Terahertz mode converters using LIGA technique

The gyrotron, an ideal millimeter or terahertz source, requires mode converter. The mode converter is used to launch the desired wave to interact with the electron beam (e.g. traveling-wave amplifier) or to extract a wave power from the interaction region (e.g. backward-wave oscillator). A poor mode-converter, however, excites unwanted modes resulting in severe mode competition. In the previous study, Computer Numerical Control (CNC) lathe was employed. However, the machining error leads to a low yield rate at sub-terahertz region, which significantly hinders the progress of high-power terahertz gyrotrons. In this work, X-ray micro-machining (LIGA technique) is explored. LIGA technique is capable of fabricating devices with high precision, high aspect ratio, and low surface roughness. The prototype of the mode converter, TE₀₂ (190-220 GHz), is ready for the cold test.