Stepped Sub-THz Gyrotron Cavity with Improved Output Mode Purity

Stepped cavity resonators are used in terahertz gyrotrons for better mode selection. They can satisfy the resonance condition for two modes with different radial indices. Such sectioned resonators, or complex cavities, were analyzed in detail by several authors in the past. One common problem for such designs is the excitation of unwanted modes by both the stepped cavity structure and the cavity output taper. Mode conversion becomes especially a severe problem for higher order resonant modes in highly oversized resonators. We introduce ways to minimize the unwanted mode conversion in both, the stepped cavity as well as in the output taper.