Simplified THz Instrumentation for High-Field DNP-NMR Spectroscopy.

We present an alternate simplified concept to irradiate a nuclear magnetic resonance sample with terahertz (THz) radiation for dynamic nuclear polarization (DNP) experiments using the TE(01) circular waveguide mode for transmission of the THz power and the illumination of the DNP sample by either the TE(01) or TE(11) mode. Using finite element method and 3D electromagnetic simulations we demonstrate that the average value of the transverse magnetic field induced by the THz radiation and responsible for the DNP effect using the TE(11) or the TE(01) mode are comparable to that generated by the HE(11) mode and a corrugated waveguide. The choice of the TE(11)/TE(01) mode allows the use of a smooth-walled, oversized waveguide that is easier to fabricate and less expensive than a corrugated waveguide required for transmission of the HE(11) mode. Also, the choice of the TE(01) mode can lead to a simplification of gyrotron oscillators that operate in the TE(0n) mode, by employing an on-axis rippled-wall mode converter to convert the TE(0n) mode into the TE(01) mode either inside or outside of the gyrotron tube. These novel concepts will lead to a significant simplification of the gyrotron, the transmission line and the THz coupler, which are the three main components of a DNP system.