What is a Gyrotron?

A gyrotron is a vacuum electronic device (VED) capable to generate high-power, high-frequency THz radiation. A gyrotron’s operation is based on the stimulated cyclotron radiation of electrons oscillating in a strong magnetic field typically provided by a superconducting magnet. A schematic, indicating the various parts of a gyrotron tube is given below.

Schematic showing how a gyrotron works

In a gyrotron, electrons that are emitted by the cathode (1), are accelerated in a strong magnetic field of a superconducting magnet (4). While the electron beam (3) travels through the intense magnetic field, the electrons start to gyrate at a specific frequency given by the strength of the magnetic field. In the cavity (5), located at the position with the highest magnetic field strength, the THz radiation is strongly amplified. The mode converter (6) is used to form a free-gaussian beam that leaves the gyrotron through a window (8) and is coupled to a waveguide. The spent electron beam is then dissipated in the collector (7).

The gyrotron is a so-called fast-wave device because the dimensions of its interaction structure is much larger compared to the wavelength of the radiation. This is in contrast to slow-wave device, which have interaction structures that are of the order of the wavelength of the generated radiation. However, especially at high frequencies, these interaction structures can be very small (sub millimeter) and therefore can easily burn out at the high power densities required to generate sufficient output power, significantly limiting the lifetime of the tube.

Since gyrotrons are typically operated in a higher mode the interaction structure (cavity) can be much larger compared to the wavelength of the radiation. Furthermore, the cavity is typically a metal tube (copper) and can be effectively cooled, due to its simple structure. Therefore, the gyrotron can provide high output power, at high frequencies and guarantees a long lifetime.

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