M. Kawai

Generation of relativistic photoelectrons induced by excimer laser irradiation

For the first time, photoelectrons from a metal surface irradiated by a pulsed excimer laser were accelerated to relativistic energy. The energy, the current density, and the pulse duration of the accelerated photoelectrons were 0.41 MeV, 0.5 A/cm2, and 20 ns, respectively.

Generation of relativistic photoelectrons induced by an ultraviolet laser and their propagation characteristics in a longitudinal guiding magnetic field

Photoelectrons from a metal surface irradiated by a pulsed ultraviolet laser were accelerated to relativistic energy to obtain a very cold and relatively high‐current electron beam. Typical values of the transverse velocity component β⊥/β∥ were measured to be less than 8×10−3, which leads to a spread of the parallel energy component Δγ∥u2009/γ∥ of 1×10−4 or less. Propagation characteristics in a longitudinal guiding magnetic field were investigated. A nonadiabatic factor was defined to estimate the conservation of the magnetic moment of the electrons moving along the magnetic flux, and this was shown experimentally to be reasonable. Spacial controllability of the cross section of this electron beam was demonstrated.

Angular velocity spread of relativistic photoelectrons induced by excimer laser irradiation

The angular velocity spread of relativistic photoelectrons induced by a pulsed excimer laser was measured. The energy, the current density, and the pulse duration of the accelerated photoelectron were 0.34 MeV, 0.5 A/cm2, and 20 ns, respectively. (The method of measurement is based on measuring Larmor radius which corresponds to the transverse component of the electron velocity.) The angular velocity spread β⊥/β∥ was found to be less than 8×10−3, which means that the energy component due to β⊥ was as small as ≲17 eV.

Observation of periodical short pulse trains in free-electron laser oscillations

Periodical short pulse trains were observed in a long pulse, low current, low voltage waveguide mode free‐electron laser. The pulse width and the pulse train intervals of the periodical short pulse trains were about a few ns and about 23 ns, respectively. These pulse trains can be explained as self‐mode‐locked oscillations of a free‐electron laser.

Measurement of the Energy Spread of the Cold Relativistic Electron Beam Using Thomson Backscattering of a High Power CO2 Laser

The energy spread of the cold relativistic electron beam was measured by the Thomson backscattering of a high power CO2 laser. The energy spread was about 1.2% of the total energy. The wavelength and the power of the scattered light were in good agreement with the theoretically calculated values.

Development and quality measurements of cold relativistic electron beam for low‐γ free‐electron lasers

A relativistic electron beam source with low temperature (cold) using a field emission cathode and uniform electrostatic acceleration has been developed for use in low‐γ free‐electron lasers. An energy of 0.51 MeV and a current of 60 A (200 A/cm2) were obtained. The energy spread and the angular velocity spread were measured to be ΔE/E=0.14% and β⊥/β∥ =4×10−2, respectively.

Measurement of homogeneously broadened spectra of spontaneous emission and small‐signal gain in a low‐energy, waveguide‐mode free‐electron laser

The spontaneous emission spectrum and the small‐signal gain have been measured in a low‐energy, waveguide‐mode free‐electron laser. Measurements are carried out at microwave frequencies (25–40 GHz) in an X‐band rectangular waveguide with a low‐energy (450–600 keV, γ∼2), low‐current (∼1.5 A), low‐energy spread (ΔE/E≪10−4) relativistic photoelectron beam. The measured spontaneous emission spectrum agrees well with the homogeneously broadened spectrum of the TE10 waveguide mode. The measured small‐signal gain spectrum is in good agreement with the theoretical prediction calculated under the assumption of operation in the low‐gain Compton regime.

Development of cold electron beam sources for a Raman type free electron laser

Abstract Two types of very cold relativistic electron beam sources for Raman FEL experiments have been developed. Using one of them as an electron beam source for a Raman type FEL, intense electromagnetic radiation pulses of about 100 kW were obtained in the millimeter wavelength region with an energy conversion efficiency of about 2%.

Experimental studies on various characteristics of a waveguide mode free electron laser using cold relativistic electron beams

Abstract Periodic short pulse trains were observed in the waveguide mode free electron laser oscillations, which could be explained as self-mode-locked oscillations of the free electron laser. The spectra of the spontaneous emission and the induced emission were observed, and were in good agreement with theoretical calculations. The pulse train intervals were in good agreement with the theoretical calculation for the lower branch oscillation, though the oscillation was confirmed to be occurring on the upper branch.

Self mode-locked oscillation of a free electron laser

Abstract Self mode-locked oscillations were observed in a long pulse, low current, and low voltage wave guide mode free electron laser. The pulse width and the pulse train intervals of the mode-locked oscillation were about a few ns and about 23 ns, respectively. It was shown qualitatively that free electron lasers can have the mechanism of saturable absorption, which is essential for getting the self mode-locked oscillations, by controlling the spectrum of the electron beam.