FEL gain as a function of phase displacement induced by undulator intersection gaps

FEL gain characteristics are considered for the FEL construction based on a multisection undulator with small intersection gaps. It is shown that induced by these gaps phase displacements between optical wave and electrons can noticeably change the gain resonance frequency as well as the gain curve shape. The effect can be used for fast resonance frequency tuning of the FEL undulator and for its step tapering.