Multipole analysis for absolute magnetic field measured by multi-probe Pulsed-NMR method

For accelerator dipole magnets, the study on multipole coefficients in a lo cal region is useful for understanding magnet characteristics and checking construction procedure. The multi-probe pulsed-NMR-method is a candidate fo r measurement of the local multipole coefficients. For the absolute fields measured by the multi-probe pulsed-MMR method, a rigorous application of th e Fourier expansion is found to yield multipole coefficients. It is made cl ear that the direction of the dipole field is unable to be determined by th e Fourier expansion alone. For estimating alignment accuracy of pulsed-NMR probes, the relationship between errors of multipole coefficients and those of probe position is derived by the Fourier expansion method. For applying of multi-probe pulsed-NMR method to accelerator dipole magnets, we designe d and fabricated a field measuring system using two probes. The probes had NMR coils wound around glass ampuls with a diameter of about 1.4 mm. The NM R signal was processed with a phase lock loop circuit to produce the NMR fr equency. Dipole field uniformity and multipole coefficients were measured a t 0.35 T for a 1.5 m long dipole magnet. It was confirmed that the field da ta by this MMR system had an accuracy of 10(-6).