An adjustable permanent magnet focussing system for heavy ion beams

A system of permanent-magnet quadrupoles and dipoles is described for transporting a heavy ion beam of the Laboratory´s K800 superconducting cyclotrons from the extraction system exit, through the return yoke, to the beginning of the external beamline. After a brief summary of the beam line layout, the authors discuss tests carried out on six candidate materials (Sm₁Co₅, Sm₂Co₁₇, NdFeB, and three types of NdFeB with various additives used to increase the coercivity) that resulted in the choice of SmCo permanent magnet material. The beam optics utilize adjustable quadrupoles and dipoles constructed from rings of permanent magnet material. To adjust the strength, the rings are rotated about the beam axis in opposite directions. Ray-tracing calculation results confirm the absence of x and y phase space mixing.