On a theory of an FEL amplifier with circular waveguide and

We consider an FEL amplifier with an axisymmetric electron beam, circular waveguide, helical undulator and guiding magnetic field. Such an FEL amplifier configuration possesses some peculiarities when it operates in a regime with the negative longitudinal mass (i.e. when μ−1∞dvzd¢ < 0). It is shown that in the presence of strong space charge fields, the “negative mass” instability significantly influences the FEL amplifier operation resulting in a significant increase in the FEL amplifier efficiency.