Influence of reflected or external signal on gyrotron operation

Gyrotrons are the most powerful radiation sources in sub-THz and THz bands, which are of great importance for many applications, such as electron-cyclotron plasma heating, DNP/NMR spectroscopy, plasma diagnostics, biomedical applications, etc. All of these applications require frequency-stable, single-mode operation. Apart from the traditional methods for frequency stabilization, several other approaches have attracted a considerable interest. In this paper, we present a review of recent studies aimed at deeper understanding of injection locking of a gyrotron by an external signal, as well as of self-injection locking by partial reflection of the output power from a remote load. The increasing of frequency stability, expanding of the frequency tunability range, and the possibility of suppressing of the parasitic modes are discussed.