Beam diagnostics using a chopped beam

A new type of beam chopper has been developed and installed at HIMAC (Heavy Ion Medical Accelerator in Chiba) in NIRS (National Institute of Radiological Sciences). The chopper has only 11.6 cm length and has been set between the ion sources and the RFQ. It is similar to a beam transformer, and modulates the beam kinetic energy by 10%. The beam is successfully chopped because the RFQ has a rather small energy acceptance. A chopped beam was injected into the synchrotron and a beam study has been performed. A very short bunch was formed and accelerated to the top energy. The behavior of the short bunch shows that a dipole oscillation was excited because of an unexpected frequency error of the RF system. It was caused by noise in the B-clock (Gauss Clock) circuit. The dipole oscillation caused by the frequency error was significantly reduced and the acceleration efficiency was improved by replacing the B-clock module.