The development and beam test of an RF chopper system for J-PARC

The linac of the Japan Proton Accelerator Research Complex (J-PARC) is now under construction. It is a high-intensity linac with a peak H− current of 50 mA and a 1.25% duty factor. As a key component of the medium-energy beam-transport line of the linac, a chopper is necessary to obtain the required beam structure for the following 3-GeV rapid-cycling synchrotron, and also plays an important role in beam-loss control in such a high-intensity accelerator. Clean chopping requires a short rise time and a high deflecting field. An radio frequency deflector (RFD) was proposed as a chopper for the J-PARC linac, because of its merits of high deflecting field, compact structure, ease to manufacture and both reliability and flexibility of the total chopper operation. An RF chopper, consisting of two RFDs, has been developed. In a low-level RF test, the fundamental RF properties show good agreement with those of a simulation in the design. In a high-power test, the chopper works well without any discharge under 36 kW peak driving power. In a test of the coupled RFD system, the longer rise time of the second RFD and the delay of the second RFD excitation were observed. A beam test of the RF chopper was also successfully performed up to a peak current of 25 mA. The chopper system worked very stable during the beam test, and the sufficient separation between chopped and normal beam was obtained, with a short rise/fall time (10 ns) of the chopped beam.