Design of the experimental setup for Overhauser dynamic nuclear polarization in stray field of a superconducting magnet

We designed and constructed a low-field setup for the study of nuclear magnetic resonance (NMR) and Overhauser dynamic nuclear polarization (DNP) in the shimmed stray magnetic field of a superconducting magnet with a variable magnetic field (0-8.5 T). The experimental configuration is capable of exploring magnetic fields in the range of 0.5-30 mT by displacing the NMR/DNP probe from the superconducting magnet. Thanks to a constructed 3D gradient system comprising three coils wound on 3D-printed plastic holders, sufficient field homogeneity can be achieved (325 ppm in 0.7 ml volume, 98-fold homogeneity improvement) for strongly DNP-enhanced NMR signals or for measurements in concentrated solutions and porous media with broad NMR spectra. This setup allows us to study the magnetic field dependence of the Overhauser effect and underlying physical processes in aqueous solutions of radicals with an easy-to-built, low-cost, and low-power consumption magnet system. The design parameters of the gradient coils generating linear magnetic field gradients were optimized using the downhill simplex method in conjunction with magnetic field computations using Magpylib (a Python package). The NMR/DNP excitation chain allows for the use of a single broadband power amplifier to excite both nuclear and electronic spins and incorporates a custom-built NMR spectrometer for the generation and control of NMR and electron saturation pulses. In addition, we present a two-stage design for an NMR preamplifier, achieving a total gain of 68 dB, which combines a low-noise bipolar MMIC amplifier INA-02186 with an AD797 operational amplifier, enabling the detection of NMR signals over a frequency range of 30-1200 kHz. DNP tests performed using a TEMPOL solution in water at a field strength of 4 mT demonstrate the efficacy of the assembled system for studying the DNP effect at low magnetic fields with high sensitivity.