# 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
#### 3.1.1 DNP-NMR and ESR Spectroscopy
Additionally, the DNP-NMR spectroscopy demands a well-collimated (Gaussian) wave beam with a linear polarization, which has to be transmitted and coupled to the sample with low losses. In order to satisfy these requirements, two new clones of the FU CW series have been developed, namely FIR FU CW C and FIR FU G. In their notations, the symbols C and G stand for \"compact\" and \"Gaussian beam\", respectively. For instance, the overall height of FU CW CI is 1.02 m, while that of CII is only 0.86 m. FU CW CI has been designed having several applications in mind, including 600 MHz DNP-NMR spectroscopy. It utilizes a compact He-free 8 T SCM, operates at a frequency of 395 GHz, and provides radiation with an output power of 120 W. The gyrotrons of the CW G series have internal (built-in) mode converters and deliver Gaussian-like beams with an almost circular cross-section. Some of them are designed and optimized especially as radiation sources for 600 MHz (FU CW GII at 395 GHz) and 700 MHz (FU CW GVI and GVIA at 460 GHz), respectively. At IPR, where these gyrotrons are installed, a slightly different nomenclature is used. According to it, FU CW VI and VIA are denoted as FU CW GO-1 and GO-II, where \"GO\" indicates that the tube delivers an optimized Gaussian output beam. The collage in Figure 3 shows photos of some of the gyrotrons belonging to the FU CW series and used for spectroscopic studies. A second-harmonic gyrotron for DNP-NMR spectroscopy that provides radiation with a frequency of 0.26 THz and power of 100 W has been developed at IAP-RAS [25]. This radiation source has been embedded in an NMR spectrometer at the Institute of Biophysical Chemistry of Goethe University (Frankfurt-am-Main, Germany). The transmission line (13 m long), which delivers the radiation from the output window of the gyrotron to the spectrometer, includes an external mode converter that transforms the operating mode TE23 into the hybrid linearly polarized mode HE11; a corrugated waveguide section; four miter bends; a variable polarizer-attenuator; a water load combined with a calorimeter; and a two-position power switch. The experiments conducted at this spectrometer have demonstrated an increase of both the sensitivity and the resolution by more than 80 times. A successor of this tube is another gyrotron for DNP-NMR spectroscopy operating at the same frequency (0.26 THz) but at the fundamental resonance and therefore at higher levels of both the output power and efficiency [42]. It has demonstrated a maximum CW power up to 1 kW at high beam  
Figure 3: Photos of some of the gyrotrons of the FU CW series developed as radiation sources for spectroscopic studies. current (0.4 A) and an output power of about 10 W (which is sufficient for most of the spectroscopic applications) in a low-current (0.02 A) regime. There is an empirical law according to which the strength of the magnets for NMR (measured by the frequency of the proton resonance) increases by 100 MHz every 5 years. It is anticipated that the next generation of the NMR spectrometers will reach 1.2 GHz, and Bruker has already announced that such a magnet (with a field intensity of 28 T) is already commercially available [69]. DNP-NMR spectroscopy at such magnetic fields requires a radiation source that provides a sufficient output power at a frequency of 0.8 THz. Motivated by this prospective, an international research team has developed the first second-harmonic double-beam gyrotron with appropriate characteristics for such an application [70]. This has been made possible due to the specific additional means for mode selection, which this concept offers in solving the problem of mode competition between fundamental and second-harmonic resonances. The experimental results of the investigations of this radiation source [71] are in agreement with the theoretical expectations and prove the potential of the double-beam tubes for further advancement to higher frequencies.