Temporally-coherent terawatt attosecond XFEL synchronized with a few cycle laser

Attosecond metrology using laser-based high-order harmonics has been significantly advanced and applied to various studies of electron dynamics in atoms, molecules and solids. Laser-based high-order harmonics have a limitation of low power and photon energies. There is, however, a great demand for even higher power and photon energy. Here, we propose a scheme for a terawatt attosecond (TW-as) X-ray pulse in X-ray free-electron laser controlled by a few cycle IR pulse, where one dominant current spike in an electron bunch is used repeatedly to amplify a seeded radiation to a terawatt level. This scheme is relatively simple, compact, straightforward, and also produces a temporally and spectrally clean pulse. The viability of this scheme is demonstrated in simulations using Pohang accelerator laboratory (PAL)-XFEL beam parameters.1183Ysciescopu

Using irregularly spaced current peaks to generatean isolated attosecond X-ray pulse in free-electron lasers

A method is proposed to generate an isolated attosecond X-ray pulse in free-electron lasers, using irregularly spaced current peaks induced in an electron beam through interaction with an intense short-pulse optical laser. In comparison with a similar scheme proposed in a previous paper, the irregular arrangement of current peaks significantly improves the contrast between the main and satellite pulses, enhances the attainable peak power and simplifies the accelerator layout. Three different methods are proposed for this purpose and achievable performances are computed under realistic conditions. Numerical simulations carried out with the best configuration show that an isolated 7.7keV X-ray pulse with a peak power of 1.7TW and pulse length of 70as can be generated. In this particular example, the contrast is improved by two orders of magnitude and the peak power is enhanced by a factor of three, when compared with the previous scheme.1152Ysciescopu

Accurate Method for Estimating of the Charge in the Bunching Part of an Electron Beam by Using an Electro-Optic Crystal

Electro-optic sampling (EOS) is a promising method to measure various properties of an electron beam non-destructively. An analysis procedure for electro-optic (EO) measurements to estimate the charge in the bunching part of an electron beam whose shape is not Gaussian is introduced in this paper. Experimental results measured by using the EO method are analyzed in terms of the relative phase shift between the horizontal and the vertical components of the laser. A simulation study is done with the pulse propagation method, which utilizes the Fourier transform to investigate the evolution of an electromagnetic pulse inside the EO crystal. The analysis result of the EO measurement expressed in terms of the relative phase shift is compared with the simulation and it shows a good agreement with the beam charge. The charge in the bunching part of the electron beam can be estimated accurately by using the analysis procedure in this paper.11sciescopuskc

Analysis of the electro-optic measurement of an electron beam with new refractive indices

Electro-optic sampling (EOS) is a promising method of measuring the electron bunch length and arrival time nondestructively. When an electric field is applied to the EO crystal, the EO crystal shows birefringence which is described by an ellipsoid of refractive indices. The ellipsoid can be expressed with the principal axes associated with the crystallographic axes. If a linearly polarized laser is passing through the EO crystal with external electric field, the polarization of the laser is modulated elliptically or even circularly. The polarization modulation of the laser probe beam should be expressed in terms of the relative phase shift between the decomposed vectors of the laser. Conventional analysis assumes that the polarization direction of the laser probe beam is to set to be at an angle of 45 degrees with the principal axes. Only the principal refractive indices have been used until now with this assumption. However, if the position of the electron beam is changed in experiments, the electric field applied to the EO crystal is also changed. Thus, the directions of the principal axes of the isotropic crystal are also changed. The principal axes do not make an angle of 45 degrees anymore with the fixed polarization vector of the laser probe beam in this situation. New refractive indices are needed to explain the experimental result with such deviation properly. In our work, new refractive indices are derived and simulation studies are carried out with the pulse propagation method, which utilizes the Fourier transform to investigate the evolution of the electromagnetic pulse inside the EO crystal. Experimental results are in good agreement with the prediction from the simulation with newly derived refractive indices.11sciescopu

A Study of the Energy Spread Effect for an Electron Diffraction Experiment

A 2.5 MeV electron beam for femtosecond electron diffraction is generated from a RF photocathode gun with the Brookhaven National Laboratory (BNL) gun IV type at the Pohang Accelerator Laboratory. We use a Ti:Sapphire laser system to illuminate the Cu photocathode with a 100 fs laser pulse. The energy spread of the electron beam is measured to be lower than 0.2% with a 23 degrees RF phase for oblique laser injection. Beam charge is estimated to be its low as 1 pC. The chromatic coherence length is estimated to be 0.106 angstrom. The diffraction patterns for polycrystalline aluminium are studied numerically with several electron beam energies with finite, energy spread. The blurred pattern is quantified to investigate the effect; of the energy spread on the diffraction pattern. A higher energy electron beam blurs the diffraction image more than a lower energy one.11sciescopuskc

Reduction of the higher-order field distribution in a photocathode rf gun for the X-ray free electron laser

The mechanism of the higher-order rf field elimination in the photocathode rf gun used for the X-ray free electron laser (XFEL) machine is discovered. The analysis and the measurement results of the rf field in several models of the rf gun with several holes at the cavity wall are presented. The contribution of the holes to the asymmetrical distribution of the rf field along the azimuthal angle is measured with several model cavities. Using a comparison between the experimental results and the numerically-obtained rf field distribution, we can reveal that the origin of the quadrupole component growing at the cavity with two holes and of the octapole component growing at the cavity with four holes is the superposition of the rf fields in the cavity. Two kinds of model cavities with several holes at the cavity wall have been fabricated, and the rf field distributions of the model cavities have been measured to compare with the theoretical analysis and the numerically-obtained rf field. From the analyses, we decided to adopt an rf gun that has dual feeds and two pumping holes for the Pohang Accelerator Laboratory (PAL-XFEL) project.11sciescopuskc

Reconstruction of electron beam distribution using phase-retrieval algorithm

The shapes of light sources such as electron beams can be reconstructed by inverse Fourier transformation of the complex degree of spatial coherency, which can be measured using Young's interferometer. The application of the phase-retrieval algorithm to reduce phase measurement errors in the complex degree of spatial coherency is numerically studied using an electron beam with an asymmetric distribution. This application is demonstrated with experimental data measured at the diagnostic beamline at the Pohang Accelerator Laboratory.11sciescopu

The coherency of synchrotron radiation at Pohang Accelerator Laboratory

The coherency of the synchrotron radiation at Pohang Accelerator Laboratory has been investigated using Young's interferometer. The electron beam size can be measured precisely using the interferometer. An interferogram using 650 nm light at the diagnostics beamline at Pohang Light Source (PLS) has been measured to determine the electron beam distribution and the spatial coherence length. Interferograms obtained by numerical study are compared with experimental results in order to understand the measured data. From this comparison, the electron beam at PLS is revealed to be a Gaussian distribution with a standard deviation of 210 mm. The spatial coherency length of 650 nm light at PLS is measured to be 0.57 cm, and that of 0.1 nm light at PLS is predicted to be 0.88 mm by the same numerical study.11sciescopu

A study of detection schemes in electro-optic sampling technique

Electro-optic sampling (EOS) is a powerful tool for the measurement of the electron beam or THz radiation. A new detection scheme called "near crossed polarizer (NCP)" scheme in the EOS technique is developed to increase the signal-to-noise ratio (SNR) in the experiment. The new detection scheme is studied in detail and the 3D scanning result with electron beam in FLASH is compared with the theoretical result of the detection scheme. (C) 2007 Elsevier B.V. All rights reserved.11sciescopu

Soft X-ray FEL simulation in PAL-XFEL

The soft X-ray beamline in the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) will provide a photon beam with a wavelength from 1 nm to 3 nm in the self-amplified spontaneous emission (SASE) mode by using an electron beam with a 3.15-GeV beam energy. Linearly polarized radiation will be supplied by using six planar undulators (PUs). The linearly-polarized radiation powers at 1 (3) nm can reach 10.2 (30) GW. Polarization of the radiation will be controlled by applying the reverse undulator tapering scheme to the PUs and adding two helical undulators (HUs). The circularly-polarized radiation powers at 1 (3) nm will be 3.11 (11.73) GW. The degrees of circular polarization will be larger than 0.99 for both wavelengths. Three options for the future upgrade of the beamline to increase the radiation power are proposed.1111sciescopuskc