Design of step field magnets for accelerator driven sub-critical system

In order to extend the life span of the proton beam target for the accelerator driven sub-critical system (ADS), a design scheme based on the step field magnets (SFMs) was used to expand the proton beams both along horizontal and vertical directions. 3D finite element method was used to simulate one single SFM and the two adjacent SFMs to get the expected step field. With the aid of the 3D field data, the effects on proton beams were investigated with beam tracking. Furthermore, a kind of compact SFM providing more obvious step field was illustrated. (authors)