Abstract
Terahertz waves generated by vacuum electron devices have been successfully applied in dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) technology to significantly enhance the sensitivity of high-field NMR. To reduce the magnetic field interference, the high-power terahertz wave source and the NMR spectrometer need to be separated by a few meters apart. Corrugated horns and directional couplers are key components for shaping high linearly polarized terahertz Gaussian beam and accurately coupling electromagnetic power in the transmission system. In this paper, a corrugated TE11-HE11 mode converter and a three-port directional coupler realized by its inner cylindrical wire array are proposed for a 330 GHz/500 MHz DNP-NMR system. The output mode of the mode converter presents a characteristic of highly linear polarization, which is 98.8% at 330 GHz for subsequent low loss transmission. The designed three-port directional coupler can produce approximately −33 dB electromagnetic wave power on port 3 in the frequency range between 300–360 GHz stably, which can be used to measure the electromagnetic wave power of the transmission line in real-time. The designed mode converter and direction coupler can be installed and replaced easily in the corrugated waveguide transmission system.
Highlights
Nuclear magnetic resonance (NMR) technology is a spectrum measurement method, which is widely used in the fields of biomedicine and materials science [1]
THz transmission system proves to be a key factor for the successful application of the whole dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP-NMR) system
We modeled and simulated the three-port directional coupler in CST Studio to verify the performance
Summary
Nuclear magnetic resonance (NMR) technology is a spectrum measurement method, which is widely used in the fields of biomedicine and materials science [1]. The high-order mode generated by the THz wave source is converted into the low-order circular waveguide TE11 mode through the mode converter. Due to the requirements of low heat dissipation and high-power capacity in practical applications, a linear coupling holes array was proposed for the 140 GHz transmission line in Frascati-Tokamak [17]. A three-port directional coupler is designed based on the cylindrical wire array and the overmoded corrugated waveguide theory in this paper. The transmission system designed in this paper is aimed to transmit 330 GHz EM waves generated by THz source to the NMR spectrometer which includes a TE11 -HE11 converter, corrugated waveguide, three-port directional coupler, and 90◦ miter bend. 1 THz wave source, 2 TE11 HE11 mode converter, 3 circular corrugated waveguide, 4 direction coupler, 5 90◦ miter bend,.
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