**Preface to "Design, Technologies and Applications of High Power Vacuum Electronic Devices from Microwave to THz Band"**

It is generally accepted that the 20th century was the age of electronics. Humanity has not yet decided on the topic that will dominate the current century, but many argue that it will be the century of T-rays—the century of terahertz radiation. Coherent electromagnetic radiation of subterahertz and terahertz frequency ranges with relatively high power has some specific features that make it very attractive for a wide range of fundamental and applied research areas in physics, chemistry, biology, and medicine. In particular, the enhancement of sensitivity for spectroscopic applications (electron paramagnetic resonance spectroscopy; dynamic polarization of nuclear spins in nuclear magnetic resonance spectroscopy); plasma applications (diagnostics of dense plasmas in fusion devices, the creation of compact plasma objects); biochemical applications (management of flow rate of reactions in organic chemistry, conformational changes of protein molecules); standoff detection and imaging of explosives and weapons; new medical technology; atmospheric monitoring; production of high-purity materials; deep space and special satellite communication, etc., can be mentioned.

For a long time, the intermediate position of THz waves between the microwave and optical portions of the electromagnetic spectrum did not achieve notable results in mastering powerful radiation sources and seemed to be too short in wavelength for the methods of classical vacuum electronics (due to the necessity of small-scale elements for slow-wave structures) and too low in frequency for the methods of quantum electronics (due to quant energy) to be applied.

The most widespread and frequently used devices in the "low THz" range (up to a frequency of 1.5 THz) are low-voltage and small-size backward wave oscillators (BWOs) that provide an output power of several milliwatts in the CW regime at the highest permissible frequencies. There also exist other low-voltage vacuum sources based on stimulated Cherenkov and Smith–Purcell radiation of rectilinear electron beams, as well as solid-state devices delivering sub-THz and THz radiation at a power level from hundreds of microwatts to one milliwatt; quantum cascade lasers already provide a power of hundreds of milliwatts at frequencies down to 5 THz. At the same time, the power of coherent radiation, which can be delivered by vacuum devices based on the stimulated Bremsstrahlung radiation of curvilinear electron beams (free electron lasers (FELs) and gyrotrons), can be higher by many orders of magnitude, which opens new opportunities for many applications. High-power radiation is produced in FELs and gyrotrons due to the advantage of using electrons' interaction with fast electromagnetic waves instead of slow waves in "conventional" electron devices. As a result, FELs can provide coherent and smoothly frequency-tuned radiation in the entire THz frequency region and the bands of much higher frequencies, presumably up to X-rays. However, FELs utilize ultrarelativistic electron beams and typically require huge particle accelerators for their realization. That is why FELs can be used only in specialized research centers. Unlike FELs, gyrodevices can operate with electron beams having significantly lower energies of 10–100 keV, meaning gyro-tubes are much more compact than FELs and available for many laboratories.

Currently, the above-mentioned vacuum electronic sources cover over 12 orders of magnitude in power (mW-to-GW) and 2 orders of magnitude in frequency (0.1–10 THz). The present investigations aim to develop radiation sources, including the development of new schemes of electron cyclotron masers, with record-breaking frequencies and peak and average powers. Despite the many technical limits, such as the requirement of strong operating magnetic fields for gyrodevices, mode competition, and high ohmic losses, the number of pulsed and CW radiation sources and the range of applications are increasing rapidly.

> **Mikhail Glyavin** *Editor*
