Analysis and design of a quasi-optical mode converter for a 1-kW, 550-GHz, TE_15,2-mode gyrotron

A simple method is presented for the analysis and design of gyrotron output couplers based on a Vlasov-type launcher and a single focusing mirror. It is shown that a quasi-analytical solution to the Kirchhoff diffraction integral can be implemented to accurately model the diffraction spreading of the launched radiation with good accuracy relative to an Electric Field Integral Equation simulation. Spectral information is exploited in the design to appropriately size the mirror and to predict the conversion efficiency and Gaussian beam parameters of the converted beam. Additionally, a set of design equations, which is more general than any previously published, is derived for the reflector. A comparison of the conversion properties of a converter designed using the new equations and previously published equations is provided. For the case considered, the converter designed using the new equations generate a field distribution with almost complete isolation of the main radiation lobe at the window plane. For the reflector designed using the previous equations, the contamination of the main lobe by the side lobes is significant beginning at the 9-dB contour. The spot size of the gyrotron output beam has also been reduced by about 30% by using the new design equations.