Current injection for field decay compensation in NMR spectrometer magnets

Current injection is a method to compensate for field drift in persistent nuclear magnetic resonance spectrometer magnets by the direct introduction of incremental current into the magnet through leads attached across a portion of the windings. An applied current ramp will distribute among the sections of the windings defined by the placement of the leads according to the inductance of the circuit. As a result, a current ramp trough leads placed across an inner coil will flow primarily in that coil. The field created by the injection current may be used to cancel a quasi-persistent field decay. The injection current required for the field decay compensation is determined as a function of magnet parameters and decay rate. The drift in field uniformity that results with current injection is found to be proportional to the field decay that is compensated, the drift and the decay being related by a quantity termed the dynamic uniformity; The field uniformity drift is quantified for an example magnet design; showing that the drift in field uniformity can be significant in the context of high resolution spectroscopy. Methods that facilitate shimming, such as gradient shimming, are identified as a natural compliment to current injection.