The Cerenkov free electron laser
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.