What Is a Gyrotron?
A gyrotron is a type of vacuum tube device in which electrons orbit around a magnetic field generated by a superconducting coil. These electrons are accelerated by high-speed rotational energy and then converted into high-power millimeter-wave microwaves, which are emitted in a cavity resonator.
The term “gyro” refers to rotation and involves the Cyclotron Resonance Maser (CRM) phenomenon. CRM is a process where the kinetic energy of electrons, rotated by electromagnetic force, is transformed into microwaves.
The millimeter wave band, ranging from 1mm to 10mm in wavelength and 30GHz to 300GHz in frequency, is known for its high linearity and large information-carrying capacity.
Uses of Gyrotrons
Gyrotrons have several applications:
- Industrial Fields: Ceramic sintering.
- Research and Development Fields: Plasma-related processes (heating, measurement, etc.) for fusion experimental devices in laboratories.
- Sub-THz Band: Satellite communications, simple radio, subscriber radio access (38 GHz band), various automotive radars, LiDAR, ADAS, autonomous driving, etc.
Gyrotrons, as high-power radio wave sources in the millimeter wave band, are being increasingly researched for applications in advanced Beyond 5G/6G communications.
Principle of Gyrotrons
Gyrotrons operate on the principle of the cyclotron resonance maser phenomenon. Electrons emitted from an electron gun inside the gyrotron gain spiral kinetic energy as they pass through a superconducting magnetic field. This energy is converted into high-power electromagnetic waves in the millimeter wave band inside a cavity resonator.
Electrons, accelerated by a high voltage (about 100 kV) in an electron gun, obtain high-speed rotational energy when passing through a magnetic field generated by a superconducting magnet (up to 10 T (tesla)). These electrons spiral into a collector in the vacuum tube, which ultimately captures them.
As the spiraling electrons pass through a resonator, they resonate and lose some kinetic energy, which is then converted into electromagnetic waves. These waves are reflected multiple times within the gyrotron and finally emitted through a window, such as an artificial diamond, as high-power millimeter-wave electromagnetic waves.
Other Information on Gyrotrons
1. Development of Nuclear Fusion and Gyrotrons
Nuclear fusion technology is regarded as a promising future power generation method. High-power sub-THz millimeter waves from gyrotrons are transmitted to a fusion reactor, about 100 meters away, to heat the plasma and initiate nuclear fusion.
The International Thermonuclear Experimental Reactor (ITER), a global collaboration project for clean energy generation, is expected to start operation in 2025. Developing gyrotrons for heating and various measurements in fusion facilities is being actively pursued.
2. Gyrotron Frequency
One of the most promising fusion facilities is the Tokamak fusion reactor. The plasma must be heated to ultra-high temperatures in a strong superconducting magnetic field in this reactor. The superconducting magnetic field varies between the center and the edges of the reactor. Therefore, having a gyrotron that can select multiple resonance frequencies is advantageous for maximizing the use of the reactor’s interior.
As per a 2022 announcement by Japan’s National Institute of Quantum Science and Technology, improvements in gyrotron components will enable one megawatt-class operation for 300 seconds at three millimeter-wave frequencies (170 GHz/137 GHz/104 GHz). This advancement is a significant step towards the practical application of nuclear fusion. Furthermore, a breakthrough in increasing the gyrotron’s oscillation frequency to 1013 GHz (THz band) was achieved in 2005 at the University of Fukui’s Center for Far Infrared Research and Development. Joint research for various applications continues both in Japan and internationally.