Upgrade of the sextupole field for beam instability mitigation in rapid cycling synchrotron of China Spallation Neutron Source

Background The China Spallation Neutron Source incorporates a rapid cycling synchrotron (RCS) that operates by accumulating protons at 80 MeV and subsequently accelerating them to 1.6 GeV within a time span of 20 ms. The beam is guided to striking the tungsten target for neutron source.
Purpose As a space charge dominated machine, the RCS is subject to space charge effects and momentum spread, hereby influencing the tune spread. To address this issue, sextupole magnets, powered by two families of DC power supply, were initially employed to decrease the absolute value of chromaticity and to control the tune spread. The head-tail instability has been observed during the RCS beam commissioning.
Method The beam tests and simulations were conducted, revealing that tuning the chromaticity proved to be an effective mitigation strategy. However, to achieve better control over the tune spread and further suppress the instability, the DC sextupole field has been upgraded to an AC sextupole field, aiming to provide dynamic for controlling the chromaticity over an acceleration cycle.
Results and conclusion Thanks to the upgraded of AC field, the instability has been fully mitigatedwith beam power of 100 kW and the transmission in the RCS has been improved by ~ 2% from 96 to 98%. With help of AC sextupole at present, the beam power in the RCS is increased to 140 kW.