Damping Test of the Higher-Order Modes of the Re-Entrant Accelerating Cavity

Higher-order modes were studied in a model cavity (500 MHz re-entrant single cell cavity) which was designed for the KEK-PF 2.5 GeV electron storage ring. The longitudinal electric field and the transverse electric and magnetic fields were measured near the beam axis for the higher-order modes. The observed parameters of the TMO-mode resonances were in reasonable agreement with the SUPERFISH calculation. The TM011-like resonance at 758 MHz has the highest shunt impedance of 3.0 M¿. The observed transverse coupling impedances of the HEMl modes at 829 MHz (TM110-like) and 1071 MHz (TN111-like) were 12 N¿/m and 27 M¿/m, respectively. Damping couplers are being developed to prevent the cavities from driving the coupled-bunch instabilities. The impedances were reduced to less than 0.01 M¿ and 0.2 M¿/m for almost all of the TM0- and HEMl-mode resonances, respectively, in a low power test.