Investigation of intrinsic aberrations introduced by the fringing field in the proposed 300 MeV proton microprobe in Harbin

In Harbin, the 300 MeV proton microprobe system with long-length quadrupole lenses, as a component of the Space Environment Simulation Research Infrastructure (SESRI), is under development driven by many applications in space science and proton therapy. Significant challenges have previously been addressed including minimization of ion scattering from collimators and the development of promising focusing systems compatible with the large beam magnetic rigidity. Here the intrinsic aberrations contributed by the fringe field regions of quadrupole lenses in the probe forming lens system are investigated with respect to the model used for the fringe field profile. The investigation compares the rectangular model, commonly used with the matrix method, and a more sophisticated model employing a multiphysics code for the full field profile used with a numerical raytracing method. For computational and constructional efficiency the proposed triplet system of long-length quadruple lenses required to achieve strong focusing was split into three pairs of two identical short quadrupole lenses, forming a sextuplet microprobe system. The comparison of the two fringe field models show large variations in the aberration coefficients of this system, however the rectangular model is suitable for a broad survey of possible systems to identify promising configurations for further optimization.