Coherent effect in undulator radiation from a waveguide-mode free-electron laser

Abstract

The coherent effect of undulator radiation has been investigated using a low-energy, low-gain, waveguide-mode Free-Electron Laser (FEL). Using a relativistic photoelectron beam produced by a Q-switched Nd-YAG laser beam, the coherent undulator raidation was observed and its measured power was $10^3$ times stronger than that of theoretically predicted incoherent radiation. For further increase of the radiation power, an ultra-short and high current photoelectron beam was generated by 0.5 ps UV excimer laser pulse and the emitted radiation power reached $10^6$ times that of incoherent radiation. These results show that the high power coherent undulator raidation can be generated when the duration of RPE is comparable to or shorter than the wavelength of the undulator radiation. Analytic consideration of the undulator radiation power shows the possibility of the coherent radiation, which scales as square of the electron number in a bunch. The generation of the coherent undulator radiation was achieved using a RPE produced by the fourth harmonics of a Q-switched NdYAG laser. The Q-switched laser pulse contains a burst of 30 ps micropulses superposed with a weak 8 ns pulse. The current and shape of the RPE was controlled by changing the incident laser intensity. The temporal structure of the RPE was measured using the Cherenkov radiation emitted by relativistic electrons impinged on an optical fiber. When the irradiated laser intensity is stronger than 5 MW/$cm^2$, the wave formm2 of the RPE does not show any microstructure, which is due to the saturation of the RPE current density by the space-charge effect. The measured radiation power from the RPE having no microstructure was near the noise level of a microwave diode, even though the current was much larger than that of the RPE having micropulses. With irradiated intensity less than 1 MW/$cm^2$, the temporal structure of the RPE closely follows that of the laser pulse. The measured power of the undulator radiatio...