# 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  
The spectroscopy based on nuclear magnetic resonance (NMR) is a powerful and widely used method for studying a big variety of compounds (e.g., complex biomolecules such as proteins) in biomolecular research, material, and pharmaceutical sciences. However, its main drawbacks are low sensitivity, low signal-to-noise ratio, and, respectively, long spectrum acquisition times. A technique developed at MIT [66] for enhancement of the NMR signal through a dynamic nuclear polarization has radically solved these problems. It involves adding a polarizing agent to the sample, irradiation by gyrotron radiation, and transferring the polarization of the unpaired electron spins to the neighboring nuclear spins. In other words, DNP-NMR is a combination of two techniques, namely electron paramagnetic resonance (EPR), aka electron spin resonance (ESR), and NMR, which provides a significant (theoretically, up to several orders of magnitude) increase of the sensitivity and decrease of the acquisition time. After the pioneering breakthrough [66], a series of gyrotrons for DNP-NMR spectroscopy have been developed at MIT [24], where the first 140 GHz system has been followed by a series of spectrometers utilizing gyrotrons with output frequencies of 250, 330, and 460 GHz and operating at the second harmonics of the corresponding cyclotron resonances. For coupling of the gyrotron radiation to the sample, low loss transmission lines are being used. For application in the time-domain DNP-NMR techniques, two gyro-amplifiers operating at 140 GHz and 250 GHz, respectively, have been developed as well. At FIR UF, a series of CW gyrotrons (the so-called FU CW series) has been developed [23; 26; 67; 68]. They occupy a wide area in the parameters space of the output power and frequency and are being used in various studies ranging from materials processing and characterization to novel medical technologies. However, the biggest portion of this series consists of the radiation sources dedicated to advanced spectroscopic techniques. Photos of some of them are shown in Fig. 3. One of them, the gyrotron FU CW II, is built using an 8 T liquid-He-free superconducting magnet (SCM) and generates radiation with a frequency of 394.6 GHz (operating at the second harmonic of the cyclotron frequency) that corresponds to the 600 MHz proton NMR at a magnetic field intensity of about 14 T. An optimized version of this tube (FU CW IIB) is characterized by a high stability of the output power (fluctuations lower than 0.5% during 18 h CW operation) achieved by using PID feedback control. The next radiation source, FU CW IV, for 200 MHz DNP-NMR spectroscopy uses a 10 T cryogen-free SCM and has demonstrated continuous tunability in a broad frequency band from 134 to 140 GHz simply by changing the magnetic field in the cavity from 4.9 to 5.2 T and operating as a gyro-BWO on a sequence of high-order axial modes (HOAM). The same technique for smooth frequency tunability has been achieved by the gyrotron FU CW VI, which has a SCM with a maximum field intensity of 15 T. It has been designed as a radiation source for the 600 MHz DNP-NMR spectrometer of the Institute for Protein Research (IPR) at Osaka University. One of the most versatile tubes, namely FU CW VII, can be used for both 300 and 600 MHz DNP-NMR at output frequencies of 200 GHz (fundamental operation) and 400 GHz (second harmonic), respectively. This gyrotron is step-tunable in a wide range (from86 to 223 GHz), since many operating modes can be excited in this frequency band by varying the electron beam parameters and the magnetic field. Specific requirements imposed on the gyrotrons used as radiation sources for spectroscopic studies stem from the fact that they have to be embedded in sophisticated laboratory infrastructure, and thus, it is desirable to minimize their weight and dimensions.