The linear theory of generation of electromagnetic radiation in free electrons laser (FEL) with electron beam, which is formed by electrostatic accelerator (ECA) in term of process of forced coherent dispersion with use of magnitostatic field of undulator and quasi-optical resonator, is considered. Spectrums of oscillations of cylindrical resonator, ohmic losses in walls and mirror of resonator, diffraction losses in output split of resonator are determined. Dependences of increment and intial current from value of unbalancing between beam velocity and phase velocity of wave of ponderomotive potential are investigated. The influence of high-frequency space charge of electron beam on process of generation of oscillation is determined.