A gyrotron is a form of electron tube or vacuum tube that is often referred to as a cyclotron resonance maser due to the fact that one of its most frequent uses is in high-energy physics research in cyclotrons. The advantage that a gyrotron offers is that it can generate enormous amounts of radio frequency (RF) energy in the megawatt range at very small wavelengths of only a few millimeters, which is not possible for standard vacuum tubes. The process can generate an enormous amount of heat, which can be used to sinter ceramics or heat plasma in fusion research reactors. Gyrotrons are also directly employed in nuclear magnetic resonance (NMR) imaging for observing quantum mechanical effects at the atomic level or in magnetic resonance microscopy (MRI) for medical diagnoses.
The principle behind how a gyrotron functions was first theoretically composed in the late 1950s, when relativistic effects of electron energy were being studied in cyclotrons for the first time. By injecting streams of electrons into the electromagnetic field of a cyclotron with an equal frequency, an effect known as negative mass instability was observed. The electron stream would tend to bunch together from a standard gyroradius or Larmor radius, causing the electrons to decelerate and release kinetic energy in the process as millimeter wavelength radio frequency energy or radiation.
Early electron cyclotron resonance energies demonstrated the potential to heat plasmas in fusion research, but the technology and scientific understanding to create a gyrotron system that was reliably capable of this did not become a mature science until the first decade of the 21st century. As the science and technology advanced, gyrotron applications split into high-energy megawatt systems for fusion research, and low-energy 10- to 1,000-watt systems for NMR spectroscopy. Where the devices produce terahertz radiation in the 100 gigahertz to 1 terahertz range, they are used in industrial applications such as plasma diagnostics and high-temperature heating of ceramic compounds. Research in Japan has also increased the efficiency of mid-range to high-power gyrotron devices by 50% as of 1994 by using an integrated mode converter to more efficiently convert electron beam energy to heat.
Since a gyrotron is a form of Microwave Amplification by Stimulated Emission of Radiation (MASER) device or free electron laser that generates electromagnetic fields, it has some similarity to the principle behind how a standard microwave oven operates. A portable gyrotron can be operated in a range of frequencies typically from 2 to 235 gigahertz, and this makes them useful devices for non-lethal weapons systems that the US military refers to as Active Denial System (ADS) technology. An ADS device based on a gyrotron can be targeted against human beings with the effect that it heats up water molecules under the skin without causing permanent damage to tissue. This acts as a deterrent field that has theoretical applications in crowd control to prevent riots, or to keep enemy soldiers or civilians from approaching military installations and downed aircraft.