Physical design of the APEP beam line at CSNS

The Associated Proton beam Experiment Platform (APEP) beam line that is under construction at the China Spallation Neutron Source (CSNS) linear accelerator (linac) will begin commissioning in 2022, eventually providing nearly 4800 h of proton beam time every year. It will be the first proton irradiation facility to use naturally-stripped protons - H- beams from the linac are stripped by the residual gas in the beampipe. A number of APEP applications have been proposed, including proton irradiation experiments. We completed the design of the beam line - including proton beam transport, collimation, energy degradation, and shielding - based on the requirements developed in test experiments. Both the proton beam spot sizes and the proton energy can be adjusted continuously across a broad range. The background and residual radiation dose rates around the beam line are maintained at a low level.