Innovative microwave transmission line for DNP-NMR experiments

see uploaded PDF: Performing Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance (DNP-NMR) requires low-loss transmission lines (TL) for efficient transfer of the microwave excitation signal between source and spectrometer. TLs based on traditional technologies often suffer from significant losses, thus limiting the DNP enhancement and making costly sources necessary. We present an innovative 4-meter long low-loss TL based on a circular corrugated waveguide (see Fig. 1). A novel modular design enables the TL to fit different experimental set-ups. As part of a new set-up recently installed at EPFL, it currently connects a custom-made frequency-tunable Gyrotron source working at ~263 GHz with an NMR spectrometer for DNP-NMR experiments at 9.4 T. This state-of-the-art DNP-NMR system is dedicated to the development of a new family of planar DNP- NMR probes to study liquids and surfaces at room temperature. Furthermore, the TL comprises a universal matching unit (MOU) as well as a full titanium probe waveguide. The probe waveguide has a fast connection system allowing for an easy manipulation of the probe in and out of the spectrometer. In this work we present the experimental characterization of the TL on a THz test bench for DNP-NMR hardware development. The TL losses as well as the mode content are reported. Moreover, we show how the MOU can be used to fully control the polarization of the microwave beam. In particular, we demonstrate that complete circular polarization can be achieved, which is crucial to make full use of the microwave power to couple to the spins. Fig. 1: (a) Waveguide connected to Gyrotron output window via MOU, where (i) is a flat metallic mirror for beam steering, (ii) and (iv) are corrugated mirrors for polarization control and (iii) is a concave mirror for beam shaping. (b) Installed waveguide viewed from the Gyrotron source, with NMR spectrometer in the background.