High efficiency quasi-optical mode converters for overmoded gyrotrons

This thesis reports on the design and experimental verification of two novel high efficiency quasi-optical mode converters for gyrotron oscillators. Gyrotrons produce high order TEm,p modes (m, p > > 1) at high frequencies (100-300 GHz). These modes must be converted to low order linearly polarized modes for transmission in corrugated waveguides or open mirror transmission lines. Because traditional waveguide converters are inefficient and impractical for very high order modes, quasi-optical mode converter techniques must be developed. Both converters described in this thesis achieved 95% conversion efficiency from high order waveguide modes to Gaussian-like beams in free space, allowing them to be used in practical high power continuous wave (cw) gyrotrons. A Vlasov-type converter was designed to transform the TE1 6,2 cylindrical waveguide mode at 146 GHz to a Gaussian beam in free space. The converter consists of a helically cut waveguide launcher and a doubly curved focussing reflector. The preliminary design of the converter was made with the geometric optics approximation. A vector diffraction theory was developed to simulate the launch and propagation of the beam through the converter system and to modify and improve upon the design made with geometric optics theory. The analysis reported in this thesis was the first full diffraction theory calculation to predict the output of the Vlasov-type converter. The converter was built and tested on a MW power level, 3/us pulsed gyrotron operating in the TE16,2 mode at 146 GHz. Calorimetric power measurements and focal plane scans showed that 95% of the TE16,2 power leaving the gyrotron was directed to a Gaussian-like focus in the far field. Beam expansion measurements indicated that 88% of this power was contained in the TEMoo fundamental Gaussian mode, giving an overall conversion efficiency of 83%. The beam was then directed into a 1.25 in. diameter corrugated waveguide and calorimetric measurements showed an efficiency of 82.5% for converting the TE1 6,2 to the HEll mode of the corrugated guide. Experimental results were compared to predictions of vector diffraction theory and good agreement was obtained. The Vlasov-type converter also provided a novel method for determining the mode mix of the radiation leaving the gyrotron. A pre-bunching converter, which is a modified Vlasov-type, was designed to transform the TE22,6 cylindrical waveguide mode at 110 GHz to a Gaussian beam in free space. The converter consists of an irregular waveguide section followed by a step cut launching aperture and two focussing reflectors. The irregular waveguide section, which was designed with coupled mode theory, is used to bunch the radiation into Gaussian bundles prior to the launch. The reflectors that follow were designed with Gaussian optics and vector diffraction theory, and their surfaces are simple toroidal shapes. The converter was built and tested on a MW power level, 3s pulsed gyrotron operating in the TE22,6 mode at 110 GHz. Experiments showed that the TE2 2,6 power leaving the gyrotron was directed to a small, Gaussian-like focus in the far field. Beam expansion measurements indicated that virtually all the power in the beam was contained in the TEMOO mode. The beam was directed into a 1.25 inch corrugated waveguide and calorimetric efficiency measurements showed that 98% of the TE22,6 power leaving the gyrotron was coupled to the HEI1 mode of the corrugated guide. The experimental results were compared to predictions of vector diffraction theory and good agreement was obtained. Four additional reflectors were built to transform the fundamental Gaussian beam, produced by the launcher and first two reflectors, into two fundamental Gaussian beams of approximately equal power level. The surfaces of the beam splitting reflectors are simple sinusoidal and toroidal shapes, which are compact and easy to manufacture. Measurements showed that the mirror relay produced two Gaussian-like beams, one containing 52% of the power leaving the gyrotron and the other containing 42%, giving a total efficiency of 94%. The results show that simple sinusoidal and toroidal mirrors can be used to produce high quality Gaussian beams. The simplicity of the mirror surfaces greatly reduces the large computing times necessary for a full diffraction theory analysis of the system. The results presented in this thesis show great promise for the use of quasi-optical mode converters in high power cw gyrotrons. Thesis Supervisor: Dr. Richard J. Temkin Title: Senior Research Scientist, Physics Department Thesis Supervisor: Dr. Kenneth E. Kreischer Title: Principal Research Scientist Thesis Reader: Professor Hermann A. Haus Title: Professor of Electrical Engineering and Computer Science Thesis Reader: Professor Frederic R. Morgenthaler Title: Professor of Electrical Engineering and Computer Science