GENERATION OF COHERENT UNDULATOR RADIATION USING A RELATIVISTIC PHOTOELECTRON BEAM WITH PICOSECOND MICROPULSES

The coherent effect of undulator radiation is investigated using relat ivistic photoelectron beam (RPE) produced by the fourth harmonics of a Q-switched Nd-doped yttrium aluminum garnet (Nd-YAG) laser. The Q-swi tched laser pulse contains a burst of 30 ps micropulses superposed wit h a weak 8 ns pulse. The current and shape of the RPE were controlled by changing the incident laser intensity. The temporal structure of th e RPE was measured using the Cherenkov radiation emitted by relativist ic electrons impinged on an optical fiber. When the irradiated laser i ntensity is stronger than 5 MW/cm2, the wave form of the RPE does not show any microstructure, which is due to the saturation of the RPE cur rent density by space-charge effect. The measured radiation power from the RPE having no microstructure was near the noise level of a microw ave diode, even though the current was much larger than that of the RP E having micropulses. With irradiated intensity less than 1 MW/cm2, th e temporal structure of the RPE closely follows that of the laser puls e. The measured power of the undulator radiation generated by micropul ses was more than 10(3) times stronger than that of the theoretically predicted incoherent radiation and tends towards a quadratic dependenc e 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).