Operational characteristics of RF system affecting PLS beam stability

The fourth RF station was installed to the Pohang Light Source (PLS), whereas its station consists of a 60 kW klystron amplifier and a nose cone type cavity connected by a coaxial transmission line. Total 240 kW RF power is available for storing electrons up to 400-mA at 2-GeV and 200-mA at 2.5-GeV of beam energy. During the normal operation, some dangerous higher order modes (HOMs) induced by cavities have been identified. Making an effort to suppress instabilities caused by HOMs has been started by enhancing the capability of cavity cooling temperature control and adding the transverse and longitudinal feedback systems. Besides these efforts, characterizations of the beam loss transient due to RF have been underway. A modulation experiment of the accelerating voltage is also planned using the fourth cavity. We present performance test results with newly installed low level system.