Coherent radiation from bunched electrons and prebunched FEL in the millimeter wavelength region

Synchrotron radiation (SR) emitted from a short-bunch beam of electrons of a linear accelerator is coherent in the millimeter wavelength region. When the wave packets of coherent SR are superposed on the subsequent bunches in an open resonator, the output has intense and quasi-continuous spectrum. The amplification of the radiation due to superposition of the radiation field is confirmed by experiment. The output spectrum is discussed on the basis of the phase of the resonant mode.