Fusion plasmas require many megawatts of heating to reach the temperatures at which fusion reactions occur. Gyrotrons are one of the  possible solutions. They are sources of high power, high frequency microwaves which can be absorbed in a very localised region of the plasma. The technique is attractive for various plasma physics reasons, and it also has some significant practical advantages linked to the high frequency of the radiation (which corresponds to a few millimetres wavelength): the radiation can be transported efficiently over large distances and very high power can betransmitted through the small  apertures which are permitted in the wall of the plasma vessel. Gyrotrons use the interaction between a high energy electron beam and radiation in a magnetic field to generate the microwaves. High power is possible because the interaction cavity is large, unlike low power devices which use a fundamental mode cavity. The cavity is carefully designed to support a single high-order waveguide mode which must be converted to a low order mode for transmission to the plasma through waveguides.

The big problem has always been the severe technological difficulty of making sources  which reliably combine high power output, long pulse length and high frequency. For a machine like ITER, the targets are at least 1 MW per gyrotron for many tens of seconds at up to 170 GHz. A practical device also needs an overall efficiency of several tens of percent.

Fusion research has been pushing gyrotron development for over two decades. In this time the progress has been astounding. For example, in 1980 a low frequency (28 GHz) output of 200 kW was an achievement. Today, gyrotrons with 0.5 MW for about 10 s at 100 GHz, and an efficiency greater than 30% are commercially available. Tubes which will meet the ITER requirements are currently in development.