The Cerenkov free electron laser

Petichakis, Christos. (2003) The Cerenkov free electron laser. PhD thesis, University of Liverpool.

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Abstract

This thesis reports on an investigation into Cerenkov Free Electron Lasers. These devices are basically travelling wave tubes but having a dielectrically lined cylinder as the slow wave structure rather than a helix. If an electron beam is injected into the centre of this structure, an interaction between the electrons and the electromagnetic (e-m) TMo I mode can occur which can lead to amplification of the e-m wave. Two different systems have been constructed. The first one was designed to operate as an oscillator at 12.4GHz and used a rectangular X-band waveguide microwave coupler. It was thought that the non-operation of this device could have been due to a lack of net gain, and so a second system was designed having a smaller diameter dielectric liner in order to achieve higher gain but at a slightly higher frequency of operation (l6.9GHz). In both systems, the interception of the electron beam with the dielectric liner was small. Unfortunately, even though a maximum electron beam current of 120mA was achieved, leading to an expected small signal gain of 1200%, no microwave output was observed either. At this stage it was considered that there must he something more fundamental at fault with these systems. After a thorough investigation. it was discovered that the small gap which always exists between the dielectric liner and the waveguide affected the dispersion relation of a Cerenkov system. Theoretically, gaps as small as 1 % of the diameter of the waveguide were found to have a serious effect, and although these gaps would not stop the operation of the Cerenkov device, microwave output would only be expected at a voltage far from that expected. It was found that the problem could be overcome by coating the outer surface of the dielectric tube with a layer of conducting material, such as silver paint, which effectively removes the gap. Further tests of a Cerenkov free electron laser with this improvement are in progress.

Item Type:	Thesis (PhD)
Depositing User:	Symplectic Admin
Date Deposited:	20 Oct 2023 15:43
Last Modified:	20 Oct 2023 15:56
DOI:	10.17638/03175044
Copyright Statement:	Copyright © and Moral Rights for this thesis and any accompanying data (where applicable) are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge.
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3175044