Numerical Investigations into Higher Order Axial Modes of Terahertz Gyrotron Resonator with Significantly Up-Tapered Midsection

Abstract

This article presents the effect of significant up-tapering of the resonator midsection on diffractive quality factor, resonant frequency, axial field profile shift, and axial separation between peaks of higher order axial modes (HOAMs) in a gyrotron resonator. Significant up-tapering of the resonator midsection causes the quality factor curves of HOAMs to come close to each other and eventually crossover. This allows HOAMs to have comparable or even higher quality factor value than the lower axial modes—a situation which is not possible in a conventional resonator. Due to this, the operating current for all axial modes can keep an order of magnitude. The up-tapering also shifts the axial field profile toward the output section of the resonator and leads to the loss of spatial symmetry that exists in the axial separation between the peaks of HOAMs in a conventional resonator. Computations have been performed for 0.2-, 0.5-, and 1-THz resonator using single-mode cold cavity solver code developed at CSIR-CEERI. Numerical validation with published results, for 0.2- and 0.5-THz conventional resonators, has also been carried out.