New slow wave structures for travelling wave tubes

• Main Author: Hazell, Jonathan
• Other Authors: Lucyszyn, Stepan
• Published: Imperial College London 2017
• Subjects: 621.3
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.745292

This thesis covers the design of slow wave structures for travelling wave tubes, with a specific focus on those that could be used for operation at millimetre or shorter wavelengths. Serpentine and a coupled cavity photonic crystal structure are covered in detail, together with the interaction between the electromagnetic waves they support and the electron gun and magnetic beam focusing systems needed for a travelling wave interaction. In Chapter 2, the existing small-signal theory of the travelling wave interaction is introduced and applied to a serpentine travelling wave tube. A set of synthesis equations for the serpentine structure are then derived from the analysis and verified with simulation. In Chapter 3, possible improvements to the serpentine structure for high frequency operation, and operation on harmonics other than the fundamental (for both the phase and the interaction impedance) are considered. From the investigation it can be concluded that higher harmonics allow a larger beam current than the fundamental. In Chapter 4, slow wave structures based on photonic crystals are proposed for use in travelling wave tubes. A specific photonic crystal arrangement – the coupled resonator optical waveguide (CROW) - that does not appear to have been studied previously in this application is then investigated. The conclusion is that a CROW is suitable for use in a travelling wave tube and is significantly more manufacturable than existing approaches. In Chapter 5, the design of a full electron beam system for use with both the original and the improved slow wave structures is presented. The design of an electron gun, cathode and collimating magnet using an immersed flow insertion are all covered in detail. In Chapter 6, conclusions are drawn and avenues for possible future work are presented.