An improved design for quasi-optical mode conversion of whispering gallery mode gyrotron radiation

Abstract

Results are reported of a theoretical and experimental investigation of a quasi-optical mode converter for the transformation of whispering gallery mode gyrotron output into a Gaussian like beam. The mode converter consists of a helically cut waveguide launcher, similar to that originally proposed by Vlasov et al, followed by a focusing mirror. Theoretical results using aperture field methods indicate that the length of the waveguide launcher is of critical importance in providing a confined radiation pattern. Experimental results on the radiation pattern were obtained for several launcher lengths using a 0.6 MW, 149 GHz pulsed gyrotron operating in the TE 1 6 , 2 mode. Radiation pattern results for the best launcher length agree well with theoretical calculations using the Stratton-Chu aperture radiation theory for unperturbed waveguide modes. A mirror focusing in the azimuthal direction was designed by a geometrical optics approach to focus the radiation coming from the launcher.Good focusing with 91.4% efficiency (power in the focused beam divided by gyrotron power) was found experimentally using the combined launcher and mirror with the pulsed gyrotron. These results indicate that quasi-optical antennas are useful for transforming high order, high frequency gyrotron modes into directed beams in free space.

The design of a Vlasov mode converter for 94GHz TE<inf>02</inf> mode gyrotron

“…In Fig.1, the circular waveguide radius is a , and radiation from the half-cylinder waveguide can be regarded as being composed of plane waves with the pitch angle(Brillouin angle) Rays from the Vlasov launcher also require focusing and profile shaping by reflectors before it is suitable for transmission as a Gaussian-like beam [5]. The elliptical reflector and the parabolic reflector, shown in Fig.2, are designed for focusing wave rays in both x-direction and zdirection.…”