A Temperature Stable Inside-Out NMR Sensor

Inside-out nuclear magnetic resonance (NMR) sensors provide a practical way to conduct porous medium materials measurement. However, they usually suffer from poor temperature stability, thus limit their applications outside the temperature controlled environment. This article describes the design and implement of an inside-out NMR sensor useful to overcome the sensitive volume fluctuation caused by temperature variation. The use of a ring made of a low permeability ferrimagnetic material (LPFM) simultaneously decrease the inhomogeneity along the axial direction, adjust the static gradient $G_{0}$ and enhance the strength of radio frequency (RF) field $B_{1}$ . In this way, the sensor can maintain the shape and position of the sensitive volume to achieve stable measuring capability under fluctuating working temperature. The uniform design (UD) approach is applied to obtain the optimal structure of magnet and RF coil. The performance of the LPFM-based sensor in terms of field distribution, coil efficiency, and sensitivity map is evaluated and compared with the structure without the ring based on finite element method (FEM) simulation. When the working temperature ranging from $10~^{\circ} \text {C}$ to $50~^{\circ} \text {C}$ , the sensitive volume of the LPFM-based sensor maintains the shape of a toroidal crescent with an axial extension of $10{-}14\,\,\text {mm}$ and a radial extension of $3{-}4\,\,\text {mm}$ . Moreover, the position of the sensitive volume is stably located in $\sim 5~\text {mm}$ from the magnet surface without being affected by working temperature. Then, we built a prototype sensor to conduct the $T_{2}$ relaxation experiments on $\text {CuSO}_{4}\cdot \text {H}_{2}\text {O}$ samples. The results demonstrate a stable NMR signal strength over a working temperature range of $10\,\,^{\circ} \text {C}{-}50\,\,^{\circ} \text {C}$ . The proposed NMR sensor construction can be applied to borehole logging and soil moisture measurements.