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

We report on the assembly and performance evaluation of a 180-GHz/6.4 T 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 9 T and equipped with cryocoolers that can cool the sample space with the DNP assembly down to 1.8 K 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 1.9 K when a 180-GHz microwave source is on during DNP operation. ¹³ C DNP of [1-¹³ C] acetate samples doped with trityl OX063 and 4-oxo-TEMPO in this system have resulted in solid-state ¹³ C polarization levels of 58 ± 3% and 18 ± 2%, respectively. The relatively high ¹³ C polarization levels achieved in this work have demonstrated that the use of a cryogen-free superconducting magnet for ¹³ C DNP is feasible and in fact, relatively efficient-a major leap to offset the high cost of liquid helium consumption in DNP experiments.