Design and Operation of the SPEAR II RF System

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Abstract
The RF accelerating voltage is provided by 4 accelerator sections in each of the 4 straight sections of the storage ring. Each accelerator section consists of 5 coupled cavities oper- ating in the s mode at the 280th harmonic of the ring fre- quency, 358.5 MHz. A schematic of an accelerator section is shown in Fig 1. The shunt impedance is approximately 18 megohms (V2/2P). The cavity is constructed from 6061 aluminum alloy. The end walls of the cavities are machined from forgings. Coupling is provided by azimuthal slots in the common-end walls and cooling is provided by means of radial cooling channels bored in these forgings close to the nose cones. Each section is designed to dissipate over 75 kW of RF power. The outer shells were rolled from alumi- num sheets and their cooling is provided by channels welded externally. Each cavity is provided with a water-cooled tuner with a tuning range of 1.2 MHz. All 5 tuners are ganged together and driven by a single motor. Coupling of RF po\ver is by means of a water-cooled loop in the center cavity. Power is fed to the coax loop from WR 2100 wave- guide through a coax to waveguide transition which consists of a cylindrical ceramic window as shown in Fig. 1. This window was developed for the high power klystron. 3 During initial testing of the accelerator section, severe multi- pactoring problems were encountered. The cavity was then coated with titanium nitride. A layer between 100 and 1000 A was applied by evaporating a

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