Observations of dynamic behaviour in an electron cyclotron maser oscillator

Electron cyclotron maser (ECM) oscillators are high power sources of microw ave radiation and have applications in fusion plasma heating and diagnostic s with potential for radar and telecommunications systems. The radiation co mes from coherent bremsstrahlung emission by relativistic electrons gyratin g in a magnetic field. It has been observed that the University of Strathcl yde ECM oscillators fitted with high-current explosive electron emission ca thodes 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 investig ating this complex behaviour. In particular, it is of interest to establish whether the modulations were due to the cathode emission process or some d ynamic cavity phenomena. We will present experimental results demonstrating a connection between the automodulation behaviour and the length of the in teraction space in a 23 GHz ECM. The ECM had an ill-defined cavity consisti ng of a cylindrical copper waveguide of 1 m length. The magnetic field limi ted 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 ampli tude modulation with a period of <similar to>4-7 ns, but reducing the lengt h to 1.5 cm caused the modulation amplitude to reduce to similar to 20%, wi th a period of similar to 20 ns whilst simultaneously changing in nature fr om stochastic to cyclical.