Characterising Dielectric-Lined Waveguides used for the Manipulation of Relativistic Electron Beams

Abstract

In this thesis a characterisation of rectangular dielectric-lined waveguide used in terahertz (THz) driven manipulation of a relativistic electron beam is presented. THz driven acceleration has gained interest due to its capability of surpassing the acceleration gradient of conventional radio frequency accelerators. Dielectric-lined waveguides (DLW) allow the interaction of the THz electric field with the electron bunch. Thus, a good characterisation of the DLW, involving study of its dimensions and performance is essential for THz driven electron manipulation. Three waveguides were measured in this thesis by microscopy. From their physical dimensions, one of them was selected for an electron acceleration experiment and so further characterisation was performed, obtaining its internal surface profile using white-light interferometry. The performance of the DLW was analysed by time-domain spectroscopy observing a dispersed waveform in time. The propagation constant was also obtained and so the phase velocity as a function of frequency. The frequency corresponding to phase velocity matching the electron velocity was 0.22 THz, different than the 0.39 THz estimated by simulations for acceleration mode. Time-frequency analysis was also performed where deflection and acceleration modes coexisted. Although the aim for this DLW was acceleration, the deflection mode was clearly predominant due to a misalignment on the y-axis, which could explain the mismatch between experimental and expected frequencies. In view of these results, the study of frequency matching the electron velocity was repeated, obtaining a frequency of 0.4 THz, much closer to the 0.39 THz expected. Since the main discovery of this thesis is the realisation of the sensitivity of the waveguide mode to misalignments, further work on the evolution of DLW modes with respect to the DLW alignment would be beneficial for future waveguide characterisation and for future THz driven electron beam manipulation experiments.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Darren Graham (Supervisor) & Guoxing Xia (Supervisor)