Analysis of the Magnetization-Induced Field Error in a Superconducting Bending Magnet for a Compact, Rapid-Cycling Heavy-Ion Synchrotron

An ongoing design study of a 90-degree superconducting bending magnet (SCBM) is being conducted for a compact, rapid-cycling heavy-ion synchrotron at the National Institutes for Quantum Science and Technology (QST). The SCBM is designed to generate a 3.5 T dipole field and a 1.5 T $\cdot$ m $^{-1}$ quadrupole field for bending and horizontally defocusing heavy-ion beams. Since a heavy-ion beam is injected into the synchrotron at the field of 0.3 T, and accelerated to 3.5 T in 5 s, and then it is extracted at the field range from 1.1 T to 3.5 T, field errors derived from the magnetization at such a ramp rate will cause instability of the beam during the acceleration. In this paper, the field errors derived from both the DC and the AC effects are analyzed with numerical models in two dimensions (2D). The DC effects from the screening current in the superconducting coil and iron hysteresis behavior are estimated by the nested ellipse model and the finite element method (FEM) coupled with an inverse Jiles-Atherton model. The AC effects from the eddy current generated in the thermal shield, the beam pipe and the thermal path are estimated by a transient FEM analysis and analytical method. The change of the field error due to the magnetization is estimated to be about 1 unit at the time of beam injection.