Influence of linear fluctuations on low- and high-gain Cherenkov FELs

In a previous study it was shown that irregularities in the liner result in phase fluctuations of the ponderomotive potential which affects the gain of a low energy Cerenkov FEL. Here we investigate numerically how sensitive a Cerenkov FEL is to such fluctuations when operating in different gain regimes. Imperfections in the dielectric liner of a Cherenkov Free-Electron Laser (CFEL) result in fluctuations in the phase velocity of a radiation wave when it propagates through the lined waveguide. Random fluctuations in the phase velocity reduce the bunching of the electrons and consequently lower the gain of CFELs. Here we theoretically investigate the influence of these liner-induced phase fluctuations in the radiation field on the saturated power of low to high gain CFELs. To obtain different gain regimes, we keep the electron beam radius constant and vary the current density. As an example, we study a 50 GHz CFEL and quantify the reduction in the single-pass saturated power for different rms liner fluctuations when the CFEL is driven by an electron beam with current densities varying from 1 A/cm2 (average gain of 0.43 dB/cm) to 25 A/cm2 (average gain of 1.39 dB/cm).