Fast switching coil system for sample premagnetization in an unshielded ultra-low-field-NMR experiment

We present a system developed to premagnetize liquid samples in an ultra-low-field NMR experiment. Liquid samples of a few milliliter are exposed to a magnetic field of about 70~mT, which is abruptely switched off, so to leave a transverse microtesla field where nuclei start precessing. An accurate characterization of the transients and intermediate field level enables a reliable operation of the detection system, which is based on an optical magnetometer.