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Description
We have designed a dual frequency gyrotron targeting frequencies of 142 GHz and 208 GHz. Prior to conducting a hot test on the gyrotron, it is essential to validate the output beam patterns of a designed mode converter launcher and a quasi-optic system with a low-power source. To achieve this, we designed mode generator cavity structures with perforated features to produce identical modes as the gyrotron cavity for each frequency. The key objective of the low-power test is to produce modes with high purity. The targeted modes are TE7,2 at 142 GHz and TE9,3 at 208 GHz.
Although the azimuthal index (m) for these higher-order modes is relatively small compared to other modes in previously manufactured mode generator cavities for MW gyrotrons [1][2], the alignment process for extracting high-purity modes in actual experiments is still considered to be challenging. This difficulty arises from the interaction between the incident beam and the mode generation cavity, with a radius of approximately 4 mm for both frequencies. In the alignment process, we used an automatic controller with very fine resolution, adjusting the position in increments of 0.1 mm to optimize the alignment. The actual output mode purity was influenced not only by misalignment issues but also by reflections in surrounding components and inner rod misalignment within the coaxial structure during the coupling process.
The experimental setup for the low-power test is depicted in Figure 1. In the low-power tests, we compared measured results with simulations, obtaining Scalar Correlation Factor (SCF) and Rotating Purity (RP) values for TE7,2 and TE9,3 modes. At 142 GHz, TE7,2 exhibited SCF = 96.41%, RP = 83.33%, while at 208 GHz, TE9,3 showed SCF = 92.92%, RP = 84.14%. After obtaining optimized mode purity, we connected the output end of the cavity to the launcher's input, measuring the beam pattern at the first mirror position of the quasi-optic system for single polarization. We compared these measurements with simulation results and observed SCF and Vector Correlation Factor (VCF). The results were 89.78%, 79.75% at 142 GHz and 90.99%, 82.79% at 208 GHz, respectively. Following the attachment of the mirror system to the next mode converter, we measured the output Gaussian beam patterns at various distances from the gyrotron's window position (136 mm from the last mirror). After attaching the mirror system to the next mode converter, we measured the output Gaussian beam patterns at various distances from the gyrotron's window position. The calculated SCF and VCF for the measured Gaussian beam patterns compared to an ideal Gaussian beam were as follows at 142 GHz, SCF = 97.71%, VCF = 81.71%, and at 208 GHz, SCF = 97.62%, VCF = 79.65%. Furthermore, we performed beam fitting using the measured data for beam radius at the actual window position. This allowed us to characterize the beam's operational features, including tilt angles along each axis. The beam radius calculated through fitting closely matched the measured beam radius at the window position. The calculated beam radius along the x-y axes were 10.62 mm, 10.34 mm at 142 GHz and 8.932 mm, 8.183 mm at 208 GHz, respectively. However, there were slight differences in the waist position and tilt angles. These variations are expected to originate from alignment errors in the overall experimental setup and assembly discrepancies in the manufactured components. These differences are anticipated to be verified through subsequent hot tests. In this work, we present a comprehensive low-power validation of a dual-frequency gyrotron mode converter launcher, utilizing a Vector Network Analyzer (VNA). Our focus is on the alignment process and comparing the actual beam characteristics to simulation results. The experimental results, obtained with the VNA, demonstrate high mode purity and accurate beam patterns at both 142 GHz and 208 GHz, paving the way for further investigation and optimization in upcoming hot tests.
