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Summary: Microbunching caused by free-electron laser (FEL) interactions in an electron bunch deforms the overall bunch shape. Recent reports indicate the timing of the electron bunch overlapping with the FEL micropulse affects deformation in resonator-type FELs. The electron bunch shape is expected to change with the FEL micropulse energy because the FEL micropulse energy is enhanced within the electron beam macropulse; however, this has not yet been investigated. We evaluated the temporal evolution of the electron bunch length during FEL oscillations by observing the coherent edge radiation generated by the resonator-type FEL device, KU-FEL, Kyoto University. Experimentally verification showed that the expansion and contraction of the electron bunch changed more clearly when the energy and net gain of the FEL micropulse were high. Furthermore, the expansion and contraction behavior was consistent with a one-dimensional FEL simulation based on a well-established unaveraged method.

Abstract The detailed structure of superradiant pulses generated from a free-electron laser (FEL) oscillator was experimentally revealed for the first time. Owing to the phase retrieval with a combination of linear and nonlinear autocorrelation measurements, we successfully reconstructed the temporal waveform of an FEL pulse including its phase variation. The waveform clearly exhibits the features of a superradiant pulse, the main pulse followed by a train of sub-pulses with π-phase jumps, reflecting the physics of light-matter resonant interaction. From numerical simulations, the train of sub-pulses was found to originate from repeated formation and deformation of microbunches accompanied with a temporal slippage of the electrons and light field, a process quite different from coherent many-body Rabi oscillations observed in superradiance from atomic systems.

We formulate a theoretical approach to describe photon vortex production in synchrotron/cyclotron radiation from a helical moving electron under a uniform magnetic field in the relativistic quantum framework. In quantum theory, electron orbitals in a magnetic field are under Landau states. The Landau level density increases with decreasing the magnetic field strength, and it is practically impossible to calculate exactly the synchrotron radiation in possible magnetic fields in the laboratory. We present a method to calculate it by using asymptotic formulations of the emission amplitudes under the condition of L_{i}−L_{f}≪L_{i}, where L_{i} and L_{f} are the initial and final Landau numbers, respectively. The wave function of the generated photon is the eigenstate of the z component of the total angular momentum (zTAM) when the magnetic field is parallel to the z axis. The approximation is applicable for photon vortex production of several tens ℏ of zTAM. We also calculate numerically the energy spectra and spatial distribution of photon vortices in magnetic field strengths of 10 and 20 T using electrons with energies of 150 MeV and 8 GeV and Larmor radii of 10 and 100 µm for the helical motion. Although the Landau number becomes up to 10^{8} in these conditions, the radiations could be calculated using the approximation. The results indicate that it is possible to produce predominantly photon vortices with a fixed zTAM when we control the energy and entrance trajectory of electrons and the structure of a magnetic field. The present formula contributes to the prediction and verification of photon vortex generation.

Photon sources based on inverse Compton scattering, namely, the interaction between relativistic electrons and laser photons, are emerging as quasimonochromatic energy-tunable sources either as compact alternatives to synchrotron facilities for the production of low-energy (10–100 keV) x rays or to reach the 1–100 MeV photon energy range, which is inaccessible at synchrotrons. Different interaction layouts are possible for electron and laser beams, and several applications are being studied, ranging from fundamental research in nuclear physics to advanced x-ray imaging in the biomedical field, depending on the radiation energy range, intensity, and bandwidth. Regardless of the specific application, a reliable tool for the simulation of the radiation produced is essential for the design, the commissioning, and, subsequently, the study and optimization of this kind of source. Different computational tools have been developed for this task, based on both a purely analytical treatment and Monte Carlo simulation codes. Each of these tools has strengths and weaknesses. Here, we present a novel Monte Carlo code based on geant4 for the simulation of inverse Compton scattering in the linear regime. The code produces results in agreement with cain, one of the most used Monte Carlo tools, for a wide range of interaction conditions at a computational time reduced by 2 orders of magnitude. Furthermore, the developed tool can be easily embedded in a geant4 user application for the tracking of photons generated through inverse Compton scattering in a given experimental setup.

Generation of few-cycle optical pulses in free-electron laser (FEL) oscillators has been experimentally demonstrated in FEL facilities based on normal-conducting and superconducting linear accelerators. Analytical and numerical studies have revealed that the few-cycle FEL lasing can be explained in the frame of superradiance, cooperative emission from self-bunched systems. In the present paper, we review historical remarks of superradiance FEL experiments in short-pulse FEL oscillators with emphasis on the few-cycle pulse generation and discuss the application of the few-cycle FEL pulses to the scheme of FEL-HHG, utilization of infrared FEL pulses to drive high-harmonic generation (HHG) from gas and solid targets. The FEL-HHG enables one to explore ultrafast science with attosecond ultraviolet and X-ray pulses with a MHz repetition rate, which is difficult with HHG driven by solid-state lasers. A research program has been launched to develop technologies for the FEL-HHG and to conduct a proof-of-concept experiment of FEL-HHG.

The extraction efficiency is one of the key parameters of an oscillator-type free electron laser (FEL). An extraction efficiency higher than 5% was observed with the midinfrared (MIR) FEL at Kyoto University using a dynamic cavity desynchronization method. The observed efficiency is the highest extraction efficiency realized in MIR-FEL oscillators driven by normal conducting linacs. A numerical simulation was performed, and the evolution of the electron energy distribution in the macropulse was compared with the experimental result. The details of the evolution of the electron energy distribution were investigated to reveal the physics of this high-efficiency lasing.

Operational experience of a 500 kV photoemission gun at the compact energy recovery linac (cERL) of the High Energy Accelerator Research Organization is presented. The gun, developed at the Japan Atomic Energy Agency, was found to have failures in two out of the ten-segment ceramic insulator just after installation at cERL. The gun had been operated at 390 kV with eight segments until April 2015 and provided a 0.9 mA beam. An additional two-segment insulator was installed on the top of the existing insulators to recover the high-voltage performance. The gun was then conditioned up to 539 kV and has been operated stably at 500 kV. No discharge caused by the gun itself was observed at 500 kV once the high threshold voltage for stable operation exceeded 500 kV. A dark current of a few picoamperes was generated at 500 kV from a photocathode puck with a semiconducting wafer, while no dark current was observed without a semiconducting wafer. Stable generation of a 500 keV beam with current greater than 0.8 mA was demonstrated for more than two hours.

The polarization properties of γ -rays elastically scattered by atoms have become more observable with the development of polarized photon beams. However, systematic studies are required to explore the elastic scattering in the MeV-energy range of the spectrum where Delbrück scattering becomes more significant, especially at large scattering angles. We implement a new Monte Carlo simulation to account for the polarization effects of elastic scattering. The simulation is based on explicit expressions driven in the formalism of Stokes parameters. The scattering amplitudes of Rayleigh, nuclear Thomson, and Delbrück scattering processes are superimposed onto a two orthogonal set of complex amplitudes. This set is then exploited to construct the core of the simulation in such a way that the simulation can handle arbitrary polarization states of the incoming beam and correspondingly generate polarization states of the outgoing beam. We demonstrate how the polarization of scattered photons is affected by the polarization of incoming photons. In addition, we explain the dependence of depolarization on the azimuthal angle.

We propose a scheme to generate narrow-band GeV photons, γ-rays, via Compton scattering of hard x-ray photons in an x-ray free-electron laser oscillator. Generated γ-rays show a narrow-band spectrum with a sharp peak, ∼0.1% (FWHM), due to large momentum transfer from electrons to photons. The γ-ray beam has a spectral density of ∼10^{2} ph/(MeV s) with a typical set of parameters based on a 7-GeV electron beam operated at 3-MHz repetition, Such γ-rays will be a unique probe for studying hadron physics. Features of the γ-ray source, flux, spectrum, polarization, tunability and energy resolution are discussed.

We demonstrated the generation of a 500-keV electron beam from a high dc voltage photoemission gun for an energy recovery linac light source [N. Nishimori et al., Appl. Phys. Lett. 102, 234103 (2013)]. This demonstration was achieved by addressing two discharge problems that lead to vacuum breakdown in the dc gun. One is field emission generated from a central stem electrode. We employed a segmented insulator to protect the ceramic insulator surface from the field emission. The other is microdischarge at an anode electrode or a vacuum chamber, which is triggered by microparticle transfer or field emission from a cathode electrode. An experimental investigation revealed that a larger acceleration gap, optimized mainly to reduce the surface electric field of the anode electrode, suppresses the microdischarge events that accompany gas desorption. It was also found that nonevaporable getter pumps placed around the acceleration gap greatly help to suppress those microdischarge events. The applied voltage as a function of the total gas desorption is shown to be a good measure for finding the optimum dc gun configuration.

We propose a simple and effective synchronization technique between a reference electrical oscillator and a mode-locked laser for a narrowband picosecond Laser-Compton scattering γ-ray source by using a commercial-based 1-chip frequency synthesizer, which is widely used in radio communication. The mode-locked laser has been successfully synchronized in time with a jitter of 180 fs RMS for 10 Hz–100 kHz bandwidth.Agood stability of 640 μHz at 80MHzrepetition rate for 10 h operation has also been confirmed.We discuss in detail the design and performance of this technique (in terms of timing jitter, stability, and validity). [ABSTRACT FROM AUTHOR]

The diffraction of γ-rays by mosaic Si crystals with thicknesses of 20, 40, and 80 mm and by a 2-mm-thick perfect Si crystal in a transmission Laue geometry was measured using a high-flux 60Co source with an intensity of 2.2 TBq. The measured diffraction intensities at 1.17 and 1.33 MeV using 40- and 80-mm-thick mosaic crystals were enhanced by a factor of approximately 8.6 compared with those of the perfect Si crystal. The integrated reflectivity is well described in statistical dynamical theory by taking into account γ-ray absorption inside the crystals. [ABSTRACT FROM AUTHOR]

Diffraction of γ-rays by a flat Si crystal has been demonstrated using a high flux 60Co source with an intensity of 2.3 TBq. The diffraction intensities of the γ-rays with energies of 1.17 and 1.33 MeV have been measured as a function of the rotation angle of the crystal. Three peaks corresponding to the Si(440) and Si(220) diffractions for 1.17 MeV and the Si(440) diffraction for 1.33 MeV have been measured. The heights and shapes of these three peaks are well reproduced by taking into account Bragg’s law and the experimental geometry. [ABSTRACT FROM AUTHOR]