Generation of coherent undulator radiation using a relativistic photoelectron beam with picosecond micropulses

The coherent effect of undulator radiation is investigated using relativistic photoelectron beam (RPE) produced by the fourth harmonics of a Q-switched Nd-doped yttrium aluminum garnet (Nd-YAG) 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 were 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/${\mathrm{cm}}^2$, the wave form of the RPE does not show any microstructure, which is due to the saturation of the RPE current density by 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/${\mathrm{cm}}^2$, the temporal structure of the RPE closely follows that of the laser pulse. The measured power of the undulator radiation generated by micropulses was more than ${10}^3$ times stronger than that of the theoretically predicted incoherent radiation and tends towards a quadratic dependence on the electron-beam current. The enhancement and tendency is due to coherent radiation emitted by electron-beam micropulses whose typical bunch lengths are comparable to the radiation wavelength (7.4 mm).