Optimisation of the electron beam system of a second harmonic CARM

Summary form only given. Progress on the Strathclyde University cyclotron autoresonance maser (CARM) experiment is reported, including the implementation of an improved electron-optical system designed to minimise velocity spread and thereby maximise electronic efficiency. During previous operation of this device 4% efficiency was achieved for CARM interaction at 14.2 GHz (four times the cyclotron frequency) with a combination of harmonic interaction and Doppler mechanisms being employed to realise this frequency shift. An efficiency less than that theoretically predicted was ascribed to a combination of a relatively high spread in the axial velocities of the beam of electrons, to which the CARM interaction is particularly sensitive, and a cavity-Q greater than optimal. An improved design of the diode is now operational and a set of cavities have been tested to optimise the operating efficiency, from which results are presented. Both the design, using numerical modelling of the space-charge limited field emission in the new diode and electron-optical system, and experimental results from electron beam and microwave diagnostic measurements are demonstrated.