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.

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.

The first ever practical-level field generation for an undulator consisting of bulk high-temperature superconductors is reported and described. A high-field short-period undulator enables bright hard X-ray to be obtained in moderate-energy synchrotrons and to provide a polarized MeV gamma-ray source for producing a polarized positron beam via pair production in future linear colliders. To generate sinusoidal magnetic fields using bulk high-temperature superconductors, a novel structure has been developed in which the superconductors line up in a staggered configuration inside a solenoid. An undulator field of 0.85 T is achieved using a 10 mm period, 4 mm gap prototype in a 2 T solenoid.

Inverse Compton scattering of polarized laser photons by energetic electrons is an excellent method to generate polarized gamma rays. A 100% polarized laser can generate 100% polarized gamma rays, but polarization varies depending on the scattering angle of the gamma rays. In this study, we experimentally measure the spatial polarization distribution of circularly polarized gamma rays using a magnetized iron that can measure the circular polarization of MeV gamma rays. Measurements of the asymmetry of gamma-ray transmission relative to the magnetized iron at each scattering angle clearly show that gamma rays are circularly polarized near the central axis, and they change from circular to linear polarization as the scattering angle increases. A simple way to obtain highly polarized gamma rays is to use a collimator that transmits only the central axis of the gamma rays with such polarization characteristics. Polarized gamma rays are, indeed, generated by ${90}^{\circ }$ collisional inverse Compton scattering between an electron beam and a circularly polarized laser.

Computed Tomography (CT) using X-ray attenuation by atomic effects is now widely used for medical diagnosis and industrial non-destructive inspection. In this study, we performed a tomographic imaging of isotope (²⁰⁸Pb) distribution by the Nuclear Resonance Fluorescence (NRF), i.e. isotope specific resonant absorption and scattering of gamma rays, using Laser Compton Scattering (LCS) gamma rays. The NRF-CT image which includes both effects of atomic attenuation and nuclear resonant attenuation was obtained. By accounting for the atomic attenuation measured by a conventional method at the same time, a clear ²⁰⁸Pb isotope CT image was obtained. The contrast degradation due to notch refilling caused by small-angle Compton scattering is discussed. This study clearly demonstrates the capability of the isotope-specific CT imaging based on nuclear resonant attenuation which will be a realistic technique when the next generation of extremely intense LCS gamma-ray sources will be available. The expected image acquisition time using these intense LCS gamma rays was discussed.

The highest extraction efficiency (9.4%) of a free electron laser (FEL) oscillator has been achieved at the midinfrared FEL facility of Kyoto University. Because of the interaction between the electron beam and FEL electromagnetic field, a maximum electron energy decrease of 16% was observed. The measured energy decrease was consistent with the measured FEL spectrum. An FEL micropulse energy of ∼100 μJ with the expected few-cycle pulse duration at a wavelength of 11 μm was observed. This result is an important milestone for the high-extraction-efficiency FEL oscillator and will contribute to the strong-field physics of atoms and molecules.