Vacuum electronics have often been referred to as the birth of modern electronics and have been extensively used in applications such as satellite communication due to their significantly higher output power in comparison to solid state amplifiers. While solid state amplifiers are used widely in electronics, recent years have witnessed a resurgence of interest among researchers to develop vacuum electronics power sources. In particular, travelling wave tube amplifiers (TWTs) are being developed for the emerging applications of the millimetre wave (30-300 GHz) and sub-THz frequency (300 GHz -1THz) range where they are currently the only viable technology to access high output
power, robustness, and minimal losses. These developments hold potential from ultrafast wireless networks (5G/6G), where for instance power is needed to overcome the high atmospheric attenuation at these frequencies. To scale these devices down at the sizes imposed by operation above 70 GHz, and meet the requirements of these novel applications in terms of both performance and costs, new solutions are required for the electromagnetic design of the main component of the TWT, the slow wave structure (SWS). This thesis focuses on tackling some of the challenges of the TWT design in the upper millimetre wave spectrum by investigating the use of gap waveguide technology to realise the SWS. Gap waveguides have recently found significant application for the
control of electromagnetic propagation at millimetre wave, where they can offer a lowloss solution versus microstrip circuit technology, within structures significantly easier to fabricate in comparison with conventional metal waveguides. Additionally, they can provide flexible designs, with inherent filtering capabilities that have been previously exploited in SWS design but in a simpler, easy to assemble topology. This work mainly investigates pin-based gap waveguides, resulting in two proposed structures: the full pin gap waveguide SWS (FPGW-SWS) and half height gap waveguide SWS (HHGW-SWS).
Numerical simulations predict suitable dispersion characteristics and interaction
impedance for wideband operation and a flexible SWS design. The FPGW-SWS suggests an operation bandwidth ranging from 87 to 100 GHz based on cold simulations. Due to the additional advantages offered by the HHGW-SWS, including reduced aspect ratio pins and a compact coupler design suitable for very high aspect ratio sheet beams, this structure was chosen for detailed particle-wave interaction simulations and for experimental verification of the cold characteristics. A minimum gain of 25 dB is achieved in a single section TWT with a 10 GHz 3dB-bandwidth centred at 94.5 GHz, and a saturated output power of 160 W. The results from the test of the fabricated prototype of the SWS and couplers match the results obtained from simulations. An alternative
approach to the design of the gap waveguide based on the holey glide symmetric gap waveguide is also investigated for comparison. It was found that this topology may be more suitable for moderate power, cost-effective applications.