Here are the most important publications describing the computation engine built in Radia.
P. Elleaume, O. Chubar, J. Chavanne, "Computing 3D Magnetic Field from Insertion Devices", proc. of the PAC97 Conference May 1997, p.3509-3511.
O. Chubar, P. Elleaume, J. Chavanne, "A 3D Magnetostatics Computer Code for Insertion devices", SRI97 Conference August 1997, J. Synchrotron Rad. (1998). 5, 481-484