Relativistic effect in electron cyclotron transverse wave devices

This note presents a simple physical argument which demonstrates the importance of relativistic electron bunching interactions between rotating electrons and an RF electric field in the plane of rotation, normal to a dc magnetic field. A magnitude comparison between the relativistic angular bunching and the well-known longitudinal bunching due to the transverse RF magnetic field shows that the close-to-cutoff operation of a number of recent cyclotron-frequency transverse-wave experiments is due solely to the relativistic effect.