This thesis discusses the design and test of an overmoded W-band Traveling Wave Tube (TWT). The TWT was designed to operate in the rectangular TM31 cavity mode at 94 GHz. The unwanted lower order modes, TM11 and TM21, were suppressed using selectively placed Aluminum Nitride dielectric loading; simulations in 3-D CST Particle Studio confirmed the suppression. The TWT was designed to operate at 30 kV with 320 mA in a 2.5 kG solenoid magnet. Simulations in both 1-D Latte and 3-D CST predicted 31 dB of gain, 200 MHz bandwidth, and 300 W peak output power at 94 GHz. Test structures of 9- and 19- cavities were made via CNC direct machining. Cold test measurements showed suppression of the unwanted modes and transmission of the TM31 mode, which correlated well with HFSS simulations. Two final 87-cavity structures were built and cold tested.
The experiment was designed and built in-house at MIT. It was operated with a 3 microsecond pulsed power supply. An initial beam test was implemented which confirmed operation of the TWT at 31 kV with 306±6 mA of current detected at the collector and 88 % transmission of current. Operation of the TWT with the first structure showed 8 dB of system gain and 10 W saturated output power at 95.5 GHz. Following these first tests, the magnetic field alignment was improved and the second structure, which showed better circuit transmission in cold test, was installed. The overmoded TWT produced 21±2 dB system gain (defined as Pout/Pin) at 94.3 GHz and 27 W of saturated output power in zero-drive stable operation. The TWT was estimated to have about 6 dB of additional loss due to coupling into and out of the circuit. Taking that loss into account, the gain on the TWT circuit itself was estimated to be 27±2 dB circuit gain. CST simulations for the experimental current and voltage predict 28 dB circuit gain, in good agreement with measurements.
This experiment demonstrated the first successful operation of an overmoded TWT. The overmoded TWT is a promising approach to high power TWT operation at W- Band and to the extension of the TWT to terahertz frequencies.
Thesis Supervisor: Richard J. Temkin, PSFC