Internal mode converter mirror shaping from measured field intensity

Abstract

Summary form only given. The simulation and experimental results for a pair of gyrotron internal mode converter phase-correcting mirrors designed from intensity measurements of the feed system will be presented The feed system consists of a rippled-wall launcher that converts a 110 GHz, TE(22,6) rotating cylindrical waveguide mode into a quasi-Gaussian free-space beam and two toroidal mirrors that shape the beam. The field intensity of the beam after the second toroidal mirror was measured over several consecutive planes, and the phase of this beam was retrieved from the measurements to recover the full field structure. This reconstructed field was used to shape a pair of phase-correcting mirrors which transform the feed field into an ideal Gaussian beam suitable for transmission through a two-inch clear aperture diamond gyrotron window. An independent physical optics numerical electromagnetics code was used to propagate the feed field through the two-mirror system to the gyrotron window aperture, and the system produced the desired Gaussian beam amplitude and phase on the window, with 99.5% of the beam power passing through the aperture. Several new insights concerning the current design approach, derived from these simulations, will be presented. The shaped mirrors were manufactured from solid aluminum and installed on the mode converter in a cold test experiment. Measurements were made of the radiated field pattern before, on, and after the window plane, and the measured beam is indeed a Gaussian with parameters that match those of the theoretical design. A detailed analysis of these encouraging results will be presented.