Ego, H. Asaka, T. Inagaki, T. Nishimori, N. Ohshima, T. Tanaka, H. Tomai, T. Yamaguchi, H. JACoW Publishing Geneva, Switzerland 2673-7035 978-3-95450-224-0 10.18429/JACoW-FLS2023-TU3D4 English 74-76 cavity HOM damping impedance operation Contribution to a conference proceedings 2024 2024-01 https://doi.org/10.18429/JACoW-FLS2023-TU3D4 http://jacow.org/fls2023/papers/tu3d4.pdf A beam-accelerating RF cavity with a new HOM-damping structure was designed in order to suppress coupled-bunch instabilities in a next generation light source with an ultra-low emittance and supplying X-rays approaching their diffraction limits. The TM020 mode at 509 MHz is selected as a beam-accelerating mode because it has a high Q-value of 60,000 and a shunt impedance sufficient for beam acceleration and brings a compact HOM-damping structure to the cavity differently from massive types of cavities with waveguides or pipes extracting HOM power. Two shallow slots are cut on the cavity inner-wall and materials absorbing RF waves are directly fitted into them. They work as HOM dampers without affecting the RF properties of the beam-accelerating mode. A prototype cavity of OFHC copper was fabricated to demonstrate the HOM-damping and generating an accelerating voltage of 900 kV in the cavity. Since the cavity was successful in operation up to 135 kW, the feasibility of both the high-power operation and the damping structure was proved. Four actual cavities were produced and installed to the new 3-GeV synchrotron radiation facility, NanoTerasu in Japan.