  Short bunch proton beams are of great significance for the applications of
white neutron beams and muon beams. The accelerator complex of China Spallation
Neutron Source (CSNS) was designed to support the applications mainly based on
neutron scattering techniques where the proton pulse length is not very
sensitive. Some theoretical and experimental studies have been performed to see
if one can extract a short-bunch proton beam by bunch rotation from the rapid
cycling synchrotron (RCS) at CSNS. The experimental results at RCS have
evidently displayed the bunch lengthening and rotation process, which
demonstrates the effectiveness of this method even with a very short available
time for the RF gymnastic processes and a high-intensity beam. With a beam
power of 50 kW and normal longitudinal emittance at the injection, the proton
beam with a bunch length of about 53% with respect to the one in the normal
operation mode was obtained and transported to the spallation target. With a
reduced longitudinal emittance at injection and the beam power of 30 kW, the
shortest extraction bunch length obtained is about 26% of the one in the normal
operation mode. Different machine settings have also been tested to show the
impact of the desynchronization between the RF and magnetic fields, the
influence of the non-adiabatic risetime and the adiabatic decay time of the RF
voltage on the extraction bunch length. The experimental results are well
consistent with the theoretical and simulated ones. It is interesting to
observe that space charge has a beneficial effect on the bunch lengthening
which will result in a shorter bunch at the extraction with the later bunch
rotation. The controlled desynchronization method between the RF and magnetic
fields in an RCS was also proven successful.
