the vacuum ultraviolet.

Generation of relativistic electron bunches with arbitrary current distribution via transverse-to-longitudinal phase space exchange

We propose a general method for tailoring the current distribution of relativistic electron bunches. The technique relies on a recently proposed method to exchange the longitudinal phase space emittance with one of the transverse emittances. The method consists of transversely shaping the bunch and then converting its transverse profile into a current profile via a transverse-to-longitudinal phase-space-exchange beam line. We show that it is possible to tailor the current profile to follow, in principle, any desired distributions. We demonstrate, via computer simulations, the application of the method to generate trains of microbunches with tunable spacing and linearly ramped current profiles. We also briefly explore potential applications of the technique.