Design of a local quasi-distributed tuning and matching circuit for dissolution DNP cross polarization

Dynamic nuclear polarization (DNP) build-up times at low temperature for low-gamma nuclei can be unfavorably long and can be accelerated by transfer of polarization from protons. The efficiency of the cross polarization (CP) depends on the B₁-field strengths, the pulse sequence chosen for cross polarization and the sample composition. CP experiments rely on high B₁-fields, which typically lead to electrical discharge and breakdown in the circuit. This problem is particularly severe in the low pressure helium atmosphere due to easily ionized helium atoms. The purpose of this study is to identify strategies to minimize voltages across components in a tuning and matching circuit of the coil to avoid electrical discharge during CP experiments. Design equations for three tuning and matching network configurations are derived. The results of the study are then used in the design of a single coil double resonance DNP probe operating at 71.8 MHz (¹³C frequency) and 285.5 MHz (¹H frequency). In the current setup we achieve 28% polarization on ¹³C in urea with a build-up time of 11.6 min with CP compared to 14% and 53 min by direct polarization using TEMPOL as the radical. Different cross polarization sequences are compared.