Pre-polarization fields for earth’s field NMR: Fast discharge for use with short T1 and large coils

[Display omitted] •Built high power pre-polarization switch for earth’s field NMR with a large surface coil.•Suitable for use with short T1 fluids such as oil, by rapid discharge of the pre-polarization field.•The discharge is softened so the spin magnetization can adiabatically follow the field. The sensitivity of earth’s field NMR is greatly increased by the use of a pre-polarizing field Bp. When used with short T1 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 Bp 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.