Toshiaki Ohsaka

Coherent synchrotron radiation at submillimeter and millimeter wavelengths

Abstract To study the coherence effect of synchrotron radiation (SR), light emission from the Tohoku 300 MeV linac, whose energy and bunch length of electrons were 180 MeV and a few millimeters, was observed in the submillimeter and millimeter wavelength regions by using a grating spectrometer. The absolute intensity of ordinary SR in the visible region was also measured. In comparison with ordinary SR, the absolute intensity of SR in the millimeter wavelength region was enhanced by about a factor of 10 6 , which corresponded roughly to the number of electrons in one bunch. The intensity of enhanced SR showed the quadratic dependence on the beam current. The enhanced SR was strongly polarized in the electron orbital plane. The relation between the electron distribution in a bunch and the spectra of coherent SR is discussed.

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.

Temperature Change of the Far-Infrared Absorption and Nature of Low-Frequency Modes in Silica Glass

The far-infrared absorption of three kinds of silica glasses with different OH concentrations has been measured at 11 K in the region 10–100 cm -1 and compared with the absorption at 297 K. Below the so-called boson peak, the absorption was markedly dependent on temperature, but by contrast above the boson peak, it was almost independent of temperature, indicating the existence of two different types of low-frequency modes. It is suggested that the temperature-dependent absorption is due to structural relaxation modes, and the temperature-independent absorption reflects the density of states of collective motions of atoms related to clusters, though it is incomprehensible why the absorption is temperature independent.

Spectrum of coherent synchrotron radiation

A spectrum of coherent synchrotron radiation was measured at wavelengths of from 0.16 to 3.5 mm. The experiment was carried out using the Tohoku 300-MeV linac, whose accelerating frequency was 2856 MHz. A bunch shape was estimated by Fourier analysis for this spectrum. This result agrees with that of simulation for the bunching process in the injector of the accelerator. The interference effects between radiation emitted by the successive bunches were observed by an interferometer. It was shown that every radiation had the same phase when it was emitted by a bunch.<<ETX>>

Effect of OH Content on the Far-Infrared Absorption and Nature of the Boson Peak in Silica Glass.

To clarify the OH effect on low-frequency modes, the far-infrared absorption of silica glass containing ∼800 wt. ppm of OH has been newly measured in the region 10–100 cm -1 at room temperature, and quantitatively compared with that of silica glasses with different OH concentrations. OH-related absorption changes showed a monotonic increase with frequency and broad peaks corresponding to the boson peak shifted to lower frequencies with OH content. It is considered that OH ions which enter silica glass by breaking Si–O bonds, change sizes of clusters constructing a glass network structure and consequently contribute to the far-infrared absorption increase. From the above OH effect, the boson peak is suggested to be due to collective motions of atoms closely related to the clusters.

Defect-Induced Far-Infrared Absorption and Vacancy Modes in Ion Conductive CeO2:CaO Solid Solution

The far-infrared absorption of pure CeO 2 , CeO 2 :CaO and CeO 2 :ZrO 2 solid solutions has been measured at 11 and 297 K in the region 10–100 cm -1 . Two apparent absorption peaks are found at 11 K in the vicinity of 50–80 cm -1 only in the CeO 2 :CaO solid solution with oxygen vacancies, not in the CeO 2 :ZrO 2 solid solution without oxygen vacancies and are assigned to be due to oxygen vacancy modes or attempt modes. The continuous absorption showing a nearly quartic frequency dependence is observed in both the CeO 2 :CaO and CeO 2 :ZrO 2 solid solutions. This continuous absorption reflects the density of states of acoustic modes and exhibits non-glass-like temperature and frequency dependencies. It is suggested from the above result that the so-called boson peak in glasses does not possess the acoustic mode-like character.

Defect-Induced Far-Infrared Absorption in Oxygen-Ion Conductor (CeO2)0.9(CaO)0.1

The far-infrared (FIR) absorption of the oxygen-ion conductor (CeO 2 ) 0.9 (CaO) 0.1 and for comparison, pure CeO 2 ceramics, has been measured in the region 10–100 cm -1 at 11 K and 297 K. Continu...

Lattice dynamics of solid CS2 based on Kihara potential

The lattice dynamics of molecular crystal CS 2 is studied within the rigid molecule and harmonic approximations, on the basis of Kihara core potential. Frequencies of lattice vibrations, dispersion curves, density of states and heat capacity at low temperatures are calculated. The results are in good agreement with experimental data and also compared with previous calculations. A role of nonaxisymmetric short-range interactions is discussed.The lattice dynamics of molecular crystal CS 2 is studied within the rigid molecule and harmonic approximations, on the basis of Kihara core potential. Frequencies of lattice vibrations, dispersion curves, density of states and heat capacity at low temperatures are calculated. The results are in good agreement with experimental data and also compared with previous calculations. A role of nonaxisymmetric short-range interactions is discussed.

Lattice Dynamics of Solid CS2Based on Kihara Potential

The lattice dynamics of molecular crystal CS 2 is studied within the rigid molecule and harmonic approximations, on the basis of Kihara core potential. Frequencies of lattice vibrations, dispersion curves, density of states and heat capacity at low temperatures are calculated. The results are in good agreement with experimental data and also compared with previous calculations. A role of nonaxisymmetric short-range interactions is discussed.The lattice dynamics of molecular crystal CS 2 is studied within the rigid molecule and harmonic approximations, on the basis of Kihara core potential. Frequencies of lattice vibrations, dispersion curves, density of states and heat capacity at low temperatures are calculated. The results are in good agreement with experimental data and also compared with previous calculations. A role of nonaxisymmetric short-range interactions is discussed.