Femtosecond Electron Bunches Research Articles

Influence of space charge effect in femtosecond electron bunch on coherent transition radiation spectrum

This paper numerically and experimentally demonstrates the influence of the space charge forces in short electron bunches on the spectrum of coherent transition radiation. To estimate and optimize dimensions of a short electron bunches produced by the KEK LUCX RF gun, ASTRA software was employed. Obtained results indicate that the modification of the femtosecond UV laser pulse transverse profile allows to decrease the space charge forces. The simulation results were confirmed experimentally with the good agreement.

Unified Time and Frequency Picture of Ultrafast Atomic Excitation in Strong Laser Fields.

Excitation and ionization in strong laser fields lies at the heart of such diverse research directions as high-harmonic generation and spectroscopy, laser-induced diffraction imaging, emission of femtosecond electron bunches from nanotips, self-guiding, filamentation and mirrorless lasing during propagation of light in atmospheres. While extensive quantum mechanical and semiclassical calculations on strong-field ionization are well backed by sophisticated experiments, the existing scattered theoretical work aiming at a full quantitative understanding of strong-field excitation lacks experimental confirmation. Here we present experiments on strong-field excitation in both the tunneling and multiphoton regimes and their rigorous interpretation by time dependent Schrödinger equation calculations, which finally consolidates the seemingly opposing strong-field regimes with their complementary pictures. Most strikingly, we observe an unprecedented enhancement of excitation yields, which opens new possibilities in ultrafast strong-field control of Rydberg wave packet excitation and laser intensity characterization.

Formation and stability of a hollow electron beam in the presence of a plasma wake field driven by an ultra-short electron bunch

A numerical investigation on the spatiotemporal evolution of an electron beam, externally injected in a plasma in the presence of a plasma wake field, is carried out. The latter is driven by an ultra-short relativistic axially-symmetric femtosecond electron bunch. We first derive a novel Poisson-like equation for the wake potential where the driving term is the ultra-short bunch density, taking suitably into account the interplay between the sharpness and high energy of the bunch. Then, we show that a channel is formed longitudinally, through the externally injected beam while experiencing the effects of the bunch-driven plasma wake field, within the context of thermal wave model. The formation of the channel seems to be a final stage of the 3D evolution of the beam. This involves the appearance of small filaments and bubbles around the longitudinal axis. The bubbles coalesce forming a relatively stable axially-symmetric hollow beam structure.

Concept of a laser-plasma-based electron source for sub-10-fs electron diffraction

We propose a new concept of an electron source for ultrafast electron diffraction with sub-10~fs temporal resolution. Electrons are generated in a laser-plasma accelerator, able to deliver femtosecond electron bunches at 5 MeV energy with kHz repetition rate. The possibility of producing this electron source is demonstrated using Particle-In-Cell simulations. We then use particle tracking simulations to show that this electron beam can be transported and manipulated in a realistic beamline, in order to reach parameters suitable for electron diffraction. The beamline consists of realistic static magnetic optics and introduces no temporal jitter. We demonstrate numerically that electron bunches with 5~fs duration and containing 1.5~fC per bunch can be produced, with a transverse coherence length exceeding 2~nm, as required for electron diffraction.

Wakefields in THz cylindrical dielectric lined waveguides driven by femtosecond electron bunches

This paper reports the wakefield effects driven by a high-intensity relativistic electron bunch in a dielectric lined waveguide (DLW). A state-of-the-art electron bunch is employed to serve as the drive bunch, which has an rms length of 10µm, i.e. 33fs, and a charge of 200pC. Such bunch parameters are comparable to those of DESY’s FLASH and SLAC’s LCLS and FACET facilities. It is demonstrated that coherent Cherenkov radiation (CCR) at the fundamental mode with frequency above 1THz and accelerating gradient as high as 2GV/m can be obtained in a single layer cylindrical diamond-DLW structure, as long as the geometrical parameters of the DLW are properly selected to match the drive bunch. Wakefield-induced energy modulations on the drive bunch itself are studied as well, which can be used to reduce its energy spread or to produce microbunches with much shorter length from it. The simulated results agree well with the theoretical predictions. Such wakefields can be used to accelerate or modulate electron bunches with ultra-high gradients, and produce high power THz radiations directly. These properties have potential applications in the fields of compact colliders and advanced radiation sources.

Measurement of<20 fsbunch length using coherent transition radiation

By monitoring coherent transition radiation (CTR) with a Michelson interferometer, bunch length measurement was investigated. Less-than-20-fs electron bunches were generated by a photocathode-based linear accelerator (linac) and a magnetic bunch compressor. From the femtosecond electron bunch of 2.1 pC and 32 MeV, CTR was measured using a Michelson interferometer that had two detectors with different spectral ranges, which enhanced the detection spectral range of CTR (ranging from 3 to 50 THz) and enabled us to determine the bunch length of the electron bunch. A model based on the interferometer's frequency sensitivity was proposed and introduced to analyze the measured interferograms. The measured interferograms and the frequency spectra were explained using the model.

Ultrafast electron diffraction from an ultracold source

Femtosecond electron bunches extracted from a laser-cooled gas cloud could become a powerful tool for macromolecular studies.

Electron acceleration in sub-relativistic wakefields driven by few-cycle laser pulses

Using particle-in-cell simulations, we study the interaction of few-mJ–few-cycle laser pulses with an underdense plasma at resonant density. In this previously unexplored regime, it is found that group velocity dispersion is a key ingredient of the interaction. The concomitant effects of dispersion and plasma nonlinearities cause a deceleration of the wakefield phase velocity, which becomes sub-relativistic. Electron injection in this sub-relativistic wakefield is enhanced and leads to the production of a femtosecond electron bunch with a picocoulomb of charge in the 5–10 MeV energy range. Such an electron bunch is of great interest for application to ultrafast electron diffraction. In addition, in this dispersion dominated regime, it is shown that positively chirped laser pulses can be used as a tuning knob for compensating for plasma dispersion, increasing the laser amplitude during self-focusing and optimizing the trapped charge.

Femtosecond 240-keV Electron Pulses from Direct Laser Acceleration in a Low-Density Gas

We propose a simple laser-driven electron acceleration scheme based on tightly focused radially polarized laser pulses for the production of femtosecond electron bunches with energies in the few-hundreds-of-keV range. In this method, the electrons are accelerated forward in the focal volume by the longitudinal electric field component of the laser pulse. Three-dimensional test-particle and particle-in-cell simulations reveal the feasibility of generating well-collimated electron bunches with an energy spread of 5% and a temporal duration of the order of 1 fs. These results offer a route towards unprecedented time resolution in ultrafast electron diffraction experiments.

Intense terahertz pulses from SLAC electron beams using coherent transition radiation

SLAC has two electron accelerators, the Linac Coherent Light Source (LCLS) and the Facility for Advanced Accelerator Experimental Tests (FACET), providing high-charge, high-peak-current, femtosecond electron bunches. These characteristics are ideal for generating intense broadband terahertz (THz) pulses via coherent transition radiation. For LCLS and FACET respectively, the THz pulse duration is typically 20 and 80 fs RMS and can be tuned via the electron bunch duration; emission spectra span 3-30 THz and 0.5 THz-5 THz; and the energy in a quasi-half-cycle THz pulse is 0.2 and 0.6 mJ. The peak electric field at a THz focus has reached 4.4 GV/m (0.44 V/Å) at LCLS. This paper presents measurements of the terahertz pulses and preliminary observations of nonlinear materials response.

Simulations and Measurements of Dipole and Quadrupole Magnets for PBP-CMU Linac System

The PBP-CMU Linac system has been established to generate femtosecond electron bunches and to study their applications, especially for generation of coherent terahertz (THz) radiations. The dipole magnet and the prototype quadrupole magnet for the system have been designed and fabricated in house. Both the simulated and the measured results were used for evaluating the magnet performance and will be used as a guideline for design and fabrication of magnet devices in the future.

Design and implementation of a flexible beamline for fs electron diffraction experiments

Ultrafast Electron Diffraction (UED) has been widely used to investigate the structural dynamics of molecules and materials. Femtosecond (fs) electron bunches are used to obtain diffraction images of a specimen upon photo-excitation by a temporally delayed light pulse. The high cross-section of electrons makes it a very flexible tool for the study of light elements, monolayers and surfaces; at the same time, electrons can travel down to few nanometers (nm) and structural information from the bulk can also be retrieved. In this article, we discuss the design and implementation of a flexible beamline for fs electron diffraction experiments in transmission or reflection geometry. By the use of a radiofrequency (RF) compression cavity synchronized to our laser system, in combination with a set of electron optics, we demonstrate that we can control the beam properties in terms of charge per pulse, transverse spot-size on the sample and temporal duration of the bunches. The characterization of the beam is performed via a light-electrons cross-correlation experiment and we demonstrate an overall temporal resolution around 300fs for bunches containing up to 105 electrons at a repetition rate of 20kHz.

Compact, low power radio frequency cavity for femtosecond electron microscopy

Reported here is the design, construction, and characterization of a small, power efficient, tunable dielectric filled cavity for the creation of femtosecond electron bunches in an existing electron microscope without the mandatory use of femtosecond lasers. A 3 GHz pillbox cavity operating in the TM(110) mode was specially designed for chopping the beam of a 30 keV scanning electron microscope. The dielectric material used is ZrTiO(4), chosen for the high relative permittivity (ε(r) = 37 at 10 GHz) and low loss tangent (tan δ = 2 × 10(-4)). This allows the cavity radius to be reduced by a factor of six, while the power consumption is reduced by an order of magnitude compared to a vacuum pillbox cavity. These features make this cavity ideal as a module for existing electron microscopes, and an alternative to femtosecond laser systems integrated with electron microscopes.

Long-Range Persistence of Femtosecond Modulations on Laser-Plasma-Accelerated Electron Beams

Laser plasma accelerators have produced femtosecond electron bunches with a relative energy spread ranging from 100% to a few percent. Simulations indicate that the measured energy spread can be dominated by a correlated spread, with the slice spread significantly lower. Measurements of coherent optical transition radiation are presented for broad-energy-spread beams with laser-induced density and momentum modulations. The long-range (meter-scale) observation of coherent optical transition radiation indicates that the slice energy spread is below the percent level to preserve the modulations.

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.

Development of femtosecond photocathode RF gun

Femtosecond electron bunches are essential for the observation of ultrafast reactions and phenomena in materials. To improve the time resolution of pulse radiolysis and ultrafast electron diffraction (UED), which involve the use of ultrashort electron bunches and ultrashort light, a femtosecond photocathode RF gun driven by a femtosecond laser was investigated experimentally. Bunch charge generated in the RF gun was studied based on the Schottky effect. Thermal emittance was estimated as 0.26 mm mrad at an rms laser spot size of 0.21 mm. Bunch length and longitudinal emittance were obtained as 180 fs and 0.87 deg keV, respectively, at a bunch charge of 3.5 pC and rms laser spot size of 0.38 mm.

Proposal for generation of high-intensity monochromatic Cherenkov radiation in THz range by femtosecond electron bunches in impurity-doped semiconductor tube

A novel method to generate high-power THz radiation is proposed. If a beam with a bunch length on the order of 100 fs is injected into an electron–hole plasma of a semiconductor with a plasma frequency on the order of THz, THz wake fields are coherently generated. If the beam moves on the axis of a hollow tube covered by a metal, the frequency spectrum of the radiation is composed of discrete components. Monochromatic radiation is obtained by making only the lowest frequency component coherent.

Ultrafast pulse radiolysis

Femtosecond time-resolved pulse radiolysis was developed using a femtosecond electron bunch and a femtosecond laser light. This development initiated the experimental study of pulse radiolysis in the femtosecond time region. The femtosecond transient absorption kinetics of hydrated electrons in water radiolysis was observed. The initial yield of the hydrated electron in water radiolysis was obtained to be G=4.6±0.3. The experimental data indicates that about 10% of the hydrated electrons undergo the spur reaction with H 3O + cations and OH radicals in the first 10 ps. The dependences of the transient absorption signal on the electron bunch length, the electron charge, and the sample thickness were also investigated.

Generating Ultrarelativistic Attosecond Electron Bunches with Laser Wakefield Accelerators

Femtosecond electron bunches with ultrarelativistic energies were recently generated by laser wakefield accelerators. Here we predict that laser wakefield acceleration can generate even attosecond bunches, due to a strong chirp of the betatron frequency. We show how the bunch duration scales with the acceleration parameters and that, after acceleration, the bunches can propagate over many tens of centimeters without a significant increase in duration.

Investigation of water distribution in proton exchange membrane fuel cells via Terahertz imaging

Coherent transition radiation in a THz regime generated from a femtosecond electron bunch is explored for its potential use in imaging applications. Due to water sensitivity, the THz imaging experiment is performed on a proton exchange membrane fuel cell (PEMFC) to assess the ability to quantify water in the flow field of the cell. In this investigation, the PEMFC design and the experimental setup for the THz imaging is described. The results of the THz images in the flow field are also discussed.