Pre-polarization fields for earth's field NMR: Fast discharge for use with short T 1 and large coils

The sensitivity of earth's field NMR is greatly increased by the use of a pre-polarizing field B . When used with short T samples, the field must be decreased rapidly to avoid loss of the pre-polarized magnetization by relaxation. Such a rapid decrease in the field requires rapid discharge (∼10ms) of a large stored magnetic field energy (∼700J). In addition, in order that the full pre-polarized magnetization be available for the subsequent pulse sequence, the field discharge should be adiabatic. This requirement is difficult to fulfill in cases where B is not everywhere parallel to the earth's field, such as with a large surface coil. Circuitry for rapid and controlled discharge is presented. Simulations and experiments confirm the importance of both of these conditions.