Low-loss Bi-directional THz Couplers to Enable Affordable MAS-DNP-NMR

Low-loss Bi-directional THz Couplers to Enable Affordable MAS-DNP-NMR Abstract The critical importance of solid-state NMR (ssNMR) was demonstrated by yielding the first atomic-resolution structures of the A40 and A42 amyloid fibrils that play a crucial role in Alzheimer’s Disease (AD). Key to that structure determination was a technique denoted as dynamic nuclear polarization (DNP) with magic angle spinning (MAS). This and many other examples show that developing transformational advances for ssNMR is crucial for both structural biology and biomedical research in general, and for progress in curing Alzheimer’s Disease and cancer. High-field (>11 T) MAS-DNP systems thus far have all required the level of microwave power that is only avail- able from gyrotrons. Gyrotrons have a narrow bandwidth. As a result, MAS-DNP then requires an NMR mag- net with superconducting sweep coils. Thus, directly, and indirectly, gyrotrons add $2-5M to the system cost. Moreover, the DNP methods are limited by the poor frequency agility of gyrotrons, and many crowded NMR laboratories simply do not have space to accommodate a gyrotron. Only 2-9% of the incident microwave power is dissipated within the lossy sample in the best published MAS- DNP designs thus far, but substantial improvements will be possible with novel overmoded cavities compatible with MAS-DNP that are under development. Such, however, would require a method of adjusting the THz tun- ing and matching to minimize the reflected wave, which otherwise could quickly destroy the solid-state micro- wave source, which though only a few percent the cost of a gyrotron is still expensive. Tuning and matching a THz cavity in a cold MAS-DNP probe requires a low-loss high-directivity bi-directional overmoded THz coupler, and such are not available. Available bi-directional couplers have 6 dB loss at 328 GHz. Using such would mean throwing away 75% of available microwave power, which would render high-field DNP impossible with solid-state sources for the foreseeable future. Preliminary simulations of a novel design indicate bi-directional THz couplers with under 0.8 dB loss should be possible. This proposed Phase I SBIR will begin with in silico optimization of novel low-loss robust overmoded THz bi- directional couplers at the frequencies required for DNP up to at least 18.8 T. Prototypes will be manufactured using novel methods involving 3D printing followed by precision machining and by gold plating onto plastics. The prototypes will be bench tested over bandwidths centered about their respective design frequencies. A bi- directional coupler for 328 GHz, driven by a 90-mW 328-GHz source, will be tested on an ultra-low- temperature (ULT) DNP probe that is under development under a separate effort. It is expected that overmod- ed bi-directional THz couplers, in combination with several other technological advances being pursued in oth- er projects, will eventually enable MAS-DNP to be added to existing ssNMR high-field systems without the re- quirement of either a specialized magnet or a gyrotron and thus will be affordable.