A.S. Smygacheva - Post-graduate Student, National Research Center «Kurchatov Institute». E-mail: sasmyga@mail.ru V.N. Korchuganov - Dr.Sc. (Phys.-Math.), National Research Center «Kurchatov Institute». E-mail: vnkorchuganov@mail.ru

A dedicated synchrotron radiation source «Siberia-2» is operated in the NRC «Kurchatov Institute». The maximum particle energy is 2.5 GeV and a beam current is about 0.2 A. In 2009 the RF system modernization was provided at the storage ring «Siberia-2». As result, two broken-down cavities were replaced by three new cavities. A scheme of a parallel power feeding two cavities from one RF generator introduced some features into the RF system operation and the beam dynamics. The RF system of the storage ring «Siberia-2» consists of two independent RF sections, and its operating frequency is 181.14 MHz. Each section includes the RF generator, a rectangular waveguide, a coaxial line and the cavity. The first RF generator power is fed to a separate cavity. The second RF generator power is fed through a six-terminal structure to two coupled cavities spaced apart at the RF wavelength in direction of the beam motion. There is its own feedback system for each section of the RF system. It keeps a constant value of a voltage on the separate cavity and of a total voltage on two coupled cavities. Also specified phase between the cavity voltage and corresponding current of the coaxial line is maintained constant for each cavity. The stable phase is needed to provide for the stability of the beam motion. It keeps due to the cavity mode frequency shifts to a lower frequency with regard to the operating frequency. We observe a different behavior of each coupled cavity during a beam injection. While the specified total voltage of coupled cavities is fixed the voltage on the first cavity is decreasing during storing the beam current, at the same time the voltage on the second cavity is increasing. In addition, the mode frequency shift of the first cavity to the lowest frequency is faster than the one of the second cavity. To understand this behavior a theoretical model of coupled cavities was developed. It is based on a transmission line theory and multi-port theory. The model takes into account the features of the six-terminal structure and the coaxial line, the differences between shunt impedances and turn ratios of two cavities. As a result of the voltage-current curve analysis the observed changes of cavity voltages are predefined by a principle of power division through the six-terminal structure. It does imply that the voltage dependence on the beam current is not eliminated in reality. For the storage ring «Siberia-2» the effective decision of the problem could become the separation of coupled cavities, that is, the installation of the third RF generator and independent feedback system for each cavity.

 

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