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Description
We propose a mode generator cavity design capable of producing a higher-order rotating mode. This cavity generates a TE7,2 mode at 142 GHz and a TE9,3 mode at 208 GHz in a coaxial geometry, which is later used as an input mode for a launcher in an internal mode converter of a gyrotron. It consists of a perforated metallic cavity with a tapered inner rod to suppress the counter-rotating and competition modes [1][2]. We designed two different cavities to understand the effect of the beam illumination on the perforated wall. In the perforated area of the cavity, the radius and period of the hole were designed as close to half-wavelength to match external and internal fields. One of the designed cavities has a perforated wall area coverage of 36 degrees, and the other has an area of 360 degrees for each 142 GHz and 208 GHz.
For these cases, we designed a quasi-optical mirror that allows the incident Gaussian beam to be reflected with an equal optical path and focused on the cavity caustic [1]. Due to the varying caustic radius at different frequencies, it is necessary to design mirrors for each frequency. The entire design process was executed using the Surf3d simulation tool [3]. The overall cavity structure is depicted in Figure 1. A coaxial resonator consists of cutoff, excitation, and radiation sections. In the cutoff section, high-order modes are reflected while low-order modes are transmitted. In the excitation section, the primary mode, determined by the cavity and inner rod radius, is excited with a high Q-factor, leading to resonance. In the radiation section, some waves reflect for resonance while the rest are transmitted as guided waves [4]. The geometric exterior of the cavity was designed using the Numerov method, a numerical technique used to solve second-order linear differential equations. Furthermore, the information of the beam reflected by the mirrors was optimized for the cavity’s perforated wall and tapered inner rod using the CST tool with a time-domain solver. After conducting simulations, Scalar Correlation Factor (SCF) and Rotating Purity (RP) were calculated for each mode. For the TE7,2 mode, SCF was 97.59%, and RP was 94.76%, while for the TE9,3 mode, SCF was 98%, and RP was 98.49%. In this study, we present a mode generator cavity design meticulously engineered based on quasi-optical system simulations. The resulting design successfully produces high-purity modes at each frequency, demonstrating its potential for facilitating gyrotron experimentation and validation.
