Contribution of iron yoke on helical coils [for RHIC]

In order to estimate the field contribution due to an axially symmetric iron yoke for a helical magnet, a three-dimensional magnetic scalar potential problem with helical symmetry is solved. It is confirmed that the asymptotic forms for potential and field coincide with those for the two-dimensional magnet, in the limit of large twist pitch length. Then, it is also confirmed that the obtained analytical expression for the magnetic field is consistent with the numerical field calculation.