It is customary today to call a generator or amplifier of electromagnetic waves a maser (Microwave Amplification by the Stimulated Emission of Radiation) if its operation is based on the stimulated emission of distributed oscillators. Electrons and ions, rotating around an ambient magnetic field, are the oscillators in so-called cyclotron masers (CMs).

There are two types of CMs in space, which differ considerably from each other. In the Earth's magnetosphere the first type operates on open field lines in the auroral region at heights between 103 and 104 km, in plasma cavities where the plasma density is so low that the electron plasma frequency is much less than the electron cyclotron frequency. Here auroral kilometric radiation (AKR) is generated by energetic electrons in such a maser system (Fig. 1.1a). The eigenmodes of these auroral CMs are electromagnetic waves with frequencies close to the electron cyclotron frequency; the wave vector k is almost perpendicular to the geomagnetic field B. These auroral CMs are rather similar to a family of laboratory devices, termed gyrotrons (Fig. 1.1b). The operation of these devices is based on the cyclotron interaction of electrons, moving along a homogeneous magnetic field through an evacuated region inside a geometrical cavity resonator. A specific feature of a laboratory CM is the cyclotron interaction of a well-organized beam of electrons rotating around a homogeneous magnetic field with a monochromatic electromagnetic wave having a spatially fixed field structure.