Tune optimization for alleviating space charge effects and suppressing beam instability at the RCS of CSNS

The design betatron tune of the Rapid Cycling Synchrotron (RCS) of China Spallation Neutron Source (CSNS) is (4.86, 4.80), which allows for incoherent tune shifts to avoid serious systematic betatron resonances. When the operational bare tune was set at the design value, serious beam instability in the horizontal plane and beam loss induced by half-integer resonance in the vertical plane under space charge detuning were observed. Simulations and experiments have shown that space charge-induced beam loss reduces as the tunes move up and away from half-integer resonance lines. However, experimental observations have shown that instability growth rates increase rapidly as the tune approaches integer from below. The tune requirements for reducing the beam loss caused by space charge effects and suppressing beam instability are different at the RCS of CSNS. The tunes over the whole acceleration process are optimized based on space charge effects and beam instability. The optimized tune pattern was able to well control the beam loss induced by space charge and beam instability. The beam power of CSNS achieved the design value of 100 kW with small beam loss.