MAXIMIZATION OF FEL GAIN FOR A HOLE-COUPLED RESONATOR

For a hole-coupled resonant cavity without FEL gain, it is possible to have amplitude cancellation of the optical Gaussian modes at the hole location and, as a result, a null is generated to give very low cavit y loss due to the presence of the hole. However, in a warm cavity, i.e . when the electron beam is present and the optical power starts to bu ild up, the FEL gain tends to amplify the modes unequally, and the unb alanced gain for different optical modes results in significant power radiated through the hole. In this paper we present a guideline, based upon variation of the Rayleigh length, for mode selection that maximi zes net gain.