Hole coupling experiments on the Mark III FEL

Abstract

Hole coupling experiments were performed on the Mark III FEL at Duke University with an experimental configuration allowing access to both the hole coupled and intracavity optical beams. These experiments were performed with two different electron beam configurations. In the first configuration, maximizing optical power through the hole maximized the percent outcoupling. The intracavity optical eigenmode was comprised mainly of the TEM00 Gauss-Laguerre mode. In the second electron beam configuration, the maximum outcoupled power did not correspond to the maximum percent outcoupling. The intracavity mode was not azimuthally symmetric, nevertheless it enhanced the electronic gain and intracavity power. The maximum percent outcoupling was obtained with an azimuthally symmetric optical eigenmode with power concentrated in the TEM10 Gauss-Laguerre mode. Experimental outcoupling efficiencies of up to 65.5% were obtained; this is more than three times the 20.0% outcoupling efficiency routimenly measured in the Brewster coupled resonator. This paper presents experimental measurements of the outcoupled and total losses, the electronic gain and the mode structure of the hole coupled Mark III FEL at a variety of wavelengths and hole sizes.