Chaotic spiking behaviour in electron cyclotron masers

Electron cyclotron maser (ECM) oscillators are high power sources of microwave radiation and have applications in fusion plasma heating and diagnostics and potentially also in radar and telecommunications systems. The radiation comes from coherent Bremsstrahlung emission by relativistic electrons gyrating in a magnetic field. It has been observed that the University of Strathclyde ECM oscillators fitted with high-current explosive-electron-emission cathodes exhibit a rapid modulation in the amplitude of the microwave output signal, whereas similar configurations using a thermionic emission cathode do not. This paper describes preliminary experiments investigating this complex behaviour. In particular it was of interest to establish whether the modulations were due to the cathode emission process or some dynamic cavity phenomena. We present experimental results demonstrating a connection between the automodulation behaviour and the length of the interaction space in a 23 GHz ECM. The ECM had an ill-defined cavity consisting of a cylindrical copper waveguide of 1 m length. The magnetic field limited the length of the interaction space to <9 cm (the length of its central plateau), adjustable using waveguide cut-off reflectors. With an 8 cm long interaction space the output signal from the ECM demonstrated a full amplitude modulation with a period ~4-7 ns, but reducing the length to 1.5 cm caused the modulation amplitude to reduce to ~20%, with a period ~20 ns whilst simultaneously changing in nature from stochastic to cyclical