Gyrotrons are microwave oscillators based on the Electron Cyclotron Maser (ECM) instability. The free energy isthe rotational energy of weakly relativistic electrons (1 < 'y < 1.2) in a longitudinal magnetic cavity field. A net transfer ofenergy from the gyrating electrons to the electromagnetic field in the interaction circuit occurs due to the azimuthal phasebunching when the wave frequency is somewhat larger than the relativistic electron cyclotron frequency L-2 or itsharmonics. Many other types of microwave sources are also based on the ECM instablility, such as gyro-klystron, gyro- travelling wave amplifier (gyro-TWA) or gyro-backward wave oscillator (gyro-BWO). Cyclotron maser devices operatingin the relativistic Doppler-shifted regime where the axial bunching mechanism can substancially offset the azimuthalbunching are called cyclotron autoresonance maser (CARM).
M. Thumm, M. Thumm,
"Progress in development of high power gyrotrons", Proc. SPIE 2104, 18th International Conference on Infrared and Millimeter Waves, 210404 (30 August 1993); doi: 10.1117/12.2298436; https://doi.org/10.1117/12.2298436