  The wakefields of a relativistic electron beam passing through a waveguide
loaded with an active medium with weak resonant dispersion have been
considered. For the calculations in this paper the parameters of the medium are
those of a solution of fullerene (C60) in a nematic liquid crystal that
exhibits activity in the X-band. It was shown that several of the TM
accelerating modes can be amplified for the geometries under consideration;
structures in which higher order modes are amplified exhibit essential
advantages as PASERs. In particular, the amplification of the highest mode
occurs in a structure loaded with a rather thick active medium layer that
maximizes the energy stored by the active medium.
