• H. Hama, M. Yasuda
  Tohoku University, School of Scinece, Sendai

To evaluate fundamental characteristics of conventional FELs with oscillator configuration, we assume an electron pulse sufficiently longer than the FEL wavelength in general (long pulse approximation). In the process of microbunching, the optical pulse gains the power exponentially. Taking finite bunch length into account, head of the optical pulse does not grow than tail due to slippage. Consequently if there is no detuning for an optical cavity, the electrons are no longer able to be microbunched because of no power growth at head of the optical pulse, which is well known as lethargy effect. Meanwhile recent progress of the linac technology enables us to have high brilliant beam in 6-dimensional phase space, accordingly the electron bunch length reaches much shorter than the wavelength of THz radiation. We have studied THz FEL employing such pre-bunched beam by applying 1-D FEL equations*. Since bunching factor of the electrons is already close to unity, non-linear power amplification is expected to occur. Bunching factor is resulted from long pulse approximation basically, so that we have to discuss the pre-bunched FEL carefully. The paper will describe consideration on it.

*M. Yasuda et al., " DEVELOPMENT OF THz LIGHT SOURCE USING PRE-BUNCHED FEL", Proc. 30th Int. FEL Conf., Gyeongju, Korea (2008).