# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
By an appropriate stabilization of the used cathode and anode high-voltage power supplies and applying a feedback control of the beam energy, relative widths of the frequency spectrum \(\Delta\)f/f and the frequency fluctuations \(\delta\)f/f lower than \(1\times 10^{-9}\) and \(3\times 10^{-10}\), respectively, have been achieved at FIR UF in the first experiment on the high-frequency stabilization of submillimeter-wave gyrotron [44]. In the subsequent studies, the technique of the output parameters stabilization based on PID control has been mastered, and now it is routinely used in the gyrotrons for DNP-NMR spectroscopy [45; 46]. It allows stabilizing the output power and frequency separately [47] as well as simultaneously [48], although in the latter case, the levels of the stabilizations are lower. Nevertheless, however, such a trade-off provides sufficiently stable radiation for the mentioned spectroscopic experiments. Generally, what is nowadays typical for the DNP-NMR gyrotrons is a long-term frequency stabilization with 0.2-0.4 MHz frequency spectrum width (stabilization is better than \(10^{-6}\)) [47]. Recently, a breakthrough result has been demonstrated by a high-power sub-terahertz (0.263 THz/100 W) gyrotron with a record frequency stability within 1 Hz, which was realized using a phase-lock loop in the anode voltage control [49]. The relative width of the frequency spectrum and the frequency stability reported in [49] are \(4\times 10^{-12}\) and \(10^{-10}\), respectively. These parameters are better (by almost three orders of magnitude) than those demonstrated so far by other high-power sources. Besides these spectral characteristics, a very important function required by many applications is the frequency tunability. In the case of gyrotrons, it can be performed in three different ways [50]. The first one is step-wise, and the tunability is being achieved switching to different operating modes by varying the cyclotron frequency (see Equation (1)), changing the accelerating voltage (and thus \(\gamma\)) or the magnetic field. The second one is a smooth continuous tunability, which is based on the excitation of a sequence of high-order axial modes (HOAM) [51]. The third possibility (which is rarely used due to the additional technical complications that it introduces) involves changing the dimensions and the shape of the cavity mechanically or by temperature control of the expansion/contraction of the resonator [52]. Efficient multi-frequency gyrotrons that are step-tunable in wide frequency ranges have been demonstrated in numerous experiments [53; 54; 55; 56; 57; 58; 59; 60]. An already classical but still impressive example of a stepwise tunability in a wide frequency band is the GYROTRON V developed at the University of Sydney [53]. Its operating frequency at the fundamental resonances has been tuned from 0.15 to 0.3 THz and extended by a second-harmonic operation to above 0.6 THz with output powers, respectively of up to 20 W and hundreds of mW in the frequency ranges 0.15-0.3 THz and 0.3-0.6 THz. The second method for frequency tuning via a successive smooth passage through a sequence of overlapping HOAM is very convenient for many spectroscopic techniques, and its successful realization has been reported in many studies [57; 58; 59; 60]. For instance, a continuous tuning in a 6 GHz band (from 0.134 THz to 0.14 THz) has been achieved in the gyrotron FU CW IV (FIR-UF) operating on a sequence of TE1,2,q modes having axial indices \(q=\pm 1,-2,\ -3,-4\) (where the negative values correspond to the gyro-BWO operation) by varying the magnetic field from 4.9 to 5.2 T [58]. Both the frequency and amplitude modulations are essential for many applications, such as for example in the remote sensing of atmosphere, communications, advanced diagnostic and spectroscopic techniques, etc. Recently, it has been demonstrated that the signal enhancement in the DNP-NMR spectroscopy can be increased significantly by using frequency-modulated CW radiation [61; 62; 63].