A linear accelerator bunch length monitor

Abstract

The Saskatchewan Accelerator Laboratory is a 300 MeV electron accelerator (Linac) and storage ring. Electrons are injected into the Linac in packets called bunches for acceleration by an electromagnetic field. The length of these bunches have a direct impact on the energy spread of the beam at the exit of the Linac. A large bunch length increases the background radiation in and around the Energy Compression System located at the end of the Linac and is an inefficient way of operating the accelerator. Bunch length is dependent on the relative phase of the microwave power used by the Linac components. A monitor has been developed that will measure the bunch length early in injection so that the relative phase of the microwave power can be adjusted for optimal bunching. The power available at two sufficiently separate harmonics in the frequency domain will give an indication of the bunch length in time. Simulations indicate the first and fourth harmonics fulfill this criteria. The bunch length monitor consists of a circular disk of copper 1 cm in diameter located 3 mm from the inside surface of the beam pipe wall. It also includes a power splitter and two bandpass filters, one at the Linac RF frequency of 2.856 GHz and the other at the fourth harmonic, 11.424 GHz. The bunch length monitor was tested in a number of trials. By varying the prebuncher phase, two minima in bunch length were observed; one minima due to the injector and the other to the first accelerating section. A reduction of 12 % in energy spread was observed and it is anticipated that with more thorough optimization, a reduction of up to 20 % could be realized.