# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
In the most advanced gyrotrons, depressed collectors are being used for recuperation (recovery) of part of the energy of the spent electron beam and increasing in such a way the overall efficiency. Both the overall weight and the dimensions of the gyrotrons are determined by the biggest component: the superconducting magnet. Compared to the conventional microwave tubes and the solid-state devices they are bulkier, but at the same time, they are much more compact than the accelerator-based sources and FEL. Recently, the availability of cryogen-free superconducting magnets with smaller dimensions has made it possible to develop compact (portable and almost table-top) gyrotrons that can be easily embedded in the laboratory infrastructure used for spectroscopy, imaging, inspection, etc. Some of them will be mentioned in the following section. The main advantages that characterize the gyrotrons as versatile radiation sources for various applications in THz science, spectroscopy, sensing, and imaging are listed in Table 1 and will be commented briefly below. Many applications (e.g., radars, ESR and NMR-DNP spectroscopies, measurements of the hyperfine splitting (HFS) of positronium) impose high requirements on the spectral characteristic of the used radiation most notably on the linewidth, mode purity, and stability of the frequency. Although there are inevitable factors that contribute to the widening of the spectrum such as fluctuations of the accelerating voltage and electron-beam noises (flicker, shot, etc. ), in a well-designed tube, their influence is minimized and the spectral characteristics are appropriate for the mentioned applications [39; 40; 41; 42; 43]. For example, the detailed measurements of the spectral characteristics of a 0.14 THz gyrotron for NMR-DNP have shown an FWHW (full width at a half maximum) linewidth of 7.2 kHz [40; 41]. In an analogous study performed using a 0.263 THz gyrotron for spectroscopy, a spectral width \(\Delta\)f of about 0.5 MHz (\(\Delta\)f/f \(\sim\) 10\({}^{-6}\)) has been reported [42]. In [43], long-term frequency stability better than 1 MHz has been \begin{table}
\begin{tabular}{l} \hline \hline \multicolumn{1}{c}{**Characteristic Features of the Gyrotrons and Their Radiation**} \\ \hline High-power (from Watts to kW) 1 radiation in both pulsed and CW (continuous wave) regimes. \\ \hline Coherent radiation with stable spectral characteristics (narrow spectral line width; small down- and up-shift of the frequency during the pulse and long operation times; possibilities for phase locking). \\ \hline Stable output parameters (power and frequency) through appropriate stabilization (e.g., PID control) \\ \hline Step-wise and smooth continuous frequency tunability in wide frequency bands. \\ \hline Possibility to modulate the output power and frequency. \\ \hline Possibility to deliver Gaussian beam output using internal or external quasi-optical converters. \\ \hline Possibility for transmission of the generated radiation by waveguides (e.g., corrugated waveguides with low losses) or quasi-optical system of reflectors and phase-correcting mirrors. \\ \hline Possibility to focus and steer the generated wave beam by quasi-optical elements. \\ \hline \hline \end{tabular}  
* Here, we omit the most powerful gyrotrons for the heating of fusion plasma that already reached megawatt levels of the output power, since this prominent application is outside the scope of the present review.  
\end{table} 
Table 1: Main advantages of the gyrotrons as versatile radiation sources in the terahertz (THz) region. demonstrated in a 0.26 THz DNP-NMR gyrotron. The radiation linewidth determined by the anode voltage fluctuations (\(\delta\)Va/Va = 0.3%) has been estimated to be as low as \(\delta\)f = 3 MHz.