Normal conducting RF structure development for CLARA

Cowie, Louise and Burt, Graeme (2022) Normal conducting RF structure development for CLARA. PhD thesis, Lancaster University.

Text (2022CowiePhD)
2022cowiephd.pdf - Published Version
Download (29MB)

Abstract

This thesis covers three RF structures that form part of the linear accelerator for the CLARA Free Electron Laser test facility project. The first structure is the RF photoinjector. The design, tuning, low power testing and work towards the RF conditioning of the structure are presented. The design includes a novel coupler and automated parametric optimisation of the cavity structure. The tuning was performed via trimming the length of each cell before brazing, and the chapter presents pre- and post-braze RF measurements. The RF conditioning chapter includes the design of a program for unmanned RF conditioning of not only the photoinjector but all CLARA RF structures. The second structure is the first CLARA linac. It is a 2 m travelling wave structure, and the RF envelope evolves as it travels through the structure due to RF dispersion. A method to predict the pulse evolution using Fourier methods is presented. The method requires two inputs to calculate the dispersion: the group velocity as a function of cell number, and the phase advance. Attenuation is included in the model with the addition of a third input: the cavity Q0. The model is tested on the first CLARA linac and shows good agreement with measurements of the RF pulse. The model can also be used to predict the beam momentum and this too shows good agreement with measurements from CLARA. The third structure is the CLARA transverse deflecting cavity. This is a dipole mode structure that is part of a diagnostic system to measure the longitudinal characteristics of the electron bunch. The structure will require field-profile tuning after fabrication. Tuning requires a method to take RF measurements and find the relative detuning of each cell. Two types of tuning method are presented, a commonly used perturbation theory based method and a novel method that finds the pseudoinverse of a matrix of measured values. Two variations of each method type are tested using 3D RF simulation results and the results compared.