LF-mode excitation in FEL caused by stimulated scattering of operating HF-mode

Conditions for resonant excitation of the parasitic LF-mode in the presence of the HF-mode operation, are analyzed. It is shown that due to the effect of stimulated scattering of the waves the HF-mode generation can lead to parasitic excitation even in the cases, when the operating electron current is significantly smaller than the starting current for the LF-mode in the absence of the HF-mode.