# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
### Advanced Spectroscopic Techniques
#### 3.1.4 Radioacoustic Spectroscopy Using Gyrotron Radiation  
Nowadays, the gas molecular spectroscopy (which is, in fact, one of the earliest applications in the terahertz spectral region) is a powerful tool used in various fundamental and applied studies as, for example, qualitative and quantitative gas analysis, non-invasive medical therapies, atmospheric remote sensing, and so on. As for any other spectroscopy, sensitivity is the main issue, since this key parameter determines the accuracy of the measurements and eventually the scope of the problems to which this technique can be applied. The current levels of the achieved sensitivity in the conventional schemes of mm-wave spectrometers have already approached the physical limits. The only known method for further increase of the sensitivity is based on the opto-acoustic (aka photo- or radioacoustic) detection of absorption. In this method, the result of the interaction of the radiation with matter is detected rather than the radiation itself. An efficient approach for increasing the sensitivity of the radioacoustic detection (RAD) by increasing the power of the radiation source has been realized in an automated facility [42]. The schematics of the spectrometer are shown in Figure 7. As a radiation source, it uses a gyrotron developed at IAP-RAS and operated in CW regime at a frequency of about 263 GHz (which can be tuned continuously within an interval of 0.2 GHz by varying the electron beam voltage and the temperature of the cavity) with an output power of up to 1 kW. The width of its radiation spectrum \(\Delta f\) is about 0.5 MHz (\(\Delta f/f\sim 10^{-6}\)) and is determined by the fluctuations of the accelerating potential instabilities of the accelerating potential provided by the high-voltage power supply. Recently, the capabilities of the RAD spectrometer have been demonstrated using as a test gas sulfur dioxide (SO2), which has a very dense and a well-studied spectrum in the mm/sub-mm range. It has been estimated that the maximum absorption sensitivity of the spectrometer is of the order of \(6\times 10^{-10}\) cm\({}^{-1}\). The bottom line of these experiments is that an increase in the scanning radiation power by about three orders of magnitude leads to a proportional increase in the sensitivity of the RAD spectrometer [77].  
Figure 6: Schematics of the pump-and-probe technique for X-ray detected magnetic resonance (XDMR) spectroscopy.  
This result clearly proves the efficiency of the outlined \"power\" approach and suggests some direction for a further realization of its potential; however, this is limited by the spectral line saturation effect. The latter problem can be substantially reduced by proper selection of the molecule, the transition, and experimental conditions. It is believed that through combining this method with complementary conventional techniques (e.g., increasing the optical path), a record-breaking sensitivity can be achieved.  
Recently, the operational performance of the gyrotron used in the RAD spectrometer has been improved significantly. Most notably, the radiation frequency has been stabilized against a reference oscillator using a phase-lock loop (PLL) system in the anode voltage control, which provides high stability, narrow bandwidth (relative width of approximately \(10^{-12}\)), and an accurate frequency control. Sample spectra have been registered at the first (263 GHz), second (526 GHz), and third (789 GHz) harmonic of the cyclotron frequency with the modulation of both the output power and frequency.