Assembly and performance of a 6.4T cryogen-free dynamic nuclear polarization system

We report on the assembly and performance evaluation of a 180-GHz/6.4T dynamic nuclear polarization (DNP) system based on a cryogen-free superconducting magnet. The DNP system utilizes a variable-field superconducting magnet that can be ramped up to 9T and equipped with cryocoolers that can cool the sample space with the DNP assembly down to 1.8K via the Joule-Thomson effect. A homebuilt DNP probe insert with top-tuned nuclear magnetic resonance coil and microwave port was incorporated into the sample space in which the effective sample temperature is approximately 1.9K when a 180-GHz microwave source is on during DNP operation. C-13 DNP of [1-C-13] acetate samples doped with trityl OX063 and 4-oxo-TEMPO in this system have resulted in solid-state C-13 polarization levels of 58 +/- 3% and 18 +/- 2%, respectively. The relatively high C-13 polarization levels achieved in this work have demonstrated that the use of a cryogen-free superconducting magnet for C-13 DNP is feasible and in fact, relatively efficienta major leap to offset the high cost of liquid helium consumption in DNP experiments.