INFLUENCE OF MULTIPLE ELECTRON-SCATTERING ON THE GAIN IN A GAS-LOADEDFREE-ELECTRON LASER

We present an analysis of the small-signal gain in the gas-loaded free -electron laser. Multiple scattering of electrons by the atoms of the gas inside the optical cavity is shown to lead to two additional effec ts, as compared to the case of a vacuum free-electron laser: a loss of coherence between different parts of the electron trajectory and an e nhancement of the phase ''jitter.'' Both effects become increasingly i mportant at short wavelengths and significantly reduce the small-signa l gain per pass. In one-dimensional approximation, analytical expressi ons are obtained and numerical calculations are made to estimate the b eam and undulator parameters necessary for lasing in the vacuum ultrav iolet.