Compact THz radiation source for DNP-enhanced NMR
- Prof. Andrei SAVILOV
- Prof. Andrei SAVILOV (Institute of Applied Physics)
- Prof. Vladimir BRATMAN (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia; Ariel University, Ariel, Israel)
- Dr. Alexei FEDOTOV (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia)
- Dr. Yury KALYNOV (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia)
- Dr. Petr MAKHALOV (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia)
- Mr. Ivan OSHARIN (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod, Russia)
- Prof. Vladimir MANUILOV (Institute of Applied Physics of Russian Academy of Sciences; 3Nizhny Novgorod State University, Nizhny Novgorod, Russia)
Nuclear magnetic resonance (NMR), which is along with X-ray crystallography one of the basic methods for determination of the protein molecular structure, greatly suffers from its low sensitivity. Therefore, the development of the instrumentation and technique for the NMR sensitivity enhancement methods such as dynamic nuclear polarization (DNP) is a topical problem. For current high-field NMR spectrometers, DNP technique exploits and requires radiation in the terahertz frequency range, namely at the frequencies of 260-650 GHz with power level of 1-100 W. This power level is far beyond capacity of solid-state devices or conventional slow-wave vacuum electron tubes and could be provided nowadays only by expensive and complicated devices such as gyrotrons and free-electron lasers, which hinders the widespread of the DNP technique. We develop an alternative and cheaper approach, which is a combining of an NMR spectrometer and a very compact low-voltage gyrotron (“gyrotrino”) in a single cryomagnet [1- 3]. This eliminates the need for an additional superconducting magnet, results in a shorter terahertz transmission line, and can make DNP systems more available for research laboratories.
The integration of the gyrotron with NMR-spectrometer in a single cryomagnet causes a number of specific features, which result from two requirements to be met for integrated version of THz oscillator, first, the matching of oscillator and DNP frequencies, and second, a very restricted space in the spectrometer cryomagnet bore. We show that the frequency matching condition can be fulfilled in the case of the gyrotron with a very low operating voltage of 1.5-2 kV. In turn, such low voltage complicates the design of the electron-optical system since it results in a very small anode-cathode distance, a low electrical field at the emitter, and strong influence of the initial velocity spread caused by emitter surface roughness. Despite these difficulties, a two-electrode electron gun was designed, which can form a laminar electron beam with the required parameters. To mitigate high-sensitivity of the electron gun to the thermal displacements of the electrodes, the mechanism of anode-cathode distance adjustment is also designed. Due to lack of free room in the spectroscopy bore, the gyrotron collector cannot be placed in a stray magnetic field, so the electron beam deposition occurs in a uniform magnetic field, which results in a power density of 10 kW/cm at the inner collector wall. Additionally, the mode converter of the designed compact gyrotron is very unconventional since we have organized the power output from the cathode end of the gyrotron cavity and placed all the converter mirrors at the one side of the electron beam.
The feasibility of the gyrotron generation under very low operating voltage was examined experimentally using an existing CW gyrotron initially designed for operation at the second cyclotron harmonic with a relatively high voltage. To form a low-voltage helical electron beam with a sufficiently large pitch-factor, a positive voltage was applied to the first anode of the gyrotron three-electrode magnetron-injection gun with a negative voltage at the cathode. CW gyrotron operation at voltages down to 1.5 kV has been demonstrated at a frequency of 252 GHz.
This work was supported by the Russian Science Foundation under project No. 16-12-10445.
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