Signal averaging in cryogenic fast-field-cycling NMR experiments

The spin-lattice relaxation time constant $T_1$ characterizes the equilibration of a spin system in a magnetic field with its environment, the lattice. Fast-field-cycling (FFC) relaxometry is dedicated to $T_1$ measurements at low field. $T_1$ frequently exhibits non-trivial field and temperature dependences, which give access to structural and dynamical information. Knowledge of $T_1$($B, T$) is also of central importance in nuclear spin hyperpolarization, where one often seeks to transfer spin polarized samples from the point of hyperpolarization to the point of detection. Here we present a new field control architecture for a unique FFC system which can probe relaxation properties between 0 and 2.5 T for temperatures from 300 down to 3 K. The field-profile is now defined directly by the NMR pulse sequence. PID control of the field improves measurement repeatability and reduces settling times, giving access to relaxation time constants down to 100 ms. The feedback control also decreases field errors to a degree that enables signal averaging and thereby the measurement of relaxation properties with improved sensitivity.