Mode converters for the RF cold test of a 250 GHz CARM cavity

Two configurations of mode converters from the TE10 in rectangular waveguide to the TE82 in circular waveguide are designed. Such conversion is required to perform the cold test of a Bragg resonator for a 250 GHz Cyclotron Auto-Resonance Maser under development at the ENEA research center. The cylindrical cavity to be tested is far more oversized than any previous CARM experiments, making the design of mode converters particularly challenging. One conversion chain uses an in-line coupling beat-wave TE11/TE81 converter, whereas the other is a sidewall coupling mode converter. Both require a final, highly oversized, step-type converter from the TE81 to the TE82 mode; its performances are computed with both a commercial software and an in-house mode matching code, whose results agree fairly well. For both configurations, simulation results are presented and the most promising solution is identified.