Optimum saddle coils for external magnetic filed compensation in field-cycling NMR

F. BONETTO; E. ANOARDO; M. POLELLO

Santos, Sao Paulo, Brasil

Congreso; XXVIII Encontro Nacional de Fisica da Materia Condensada; 2005

Sociedade Brasielira de Fisica

The magnetic field generated by a saddle coil system was exactly calculated in the whole space and experimentally tested in two different spacial 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 z_o=4R and f_o=Pi/3; where z_o, R and 2f_o 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. Deviations of the magnetic field produced by an optimum compensating saddle coil system werecontrasted with typical NMR magnet field inhomogeneities. Possible influences of such deviations were determined to be negligible in NMR experiments.