94 GHz overmoded TWT experiment

Summary form only given. A 94 GHz overmoded travelling wave tube (TWT) amplifier has been designed and experimentally implemented. The overmoded coupled cavity TWT operates in the TM₃₁ mode at 94 GHz. Lower order modes that may oscillate at lower frequencies in the cavity are suppressed using selectively placed lossy dielectric loading. The AlN dielectric loading has a negligible affect on the transmission of the TM31 mode. Operation in a higher order mode allows for a larger cavity at 94 GHz than the equivalent fundamental mode structure, permitting a larger beam tunnel and a higher current to propagate through the TWT. These advantages lead to an increase in the gain and peak power of the TWT. The overmoded TWT is designed for pulsed operation with a 30 kV, 320 mA Pierce electron gun and 2.5 kG solenoid magnet. The design has been simulated in 3-D CST Particle Studio and predicts 32 dB of gain and 300 W peak power. The design and implementation of the overmoded cavity has been verified with several cold test structures manufactured with standard CNC milling using Glidcop, a copper alloy with a small quantity of alumina. Low power S-measurements show good agreement with theory (HFSS) for the 94 GHz TM₃₁ mode and TM₄₁ mode at approximately 112 GHz. In addition, AlN composite dielectric loading with a loss tangent of 0.25 at 90 GHz was used to induce losses in the unwanted modes. The AlN dielectric, when placed around the cavity structure induced large losses in the unwanted modes and minimal losses in the TM₃₁ mode. The electron gun has been activated and a beam test in underway to verify operation of the experimental set-up for the TWT. The Pierce electron gun was designed in Michelle for use with the experiment and 2.5 kG solenoid magnet. Initial experimental results for electron gun operation agree well with theory and show very good beam transmission to the collector. Once the operation of the electron gun has been ve- ified, the overmoded TWT will be installed in the experimental set-up and stable operation at 94 GHz will be verified.