Millimeter-wave Techniques: Theory, Algorithms and Methods

Abstract

The goal of book is to present the theory, algorithms and methods to design the millimeter-wave Gyrotron beam-shaping antenna system (of good metal conductor, e.g., of aluminum or copper) to shape four roughly Gaussian beams of different frequencies into one specified high-quality Gaussian beam at the output of the mirror system. The input beams are from a quasi-optical launcher which converts each of the following mode/frequency pairs into a roughly Gaussian form: TE21,6 (107.5 GHz), TE22,6 (110.0 GHz), TE24,7 (124.5 GHz) and TE25,7 (127.5 GHz). Each of the output beams from the launcher is to be converted into a Gaussian beam with specified waist radius and with as high a coupling coefficient as possible to an ideal Gaussian beam. A design optimization procedure for the mirror system is developed and applied to the case of the four mode/frequency pairs above. The frequencies are specified because they are of interest to General Atomics. The 127.5 GHz frequency is the ITER start-up frequency. In addition, the design obtained is tested by simulation for two other mode/frequency pairs, TE23,6 (112.9 GHz) and TE23,7 (121.5 GHz). The theory, algorithms and methods required in such multi-mode beam-shaping mirror system designs are investigated. These include (1) the iterative phase retrieval technique for phase retrieval from measured magnitude-only data; (2) a theoretical formula developed to evaluate the validity of the image theorem approximation in the cylindrical geometry used during the mirror system design; (3) the Taylor-FFT algorithm developed in this research work for fast computation of the electromagnetic wave propagation and scattering; (4) the newly developed phase gradient phase correction method for mirror surface correction; and (5) the least mean square optimization method, proposed for multi-mode mirror design. Based on our theory, algorithms and methods, a coupling coefficient to the target Gaussian beam greater than 99.99% is achieved for the single-mode designs and an average coupling coefficient up to 99.50% within the window aperture of 88 mm diameter for the multi-mode design. The design has been successfully verified the Method of Moments software Surf3D.