Hole-Coupled Free Electron Lasers: a Transverse Mode Analysis.

In Free Electron Lasers (FELs) hole-coupling offers two distinct advantages over other outcoupling methods. First, holes provide a broadband and highly efficient means of outcoupling intracavity radiation that is non-dispersive and independent of the polarization of the intracavity radiation. Second, holes can provide a means to manipulate the transverse optical mode profile to increase the electronic gain and laser power. In this dissertation, we use both computer simulations and experiments to investigate FEL resonators with on-axis holes. We develop a matrix formalism based on the expansion of the optical transverse mode structure in free space modes. Simulations investigate the dependency of laser output coupling efficiency, transverse mode structure, gain and losses on hole size and resonator parameters. The simulation results suggest the possibility of hole-coupling in a non-phase degenerate resonator, and the possibility of mode control in a phase degenerate resonator. We designed and installed a hole-coupled resonator on the Mark III infrared FEL at Duke University. The experimental configuration is unique because it provides access to the intracavity mode enabling us to view directly the transverse mode profile. Results for two hole sizes and two electron beam configurations are presented and compared to the predictions of computer simulations. The computer simulations predict that, in the Mark III resonator, the optical mode profile will not be greatly affected by the presence of the hole. Experiments were performed for two different electron configurations. For the first configuration, the assumptions of the theory were satisfied and the experimental and theoretical results agreed. Unexpectedly, in the second configuration, maximum gain occurred when the electron beam axis and optic axis of the resonator did not coincide. In this case, the intracavity mode had a surprising and unusual transverse mode profile.