Universal active magnetic shielding & compensating system for NMR applications

GUILLERMO FORTE; LUIS CANALI; ESTEBAN ANOARDO

Aachen

Congreso; 6th Colloquium on Mobile Magnetic Resonance; 2006

Selected NMR applications may require the cancellation of time dependent magnetic fields within an exclusion volume usually containing the sample. A good example can be found in fast-field-cycling NMR experiments, where external magnetic field contributions must be compensated at the ULF (ultra low frequency) limit [1]. We present a system designed to compensate time dependent magnetic field components and gradients, operating at a maximum frequency of about 100Hz. The device is based on a special probe containing 10 Hall sensors, providing 6 inputs to the control system: magnetic field components and mean gradients along the three spatial directions (under the approximation of constant gradient). The control system itself was developed within PC environment. In a forthcoming step and after all relevant testing of the device will be closed, it will be transferred to an adequate hardware platform in order to pull-up the spped of the device. A set of 6 coils (four saddle-type and two Helmholtz-type) driven by 6 independent current sources are used to correct the field. Since the magnetic field generated by each compensating coil affects the lecture of Hall sensors in all directions, the control system should include a suitable treatment for a coupled multi-variable problem. Two different strategies were considered: in a first simple case, the transfer function of each coil was analyzed in order to introduce the corresponding corrections allowing controlling each variable independently. In order to gain efficiency, in a second step we directly tackled the coupled multi-variable problem.Trabajo presentado como poster.