# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 1 Introduction  
The recent years are witnessing remarkable progress and the proliferation of various applications that are utilizing electromagnetic radiation with terahertz frequencies belonging to the so-called terahertz (THz) gap, which nowadays is more frequently referred to as the last frontier of the electromagnetic spectrum. This is stipulated by the unique properties of the THz waves, which are also known as THz rays (for an insightful comment on the legitimacy and the subtle distinctions of the usages of these two terms, see [1]). Among them, the following characteristics are the most notable. First, the terahertz radiation is invisible by a naked human eye, but it penetrates dielectric materials (e.g., plastics, paper, textile, wood) that are opaque in the visible range of the spectrum. At the same time, it is strongly absorbed by water and other polar liquids (solvents) but compared with visible and IR light is less affected by attenuation due to both Mie and Rayleigh scattering because of the significantly longer wavelength. Moreover, the terahertz waves are non-ionizing (in contrast to extreme-UV light and X-rays), non-invasive, non-destructive, and therefore biologically safe and harmless at low specific absorption rates. An even more important feature of the THz waves stems from the fact that their frequencies correspond to the characteristic frequencies (resonances) of the molecularmotions (rotations, vibrations, stretching, hybrid modes of motion), and the irradiation of various gases and organic substances produces specific signatures that can be observed analyzing the resulting transmission, absorption, and reflection spectra. This allows using the THz waves for detecting/sensing many substances that exhibit such characteristic spectra. The basic mechanisms of such interactions are now well understood [2; 3; 4], and their underlying theoretical interpretations serve as a basis for the development of a wide range of methods and techniques [2; 5] in material spectroscopy [6], bio-sensing [7; 8; 9; 10], pharmaceutical industry [11], food inspection [12; 13], and security [14]. The main components of any THz system that determine its operational performance are the used radiation sources [2; 15] and detectors [16]. The terahertz region, a kind of \"no man's land\", which borders the microwaves and light and thus the domains occupied by the electronic and photonic devices, respectively, is nowadays being populated by various sources coming from both sides (\"borders\") of this frequency range. It is believed that such convergence would finally lead to filling (bridging) the gap. The clear aims in such direction have been seen well in the recently formulated THz science and technology roadmap [17]. Despite the progress demonstrated by practically all radiation sources, there is a noticeable difference in their output power levels. Although sufficient for many applications, the power of the most frequently used solid-state devices (e.g., IMPATT and Gunn diodes, quantum cascade lasers (QCLs) that are tunable in a wide frequency range) is orders of magnitude lower than that provided by the vacuum tubes (initially mm-wave sources that recently advanced toward the THz frequencies) such as Backward Wave Oscillators (BWO), and accelerator-based electron-beam sources [18] (most notably the Free Electron Lasers (FEL) and storage rings). A conventional figure of merit that characterizes the latter devices, as well as the high-power microwave tubes, is given by the product of the average output power \(P\) and the frequency \(f\) squared (\(Pf^{2}\)). With respect to this value, the gyrotrons are among the most powerful sources of both pulsed and CW (continuous wave) coherent radiation in the terahertz frequency range and recently are contributing significantly to bridging the THz gap, providing terahertz waves for different applications in the fundamental scientific research and the technologies [19; 20].