Design Study of a Fundamental Mode Input Coupler for a 372-GHz Gyro-TWA I: Rectangular-to-Circular Coupling Methods

Abstract

The design of two fundamental mode rectangularto-circular waveguide input couplers for a low-terahertz gyrotron-traveling wave amplifier (gyro-TWA) is presented. A T-junction input coupler with a Bragg reflector and a multiplehole directional coupler were optimized for operation between 360 and 384 GHz, the proposed gyro-TWA bandwidth. The T-junction coupler and the multiple-hole coupler achieved the respective bandwidths of 10% and 35%. The benefits and potential limitations of the low-terahertz wave coupler topologies are discussed alongside the challenging manufacturing methods of the submillimeter-wave components.Index Terms-Gyrotron-traveling wave amplifier (gyro-TWA), mode coupling, terahertz input coupler, waveguide coupler. Jason R. Garner received the B.Sc. (Hons.) degree in physics and the M.Sc. degree in high-power radio frequency science and engineering from the University of Strathclyde, Glasgow, U.K., in 2010 and 2012, respectively, where he is currently pursuing the Ph.D. degree with the Scottish University Physics Alliance.His current research interests include the design of passive components for millimeter-wave gyro-amplifiers.

Demonstration of a high power broadband mm-wave gyro-TWA

“…The microwave seed signal was coupled into the ultra-high vacuum system through a broadband (88-104 GHz), high transmission (-0.6 dB) pillbox microwave window. A sidewall coupler [6,7] with a T-junction structure [8] was used to convert the TE10 mode in rectangular waveguide into the TE11 mode in the circular waveguide. An optimized broadband Bragg reflector was used at one port of the T-junction to allow the electron beam to pass without interception and at the same time to minimize any millimetre-waves propagating into the electron gun region.…”

A multiple-hole input coupler for a 372 GHz gyro-travelling wave amplifier

“…The efficient coupling of the microwave radiation is important to the overall performance of the device, not only to reduce the cost of the high frequency source, but also minimize the possibility of oscillation in the system. A side-wall coupler [6] has been developed for a W-band gyro-TWA, and the simulated and measured results show that a good transmission coefficient can be achieved by using a rectangular-to-circular T-junction and a broadband reflector [7]. The W-band design can be scaled to higher frequencies, but the tolerance studies show that the bandwidth is sensitive to the dimensions of the rectangular hole.…”

Input coupling systems for mm-wave amplifiers

“…The machining tolerance and loss are two major challenges that need to be overcome. To avoid the machining difficulty of the broadband reflector, a multiple-hole input coupler was designed [10,11]. The multiple-hole coupler is a 4-port waveguide component.…”