A 400-mA Stable Beam Store with a Superconducting RF System at the PLS-II

Abstract

At Pohang Light Source (PLS)-II, superconducting RF cavities, which are used energy compensators and make up for the energy loss by emitting synchrotron radiation were developed for the high-brightness photon beam, which has a brightness two orders of magnitude higher than that of predecessor PLS because of a dramatic reduction in higher order modes. The initial beam current for beamline users’ experiments was 150 mA with one cavity, and it increased to 280 mA with two cavities and to 350 mA with three cavities. At beam currents above 320 mA a severe instability was caused by the coupler vacuum, which placed an operation limitation on the high-beam current. Through intensive study, the vacuum instability was avoided, resulting in a stable high beam current in both the decay mode and the top-up mode. The coupling between the forward power and the beam current will change depending on their intensities. When that happens, the positions of maximum induced voltage around the window would also change. The trend is for the position of the maximum induced voltage to approach the window and produce vacuum an instability when forward power is ramped up. When these revelations are considered, one can conclude that the improved relationship between the forward power and the beam current by adjusting the external Q value prevents the maximum induced voltage from occurring around window. Because of the finding of this study, stable operation with a high-beam was realized for as long as over 200 hours or more between faults.