Fujio Hinode

Exotic radiation from a photonic crystal excited by an ultrarelativistic electron beam.

We report the observation of an exotic radiation (unconventional Smith-Purcell radiation) from a one-dimensional photonic crystal. The physical origin of the exotic radiation is direct excitation of the photonic bands by an ultra-relativistic electron beam. The spectrum of the exotic radiation follows photonic bands of a certain parity, in striking contrast to the conventional Smith-Purcell radiation, which shows solely a linear dispersion. Key ingredients for the observation are the facts that the electron beam is in an ultra-relativistic region and that the photonic crystal is finite. The origin of the radiation was identified by comparison of experimental and theoretical results.

A prebunched FEL based on coherent transition radiation in the far-infrared region

Abstract A short-bunch beam of electrons of 150 MeV is led to an open resonator of semi-confocal type. The wavepackets of coherent transition radiation (TR), emitted from bunched electrons passing through mirrors of the resonator, go back and forth in the resonator and superpose on the subsequent bunches to stimulate coherent TR. The main mode is assigned as TEM 01 , and the intensity depends on the square of the beam current. The output intensity of the resonator is compared with the coherent spontaneous TR generated within the resonator to show amplification of radiation due to the stimulated emission.

Conceptual design of an isochronous ring to generate coherent terahertz synchrotron radiation

A novel coherent light source project in the terahertz wavelength region has been developed at Tohoku University. The project may involve development of high brightness electron guns employing cathode of single crystal LaB6 for production of a very short bunch length less than 100 fs. The light source has been designed based on isochronous ring optics to preserve the short bunch length. Although the ring is not a storage ring, the lattice of isochronous optics has resulted from consideration of path length differences due to the betatron motion. The coherent terahertz photons are emitted from circulating electron bunches injected from the linac. Even though the beam is bent by dipole magnets, the bunch shape does not collapse because of the nearly complete isochronous optics of the ring. Since production of the coherent terahertz radiation requires a bunch length less than 100 fs (stdv, if Gaussian), the maximum path length difference created by passing through the dipoles is controlled to not exceed a couple of tens of femtoseconds. The predicted spectrum of the coherent terahertz radiation and its characteristics are also presented.

Lattice Modification of a 1.2 GeV STB Ring for Generation of High Energy Gamma-Rays Using Internal Target Wire

A 1.2 GeV Stretcher-Booster Ring (STB ring) has been routinely operated at Laboratory of Nuclear Science (LNS), Tohoku University. The STB ring has functions of a pulse-beam stretcher and a booster-storage ring [1, 2]. In the booster-storage operation, high energy gamma-ray beam generated via bremsstrahlung from internal target wire is utilized for experiments of nuclear physics. Some fractions of circulating electrons are also deflected in the target wire due to Coulomb scattering without significant loss of the energy. The scattered electrons that are not getting out of the dynamic aperture once can circulate in the ring. Such electrons, however, would hit the chamber walls and supports of the target wire during further turns, because they have very large betatron amplitude. Consequently the Coulomb scattered electrons must be a source of significant background and may cause a degradation of gamma-ray beam quality. The quality of the gamma-ray beam has been improved by modifying the lattice functions of the ring. We present the details of the modification and the result of the improvement.

Upgrade of the 1.2 GeV STB ring for the SR utilization in Tohoku University

An electron accelerator complex, 300 MeV linac and 1.2 GeV booster synchrotron, had been routinely operated as a user facility at Tohoku University until the Great East Japan Earthquake damaged the accelerator seriously. In the last year some budgets were approved to partially reconstruct the accelerator. For the booster ring, some old power supplies of magnets are replaced. Furthermore some quadrupole magnets are also replaced to the combined function magnets of which sextupole component is included. Modifying the ring optics so as to introduce the horizontal dispersion into the position of combined magnet, this replacement will make it possible to correct the chromaticity. There has been no sextupole in the ring, so that the ring current is significantly limited due to head-tail instability. Hence this upgrade will bring the new capability into the ring as a synchrotron light source. Presently it is planned to utilize SR from the 1.3 T bending magnet for educational purpose in the field of SR application such as XAFS. The photon flux of 5.6E12 photons/s/mrad2/0.1%BW will be obtained around the peak energy region of 1 keV for this beam line in the case of 200 mA operation with the emittance of 160 nmrad. In addition to the beam line from bending magnet, a 2 m long straight section is also reserved for an insertion device for future application.

Study of Slow Beam Extraction through the Third Order Resonance with Transverse Phase Space Manipulation by a Mono-Frequency RFKO

An electron pulse stretcher ring (STB ring) [1] at Laboratory of Nuclear Science (LNS) in Tohoku University provides quasi-cw electron beam for the study of nuclear physics. The extracted beam from the ring has certain spread in time and space resulted from the emittance of injected beam from linac even if the injected beam is perfectly matched to the ring optics. However, the extracted beam emittance can be reduced by applying a phase space manipulation. Under the effect of perturbation using an RF shaker driven by a mono-frequency, the betatron amplitude of circulating beam can be controlled in order to reduce the extracted beam emittance. We have demonstrated the reduction of emittance experimentally on the STB ring.

Simulation Study of a Thermionic RF Gun for High Brightness and Short Pulse Beam

Recently sub-picosecond electron pulses are paid attention because of valious applications of THz coherent radiation emitted from such short bunches. Although photoinjetcors using RF guns are rapidly developped as high blliiant electron sources, while thermionic RF guns are still expected to have potential ability to create high-brightness and short-pulse beams [1]. In particular, components of a thermionic RF gun are simple, compact and low-cost than those of a photo-cathode RF gun. For creating such beams, a prototype of thermionic RF gun was designed and its characteristics have been studied by a 3-D simulation code based on an FDTD (Finite Difference Time Demain) method which has been developed so far [2]. The gun is consists of two independently power feeding S-band RF cavities. The first cell is a cathode cell to extract the beam and the second one is an accelerating cell. This gun can be operated at modes with different RF-power ratio and phase between two RFs. A similar way of operating RF gun has been already reported by Lewellen [3]. This paper describes the results of simulations for this thermionic RF gun.

A Far‐infrared Undulator for Coherent Synchrotron Radiation and Free Electron Laser at Tohoku University

In order to develop an intense far‐infrared radiation source, a high quality electron beam has been studied at Tohoku University, Sendai. The bunch length of the beam expected is very much shorter than terahertz (THz) wavelength, so that coherent spontaneous emission of synchrotron radiation will be a promising high brilliant far‐infrared source. An undulator consisting of permanent magnets has been designed in which optional free electron laser (FEL) will be operated in free space mode. Consequently the minimum gap of the undulator is decided to be 54 mm for 0.36 mm radiation to avoid diffraction loss, and then the period length of 10 cm is employed. The undulator may cover a wavelength range from 0.18 to 0.36 mm with the beam energy of 17 MeV. Property of coherent THz radiation from the undulator and possibility of novel pre‐bunched THz FEL is discussed.