Mode selectivity in one-dimensional co-axial Bragg structures for a high power free electron maser

Abstract

One-dimensional (1D) Bragg structures can be used in high power microwave sources to provide 1D-distributed feedback via back-scattering of incident waves in devices such as free electron masers (FEMs). The scattering occurs due to coupling of the wave on a shallow 1D corrugation on the surface of a waveguide. Analytical theory was used to investigate mode selectivity in co-axial waveguides that have 1D corrugations on the central and outer conductor. Coupled wave theory was used to predict the level of coupling between the forward and backward waves of possible mode transitions. In multimode co-axial structures with uniform 1D Bragg corrugations multiple frequency reflection bands associated with scattering of different modes have been observed in the operating frequency band of a FEM currently under investigation at the University of Strathclyde. To overcome the problem of multiple reflection of waves over the operating region of the FEM it was found that it is possible to suppress reflection bands at certain frequencies and hence obtain a single reflecting band by shifting the relative phase between the corrugations of the inner and outer conductor. Computational modelling of the co-axial 1D Bragg structures has also been performed using the 3D Particle-In-Cell code MAGIC. This code was used to calculate the transmission (reflection) bands of the structures studied and verify the results obtained from the coupled wave theory. The Bragg structures considered can be used in FEM experiments to provide a single mode reflector in a specific frequency band. We report the first results of an experimental study of a high-current accelerator designed to satisfy the requirements for a high power microwave (HPM) free electron maser. A pulsed high voltage power supply, transmission line, the diode and the guide magnet system were designed and constructed. The parameters were chosen in order to produce ~100 MWs of output radiation in the Ka (26.5-40 GHz) frequency band. Experimental study of the free electron maser (FEM) is currently under progress and the results of electron beam formation experiments are presented. (4 pages)