A novel THz-band double-beam gyrotron for high-field DNP-NMR spectroscopy

The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging

The gyrotrons are powerful sources of coherent radiation that can operate in both pulsed and CW (continuous wave) regimes. Their recent advancement toward higher frequencies reached the terahertz (THz) region and opened the road to many new applications in the broad fields of high-power terahertz science and technologies. Among them are advanced spectroscopic techniques, most notably NMR-DNP (nuclear magnetic resonance with signal enhancement through dynamic nuclear polarization, ESR (electron spin resonance) spectroscopy, precise spectroscopy for measuring the HFS (hyperfine splitting) of positronium, etc. Other prominent applications include materials processing (e.g., thermal treatment as well as the sintering of advanced ceramics), remote detection of concealed radioactive materials, radars, and biological and medical research, just to name a few. Among prospective and emerging applications that utilize the gyrotrons as radiation sources are imaging and sensing for inspection and control in various technological processes (for example, food production, security, etc). In this paper, we overview the current status of the research in this field and show that the gyrotrons are promising radiation sources for THz sensing and imaging based on both the existent and anticipated novel techniques and methods.

Experimental demonstration of the third cyclotron harmonic excitation in technological gyrotron with delayed selective feedback

The excitation and effective selection of modes at high harmonics of the cyclotron frequency are experimentally demonstrated using a delayed feedback into the gyrotron. The operation at the third cyclotron harmonic at the frequency of 40.5 GHz with the output power of 3 kW and efficiency 8% is observed in a continuous wave gyrotron with an oil-cooled magnet with a field of 0.5 T in the interaction space.

A broad range frequency measurement method for continuous and pulsed THz waves

This paper proposes a method to measure the frequency of terahertz (THz) waves based on the Zeeman effect and the high magnetic field technology with a wideband range from 60 GHz to 3 THz. As the frequency of THz waves absorbed by the sample is linear to the magnetic field in the Zeeman effect, the frequency can be measured by the magnetic field strength. A comparison study of THz frequency measurement was carried out in two magnet systems (a superconducting one and a pulsed one) to investigate the performance in two kinds of high magnetic fields. The experimental results of 60-700 GHz show that this method has high resolution (about 0.001%), excellent linearity, and good repeatability. Moreover, the proposed method can measure polychromatic signals simultaneously as well as the single pulse frequency in the order of tens of microseconds.

Perspectives on microwave coupling into cylindrical and spherical rotors with dielectric lenses for magic angle spinning dynamic nuclear polarization

Continuous wave dynamic nuclear polarization (DNP) increases the sensitivity of NMR, yet intense microwave fields are required to transition magic angle spinning (MAS) DNP to the time domain. Here we describe and analyze Teflon lenses for cylindrical and spherical MAS rotors that focus microwave power and increase the electron Rabi frequency, ν_1s. Using a commercial simulation package, we solve the Maxwell equations and determine the propagation and focusing of millimeter waves (198 GHz). We then calculate the microwave intensity in a time-independent fashion to compute the ν_1s. With a nominal microwave power input of 5 W, the average ν_1s is 0.38 MHz within a 22 μL sample volume in a 3.2 mm outer diameter (OD) cylindrical rotor without a Teflon lens. Decreasing the sample volume to 3 μL and focusing the microwave beam with a Teflon lens increases the ν_1s to 1.5 MHz. Microwave polarization and intensity perturbations associated with diffraction through the radiofrequency coil, losses from penetration through the rotor wall, and mechanical limitations of the separation between the lens and sample are significant challenges to improving microwave coupling in MAS DNP instrumentation. To overcome these issues, we introduce a novel focusing strategy using dielectric microwave lenses installed within spinning rotors. One such 9.5 mm OD cylindrical rotor assembly implements a Teflon focusing lens to increase the ν_1s to 2.7 MHz within a 2 μL sample. Further, to access high spinning frequencies while also increasing ν_1s, we analyze microwave coupling into MAS spheres. For 9.5 mm OD spherical rotors, we compute a ν_1s of 0.36 MHz within a sample volume of 161 μL, and 2.5 MHz within a 3 μL sample placed at the focal point of a novel double lens insert. We conclude with an analysis and discussion of sub-millimeter diamond spherical rotors for time domain DNP at spinning frequencies >100 kHz. Sub-millimeter spherical rotors better overlap a tightly focused microwave beam, resulting in a ν_1s of 2.2 MHz. Lastly, we propose that sub-millimeter dielectric spherical microwave resonators will provide a means to substantially improve electron spin control in the future.