Experimental research on the longitudinal coupling impedance of RF-shielded BPM-bellows assemblies designed for the Hefei Advanced Light Facility

A compact beam position monitor bellows (BPM-bellows) assembly using the finger-type shield has been designed and manufactured for Hefei Advanced Light Facility (HALF). This RF shield is applied to screen the high resonance in the bellows volume. The coaxial wire method was chosen for the measurement of longitudinal coupling impedance, and the experimental setup was placed vertically to eliminate the sag effect. In order to overcome the precision limitation of the setup when measuring the small-aperture shielded bellows assembly, 5 bellows assemblies have been connected together as one device under test (DUT). The impedance of this long DUT can be conveniently tested. This 5-bellows DUT method has better repeatability than measuring one shielded bellows DUT. In addition, the 5-bellows method can detect the small impedance of bellows assembly generated from the RF shield structure and CF flange cavities. The measurement results also reveal that the shielded bellows assembly has a low impedance close to that of a vacuum tube compared with the unshielded one, so the finger-type structure can achieve the demand of RF shielding.