Study of Coaxial HOM damper for an RF Damped Cavity

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A new type of HOM (Higher order mode)-damped cavity has been successfully developed by collaboration between ISSP of the University of Tokyo and the Photon Factory of KEK. The cavity can absorb HOM's by SiC beam ducts attached at both ends of the cavity. However, several modes, which have frequency lower than the cutoff of the beam duct, are trapped in the cavity and survive with high impedance. Frequency detuning of these trapped modes is one of proper methods to avoid the instability due to them. For a ring with large circumference, however, the method becomes less effective because of its low revolution frequency. In such a case, it is more preferable to reduce Q-values of the trapped modes.For this purpose, we have been developing a simple and compact-size HOM damper. The damper has a rod-shaped coupling antenna with a coaxial waveguide, A small SiC piece made of the same material as the absorber in the beam duct is mounted on the end of the waveguide in order to absorb the extracted HOM power.We fabricated two prototype models of the HOM damper and attached them to horizontal and vertical ports of the cavity. The RF characteristics were measured in low-power levels. In the measurement, it was confirmed that the HOM damper strongly coupled to six trapped modes and reduced their Q-values drastically without affecting the accelerating modes. We also carried out high-power test of the HOM dampers. Input power up to 60 kW was applied to the cavity successfully, and unusual heating was not observed on the HOM dampers.We will continue the high power test and try to increase the input power up to 100 kW. Furthermore, we will carry out actual beam test of the HOM damper at the Photon Factory storage ring in near future.