Design and experiment results of the LEBT pre-chopper for CSNS

In a high intensity proton accelerator complex comprising a linac and a ring, a beam chopper is often necessary in order to reduce the beam loss during injection from the linac to the ring. The China Spallation Neutron Source (CSNS) front end incorporates a pre-chopper in the Low Energy Beam Transport line (LEBT) that will remove a 530 ns section of beam at approximately 1 MHz rate. Physical design of the pre-chopper is presented in this paper. The distinguished feature of this electrostatic beam chopper is the novel design of deflecting structure, and the theoretic fast beam rise/fall time, which is an essential requirement for CSNS accelerator to avoid radioactivity induced by the lost particles due to insufficient chopping during the transient time; Taking advantage of the existing LEBT of the Accelerator Driven Subcritical System (ADS), the beam study of a similar chopper system was successfully performed and the achieved rise/fall time was less than 20 ns, which is the fastest one compared to other similar pre-choppers. This type of pre-chopper will be used in the CSNS LEBT, which is under construction.