Analysis of uniformity in magnetic fields generated by saddle compensating coils for field cycling NMR

F. BONETTO; E. ANOARDO; M. POLELLO

Torino, Italia

Conferencia; 4th Conference on Field Cycling NMR Relaxometry; 2005

Universidad de Torino, Italia

Commonly, in NMR relaxometry experiments, external uniform magnetic fields must be compensated. Thus, the three components of the magnetic field generated by the implemented compensating system should be as uniform as possible.  Saddle shaped coils have been widely used in the past to generate uniform magnetic fields and magnetic field gradients in NMR experiments.  In the present work the magnetic field generated by a saddle coil system was exactly calculated in the whole space and experimentally tested in two different spatial planes (central and top planes) of the coil. The uniformity of the three components of the magnetic field was separately studied using Taylor series expansions around the geometric center of the coil. The optimum geometry was determined taking in consideration the first two terms of the Taylor expansions. It was found that the maximum uniformity of the field produced by the saddle coil system is achieved when zo=4R and öo=ð/3 [D. M. Ginsberg and M. J. Melchner, Rev. Sci. Instr. 1, 122 (1970). ]; where zo, R and 2öo are the height, radius and the aperture angle of the coil, respectively. The fourth term of the Taylor expansion was calculated in order to obtain an approximated manipulable analytical expression of the magnetic field within the sample. Fourth order Taylor series and exact expressions were compared in different regions of the coil. A noticeable agreement between them (differences minor than 1.5 %) was encountered when distances from the center of the coil do not exceed R/2. Therefore, the magnetic field in the central zone of the saddle coil system (usually occupied by the sample) can be easily and accurately obtained with a pocket calculator via the fourth order Taylor expansion. Deviations of the magnetic field produced by an optimum compensating saddle coil system were contrasted with typical NMR magnet field inhomogeneities. Possible influences of such deviations were determined to be negligible in NMR experiments.