Observations of dynamic behaviour in an electron cyclotron maser oscillator

Electron cyclotron maser (ECM) oscillators are high power sources of microwave radiation and have applications in fusion plasma heating and diagnostics with potential for 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 rapid communication describes preliminary experiments investigating this complex behaviour. In particular, it is of interest to establish whether the modulations were due to the cathode emission process or some dynamic cavity phenomena. We will 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 of ~4-7 ns, but reducing the length to 1.5 cm caused the modulation amplitude to reduce to ~20%, with a period of ~20 ns whilst simultaneously changing in nature from stochastic to cyclical.