Longitudinal dynamics study and optimization in the beam commissioning of the rapid cycling synchrotron in the China Spallation Neutron Source

The China Spallation Neutron Source (CSNS) is a high intensity proton accelerator-based facility, and its accelerator complex includes two main parts: a H-linac and a rapid cycling synchrotron (RCS). The RCS accumulates the 80 MeV proton beam, and accelerates it to 1.6 GeV, with a repetition rate of 25 Hz. The beam commissioning of RCS began in May, 2017 and reached the target beam power of 100 kW in February, 2020. The longitudinal beam dynamics study plays the key role in the commissioning. The study and optimization include the synchronicity between the ramping magnet field and the RF system, the mitigation of the high intensity effects to minimize the beam loss and the measurement of the longitudinal dynamics. Through the optimization of the bunch factor, the strong space charge effects with high intensity is mitigated, thus the RCS transmission efficiency arrives mostly at 100 %. The beam distribution is not affected by the phase loop with a new algorithm and the beam loss is smaller than the original one. The beam commissioning in longitudinal plane is reviewed in the paper.