Development of a small-probe pulsed nuclear magnetic resonance system for measuring magnetic field uniformity

The pulsed nuclear magnetic resonance (NMR) field measurement system is suitable for high-accuracy field homogeneity measurement for accelerator dipole magnets. The measurement has to use NMR probes as small as a few mm in diameter. The coaxial cable connecting the NMR probe and preamplifier has a length of 10 m or more. The coaxial cable has to be treated as a distributed-constant circuit for high-field measurement. We studied the optimum condition of the radio-frequency signal propagation in the coaxial cable, and found that a cable length of nλ_NMR/2 (n = 1,2,...) gives optimum propagation characteristics. In accordance with this result, we designed and fabricated a prototype field measurement system using pulsed NMR. The use of a phase-locked loop circuit facilitated the measurement of the frequency of the free-induction-decay signal. To confirm the performance of this system, the magnetic field in the 2 T region of the superconducting solenoid magnet was measured with the NMR probe of which the shape was a cylinder of 4 mm in diameter.