Consideration on terahertz FEL using pre-bunched electrons shorter than the wavelength

Interaction between optical field and electron pulse, whose bunch length is much shorter than a wavelength (pre-bunched beam), has been studied by developing a simulation code based on 1-D FEL equations for oscillator configuration. Since coherent spontaneous (CSR) radiation dominates in early stage of the oscillation, we have not applied shot-noise electron population in an optical potential. Start-up of lasing is very much faster because of CSR, and the FEL interaction at head of the optical pulse becomes significant, so that the pulse width gets shorter. It is just the contrary of lethargy effect occurred at zero-detuning with the long electron pulse. In addition saturation mechanism seems to be different from that with the long electron pulse and saturated peak power is very much higher than that of conventional FELs. As a whole the FEL with pre-bunched beam is very much attractive for future light source particularly in terahertz (THz) wavelength region because of unique properties of the optical pulse evolution.