Observations of dynamic behaviour in an electron cyclotron maseroscillator

Abstract

Electron cyclotron maser (ECM) oscillators are high powersources of microwave radiation and have applications in fusion plasmaheating and diagnostics with potential for radar and telecommunicationssystems. The radiation comes from coherent bremsstrahlung emission byrelativistic electrons gyrating in a magnetic field. It has been observedthat the University of Strathclyde ECM oscillators fitted withhigh-current explosive electron emission cathodes exhibit a rapidmodulation in the amplitude of the microwave output signal, whereassimilar configurations using a thermionic emission cathode do not. Thisrapid communication describes preliminary experiments investigating thiscomplex behaviour. In particular, it is of interest to establish whetherthe modulations were due to the cathode emission process or some dynamiccavity phenomena. We will present experimental results demonstrating aconnection between the automodulation behaviour and the length of theinteraction space in a 23 GHz ECM. The ECM had an ill-defined cavityconsisting of a cylindrical copper waveguide of 1 m length. The magneticfield limited the length of the interaction space to <9 cm (the lengthof its central plateau), adjustable using waveguide cut-off reflectors.With an 8 cm long interaction space the output signal from the ECMdemonstrated a full amplitude modulation with a period of ~4-7 ns,but reducing the length to 1.5 cm caused the modulation amplitude toreduce to ~20%, with a period of ~20 ns whilst simultaneouslychanging in nature from stochastic to cyclical.