Normal conducting RF structure development for CLARA

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

Generation of ultra-short, high brightness relativistic electron bunches

IX+153hlm.;24c

A CW normal-conductive RF gun for free electron laser and energy recovery linac applications

Currently proposed energy recovery linac and high average power free electron laser projects require electron beam sources that can generate up to {approx} 1 nC bunch charges with less than 1 mmmrad normalized emittance at high repetition rates (greater than {approx} 1 MHz). Proposed sources are based around either high voltage DC or microwave RF guns, each with its particular set of technological limits and system complications. We propose an approach for a gun fully based on mature RF and mechanical technology that greatly diminishes many of such complications. The concepts for such a source as well as the present RF and mechanical design are described. Simulations that demonstrate the beam quality preservation and transport capability of an injector scheme based on such a gun are also presented

Low Energy Linacs for 3D X-ray Scanning Applications

Typical cargo scanning Linacs used to scan shipping containers are usually designed at S-band (2-4 GHz) frequencies and have energies of 3-6 MeV in order to obtain sufficient contrast during inspections. To scan smaller and thinner containers and to reduce the footprint of scanning systems, there is an interest in lower energy (1-3 MeV) and higher frequency (5-6 GHz) devices. In this thesis, a design of such a device is presented, with a compact (11.9 cm), five cell, 2 MeV, bi-periodic, C-band linac selected as the final design. Multi-objective optimisation methods and spline modelling techniques are used to optimise the cells with a Pareto analysis used to select a final design, also allowing for rapid design adjustment. A novel coupling method using nose cone slants is developed, giving an improvement of 28 % in the coupling factor between cells (0.7 % - 0.9 %), with minimal effect on the peak fields or shunt impedance (1.3 %). MO methods are also employed to optimise the cell lengths and RF amplitudes to increase the capture efficiency of the linac to over 90 % using re-capture methods. A complete thermal analysis is presented showing that the linac can handle up to 1.2 kW average power with less than 2 % error in the electric field. The study includes CFD simulations and an improved method for estimating the heat transfer coefficient by including bends when performing calculations which agrees with the CFD analysis. The design is then integrated into a full RF system that allows for three linacs to be fired and synchronised, using three frequencies with a 3 MHz separation (5.712 ± 3 MHZ) on three sections of one 10 µs RF pulse. It is then shown that this system is capable of generating quasi-3D images in a CT-like setup using 3D image reconstruction techniques