Technical Papers and Presentations

High-power THz TWTs of frequencies between 0.1THz to 0.3THz are being investigated for their use as amplifiers in ultra-broadband wireless communication systems, because of their high gain, high efficiency, wide bandwidth and high linearity. They have many other new emerging applications in medical imaging and sensing, spectroscopy, high-resolution radar, etc. as high-power THz sources. THz TWT is a very complex vacuum microelectronic device with many critical components such as electron gun for beam formation, periodic-permanent-magnet focusing system for electron beam focusing, RF slow-wave structure (SWS) for efficient energy extraction from the electron beam to the RF circuit field, input and output RF couplers, multi-stage depressed collector for collection of spent beam with minimum energy loss, and efficient packaging considering thermal dissipation and EMI/EMC aspects.

The RF performance of a TWT primarily depends on the design of its SWS as it supports THz waves which get amplified by extracting kinetic energy from the electron beam under the beam-wave synchronous condition. The SWS is therefore designed for high circuit impedance, wide circuit bandwidth, low circuit loss and minimum interference of higher modes. The complete SWS for a THz TWT consists of many periods (minimum 100) for 30dB gain. The SWS of a TWT is preferably designed in 2 sections with sever (attenuator) in-between for stability in tube performance against any reflections due to mismatch. Also, the structure has a phase velocity taper in the output for high electronic efficiency. The RF input coupler and the RF output coupler of the SWS are designed for good matching with VSWR much less than 1.5:1 over the desired operating band.

At THz frequencies, planar RF structures with sheet electron beams are preferred for high-power THz TWTs because of the significant advantages of sheet-electron beam over the cylindrical beam and planar RF SWS over the cylindrical structure. The present paper presents the beam-wave interaction analysis for planar THz TWT with sheet beam. COMSOL-3D simulation code was used for beam-wave interaction analysis of THz TWT [1] and to determine cold circuit parameters like dispersion and impedance characteristics over the desired frequency band. The simulated results on the RF output power and gain by COMSOL-3D simulation code were reported as comparable with the CST-3D simulated results over the frequency band.

In-house developed SUNRAY-SSM code [2] for small-signal (linear) beam-wave interaction analysis and SUNRAY-LSM code [3,4] for large signal (nonlinear) beam-wave interaction analysis of TWTs, are used to simulate the RF performance of a planar 0.22THz, 100W, 30dB gain TWT with sheet beam. The RF output power and gain as simulated by SUNRAY codes for the 0.22THz TWT are found comparable with the results as simulated by the CST-3D code. Efforts will be made to compare these results on dispersion and impedance characteristics of SWS along with the RF output power and gain for the 0.22THz TWT with the COMSOL-3D software simulated results over the desired THz frequency band if it may be accessible.

References [1] LR Billa, MN Akram, X Chen, “COMSOL Multiphysics Modeling and Simulation of Traveling Wave Tube Amplifiers”, IEEE UK- Europe-China Workshop on Millimeter Waves & THz Technologies, Sept 2016, pp.1-3, DOI: 10.1109/UCMMT.2016.7874022. [2] Vishnu Srivastava, SN Joshi, “Small Signal Model for TWTs considering effects of severs, attenuators & velocity tapers”, IETE Journal of Research, vol.39, pp.331-338, 1993; http://DoI.org/10.1080/03772063.1993.11437143. [3] Vishnu Srivastava, “SUNRAY-1D and SUNRAY-2.5D codes for large signal analysis of TWT,” Proceedings of the IEEE International Vacuum Electronics Conference, 2013, Paris. [4] Vishnu Srivastava, “Nonlinear analysis of beam-wave interaction in a planar THz travelling-wave tube amplifier,” Journal of Electromagnetic Waves and Applications, U.K., vol.32, no. 2, pp.190-203, 2018.