Extraction of 3D field maps of magnetic multipoles from 2D surface measurements with applications to the optics calculations of the large-acceptance superconducting fragment separator BigRIPS

摘要

The fringing fields of magnets with large apertures and short lengths greatlyaffect ion-optical calculations. In particular, for a high magnetic field wherethe iron core becomes saturated, the effective lengths and shapes of the fielddistribution must be considered because they change with the excitationcurrent. Precise measurement of the three-dimensional magnetic fields and thecorrect application of parameters in the ion-optical calculations arenecessary. First we present a practical numerical method of extracting full 3Dmagnetic field maps of magnetic multipoles from 2D field measurements of thesurface of a cylinder. Using this novel method we extracted the distributionsalong the beam axis for the coefficient of the first-order quadrupolecomponent, which is the leading term of the quadrupole components in themultipole expansion of magnetic fields and proportional to the distance fromthe axis. Higher order components of the 3D magnetic field can be extractedfrom the leading term via recursion relations. The measurements were done formany excitation current values for the large-aperture superconducting tripletquadrupole magnets (STQs) in the BigRIPS fragment separator at the RIKENNishina Center RI Beam Factory. These distributions were parameterized usingthe Enge functions to fit the fringe field shapes at all excitation currentvalues, so that unmeasured values are interpolated. The extracted distributionsdepend only on the position along the beam axis, and thus the measuredthree-dimensional field can easily be parameterized for ion-opticalcalculations. We implemented these parameters in the ion-optical calculationcode COSY INFINITY and realized a first-order calculation that incorporates theeffect of large and varying fringe fields more accurately. We applied thecalculation to determine the excitation current settings of the STQs...