Longitudinal coupled-bunch instability around 1 GHz at the CERN PS booster

The fast-growing “Ring 4” instability occurring at intensities above 6.5×10¹² protons in the top one of the four rings of the CERN PS Booster (PSB) is finally explained by an asymmetry in the 40 vacuum pump manifolds common to all rings. Measurements (by wire method) and calculations show a sharp resonant peak (Q~2000) at 1100 MHz and shunt impedances two times higher for the Ring 4 ports as compared to the other rings. This factor is sufficient to explain that the threshold of the instability falls below the maximum intensity only in Ring 4. A final, but labor-intensive and expensive, cure consists of inserting short-circuiting sleeves into all 160 beam ports. Results of beam and impedance measurements and the planned cure will be presented and discussed