Reduction of the Impedance Created by the Insulated Vacuum Flanges in the PS Booster

The original vacuum flange assembly in the PS Booster (PSB) was designed to present insulation at low frequency and conduction to the beam image current above 2 MHz. However, in order to satisfy the needs for the LHC, the RF harmonic in the PSB had to be changed from h=5 to h=1, leading to a reduction of the lowest accelerating frequency to 600 kHz. The RF component of the beam was therefore passing through the main resonance of these flanges giving a total (integrated around the ring) longitudinal coupling impedance of 1000 W at 750 kHz, taking into account some additional RF bypasses. The voltage generated by the beam current was coupling to various electronic devices which therefore had to be equipped with common-mode rejection circuits. After some non-essential insulated flanges had been replaced by conducting ones during the 1998-99 shutdown, the total impedance was lowered to about 200 ohms (still higher than the maximum value for h=5 which was 130 ohms). This was one of the improvements that made it possible to reach a new intensity record in September 1999. New RF decoupling flanges introduced in 2000 to further reduce the impedance are described here, together with the results obtained.