H. Hama

Observation of intracavity Compton backscattering of the UVSOR free electron laser

Abstract High-energy gamma rays produced by intracavity Compton backscattering of a free electron laser (FEL) on the UVSOR storage ring have been observed. In the experiment, the storage ring was operated at an electron energy of 600 MeV. The FEL wavelengths were 466 and 270 nm, and then maximum photon energies were calculated to be 14.6 and 25.2 MeV, respectively. Spectra of the photons measured by using a large NaI scintillation detector were found to be consistent with a theoretical calculation.

Lasing of a free electron laser in the visible on the UVSOR storage ring

Abstract We have conducted oscillation experiments of a free electron laser on the UVSOR storage ring. In the experiments the storage ring was operated in the two-bunch mode at an electron energy of 500 MeV and an optical klystron was employed. Dielectric multilayer mirrors with high reflectivity at a wavelength around 455 nm were used for an optical resonator. When the beam current is higher than 9 mA/bunch, lasing started at 456 nm. The lasing wavelength can be varied in the region from 430 nm to 480 nm. The linewidth of the laser is approximately 0.2 nm. The peak power in micro-pulses transmitted by the front mirror is roughly estimated to be at least 0.8 W at a beam current of 23 mA/bunch.

FEL experiment on the UVSOR storage ring

Abstract A gain-enhancement technique using a higher harmonic cavity has been developed for free electron laser experiments on the UVSOR storage ring. With this technique, we obtained lasing in the ultraviolet region at wavelengths around 300 nm. Applying gain-switching to control lasing, effective gain was derived at a wavelength of 490 nm, which was in good agreement with the theoretical prediction.

Observation of micro-macro temporal structure and saturation mechanism on the UVSOR free electron laser

Abstract The dynamic behavior of a free electron laser oscillation on an electron storage ring has been studied. We have measured time dependent micropulse evolution using a streak camera with a dual-sweep. Continuous lasing which appears under detuned condition has been ascertained to consist of discrete evolving groups of micropulses. In the gain-switching mode, bunch lengthening of the electrons due to bunch heating by the interaction with the laser was clearly observed.

Microscopic study on lasing characteristics of the UVSOR storage ring free electron laser

Abstract Characteristics of storage ring free electron laser at a short wavelength region (UV and visible) has been studied at the UVSOR facility, Okazaki. We have introduced an empirical formula to describe the gain variation based on the bunch-heating model for gain-switched laser macropulses. Considerable substructures were found in both temporal distributions and frequency spectra. There are a couple of peaks discretely located in a laser optical bunch. Such an internal substructure has been found to be established in an early stage of lasing process.

Design of a helical undulator for UVSOR

Abstract The design concept and the expected performance of a helical undulator which is to be installed in the 0.75 GeV storage ring, UVSOR, of the Institute for Molecular Science are reported. The undulator should produce perfectly circularly polarized light in the energy range of 2 – 45 eV and elliptically and linearly polarized light with energies up to 300 eV.

From the operation of an SRFEL to a user facility

Storage ring free-electron lasers (SRFELs) likely have a potentiality for scientific application as a unique light source because of the good coherence and temporal feature in addition to variable wavelength. At the UVSOR, the performance of the SRFEL has been improved aiming at users applications. Recently, an experiment using SRFEL combined with synchrotron radiation (SR) was begun. As the first experiment, the double-resonant excitation of Xe has been investigated by using SR and SRFEL as pump and probe lights, respectively. The relevance of making use of SRFEL for the pump/probe experiment is demonstrated.

Control of the bunch length on an electron storage ring

Abstract We report on an experiment to control bunch length by means of changing the momentum compaction factor on the UVSOR storage ring. The momentum compaction factor was varied by changing the dispersion function, and reduced to be approximately one hundredth of the ordinary value. The shortest bunch length estimated from the synchrotron oscillation frequency was 25 ps, which is one tenth of the ordinary value. Correction for the second order momentum compaction factor was found to be essential for operation with the extremely low momentum compaction factor. The results of measurement of the momentum compaction factor and the bunch length are presented.

Undulators at HiSOR – a compact racetrack-type ring

A compact racetrack-type 700 MeV storage ring (HiSOR) has been constructed at Hiroshima Synchrotron Radiation Center (HSRC). As the ring was planned for synchrotron radiation research on science and technology using VUV to X-rays up to 5 keV with limited size and cost, the ring was designed (i) to realize a high magnetic field (2.7 T) using conventional dipole magnets for higher critical energy, and (ii) to include two straight sections for insertion devices. A linear undulator (25-300 eV) and a new-type helical/linear undulator were installed at the two straight sections. The latter undulator consists of upper and lower jaws, as in a planar undulator; each jaw consists of one fixed magnet array at the centre and two magnet arrays on both sides. By longitudinal displacement of the side magnet arrays, the phase between the vertical and horizontal magnetic fields, and therefore the polarization (right- or left-circular, elliptical, linear) can be selected. The helical/linear undulator gives almost perfect circular polarization at 4-40 eV in the helical configuration without changing the phase of the magnet arrays, as well as linearly polarized light at 3-300 eV in the linear configuration.

Testing for qualification of a helical optical klystron for UV storage-ring free electron lasers

Abstract A new helical optical klystron, UNKO-3, for a UV-free electron laser has been installed and commissioned on the UVSOR storage ring, Okazaki. Lasings at a visible wavelength region around 470 nm and a UV region down to 260 nm were obtained in test experiments. It was found that the mirror degradation rate with radiation from a helical mode was several tens of times slower than that with a planar mode.