Effect of Magnetic Field Fluctuations in Spin-Echo NMR Experiments

Abstract

The effect of magnetic field drift and ripple on spin‐echo NMR experiments is discussed with emphasis on Carr‐Purcell spin‐echo (CPSE) experiments. It is shown that magnetic field fluctuations dephase the echo maxima with respect to the pulsed rf signal phase. This makes it difficult to use phase sensitive detection in the presence of a large field ripple even when the average field is very stable. The effect of ripple is particularly serious in CPSE experiments because the Meiboom‐Gill improvement of the CPSE sequence requires phase coherence of the echo signals. Closed equations are derived for the magnitude of the dephasing at a single echo and at the nth echo of a CPSE sequence, caused by linear magnetic field drift, sinusoidal ripple, and nonsinusoidal ripple. The results indicate that the effect of ripple is proportional to the ratio of its amplitude to its frequency, and that the effect on a CPSE sequence can be minimized by making the 180° pulse repetition rate large with respect to the ripple frequency. The results of various experimental tests are also given. They are in agreement with the theoretical predictions. The performance of several magnet systems is compared.