Simulations and components for novel fast wave microwave amplifiers and sources

• Main Author: Matheson, Kathleen
• Published: University of Strathclyde 2014
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.635538

A three dimensional parameterised model of an X-band 2nd harmonic gyro-travelling wave amplifier (gyro-TWA) with a helically corrugated interaction region has been created and optimised in the Particle-in-Cell code MAGIC-3D, to achieve an output power and saturated efficiency of ~1.0MW and ~27% respectively at 9.4GHz. This numerical model has been benchmarked to an experiment [Bratman, 2000] which demonstrated an output power and saturated efficiency of ~1.1MW and ~29% respectively at 9.4GHz, for similar input parameters. The numerical model has been coded in the Cartesian co-ordinate system which offers greater numerical stability over previous models, and has been shown to accurately and consistently reproduce results comparable to the experimental measurements. The good agreement between the simulation data and the experimental measurements naturally present the numerical model as a suitable benchmark tool to investigate potential efficiency and bandwidth enhancement of the amplifier, achieved through parameter profiling of the microwave circuit. The model predicts that a helical down taper of length 14cm to an output mean radius (r0) and corrugation amplitude (l) of ~11.3mm and ~1.8mm respectively i.e. ~80% of the original helical waveguide's r0 and l values, positioned 4cm before the end of the original uniform helical interaction region of the amplifier, could increase both the saturated efficiency of the amplifier by ~2.5% at 10.0GHz, from ~28.6% to ~31.1% and its bandwidth by 800MHz, from 1.8GHz to 2.6GHz. In addition, an X-band Marie-type mode converter has been simulated and fabricated which effectively converts from the fundamental mode in rectangular waveguide to the cylindrical TE01 mode with minimal reflections, over an optimised 2.0GHz bandwidth. This converter has been used to test a Penning cathode mesh with the experimental measurements confirming that the mesh transmitted an RF signal in the TE01 mode without reflection or mode conversion.