Study on the longitudinal impedance of BPM for KEKB and super KEKB

The longitudinal impedance of KEK B-factory (KEKB) button-type beam position monitors (BPMs) was recalculated by MAFIA in preparation for a future plan to increase the beam current. The diameter and the gap of the button electrode were 12 mm and 1 mm, respectively. For High Energy Ring (HER), an asymmetric structure was applied to extract the TE110 mode into the coaxial cable. The Q-value and shunt impedance were estimated at 91 and 17 ohm (at 7.6 GHz), respectively. In this case, the current limit was 2.6 A. Based on the experiences at the KEKB, the new BPM was designed for the Super KEKB, a future high-intensity B-factory at KEK. In order to reduce the impedance the electrode diameter was cut down to 6 mm from 12 mm. the Q-value and shunt impedance were estimated at 23 and 2 ohm (at 13 GHz), respectively. The current limit was expected to be about 7 A in full bucket operation (5120 bunches), and more than 10 kA in 4-bucket spacing operation.