Wiktionary
gyrotron
n. (context physics English) a type of microwave generator employing cyclotron resonance coupling between microwave fields and a beam of electrons
Wikipedia
Gyrotron
A Gyrotron is a high-power linear-beam vacuum tube which generates millimeter-wave electromagnetic waves by the cyclotron resonance of electrons in a strong magnetic field. Output frequencies range from about 20 to 250 GHz, covering wavelengths from microwave to the edge of the terahertz gap. Typical output powers range from tens of kilowatts to 1–2 megawatts. Gyrotrons can be designed for pulsed or continuous operation.