## 2 Methods and Hardware for Application of subTHz Instruments  
### Gyrotrons: High-Power subTHz Sources and Amplifiers  
Here, we consider approaches to the problem of propagation of high-power microwave beams using examples of currently available powerful and ultra-powerful (up to 1 MW) pulsed or continuous-wave (CW) subTHz radiation sources, which propagate in the subTHz atmospheric transparency window of \(\sim\)1.3 mm or \(\sim\)240 GHz. A line of very powerful subTHz generators and amplifiers has been developed and manufactured in collaboration with IAP RAS and GYCOM [18; 19; 20; 21; 22]. For such powerful amplifiers and generators, efficiency is an important factor. The efficiency of gyro-devices (including those based on gyro-frequency harmonics) can be increased significantly (by about a factor of one and a half) through the use of recuperation schemes, which has been demonstrated repeatedly in experiments [22]. This progress in developing unique generators, which generate powers of up to the megawatt level, and receivers [23] in the subTHz range with the noise close to the fundamental quantum limit expands the range of possible subTHz applications radically. Applications of powerful microwave sources (see Figure 1) actively developed over the recent years in plasma heating installations, future thermonuclear power plants, and accelerators [18], as well as when fulfilling several scientific and engineering tasks ranging from high-resolution laboratory spectroscopy [19] to drilling wells for geothermal energy plants and SETI problems [24], do not involve some exotic cases of channeling subTHz radiation through the atmosphere [25; 26] and, therefore, go beyond the scope of this paper. The influence of the atmosphere is essential for us in the following subTHz applications considered here. 1. Use of radar installations for identifying the positions of space debris, the volume of which has been dramatically increasing in the near-Earth orbits in recent years, and also for scanning the lunar and planetary surfaces [27,28] from the Earth (ground-based low-power radar may have some other applications in addition to the above). 2. Telecommunications. Interplanetary and deep-space missions, like flights to Mars and Venus, and the L2 missions, require high-performance space communications [4,28], which will be bound to use the THz range. Here, some old projects of sending messages from the Earth to extraterrestrial civilizations come to one's mind [24], but even short-term prospects of a new generation of terrestrial mobile communications, such as 7G, imply the use of THz waves: 7G is expected to distribute signals between mobile consumers at frequencies of about 0.7 THz [5], while the exchange between cells will be organized via fiber-optic links. 3. There are currently more exotic projects of using powerful microwave beams sent through the atmosphere not as information carriers, but as energy transport means, as contrasted with the first two cases. The examples that have appeared over recent years in the THz community include power supplied from the Earth to a spacecraft for propulsion [25] and power supplied from near-Earth solar power stations to terrestrial consumers via powerful microwave beams [26]. The signal-to-noise ratio (SNR) is the key characteristic for telecommunication and, even more so, radar tasks. The use of signal sources of extremely high power and effective ultra-high-sensitivity receivers, the technology of which has also made good progress over the last decade [23], makes it possible to provide extremely high SMR levels. The best performance parameters of the gyrotrons developed by the authors, primarily for nuclear fusion, but proposed here for other applications, are: 250 GHz/330 KW in the pulsed regime, 263 GHz/1 KW in the pulsed and continuous-wave (CW) regimes, and 527 GHz/250 W in the pulsed and CW regimes. It is definitely better than the proposals of the competitors listed here [13,14].