Tackling Transverse Coherent Instabilities of Co- and Counter-Rotating Bemis in the CERN Antiproton Accumulator

In the CERN Antiproton Accumulator, transverse instabilities set in at circulating intensities as low as 10¹⁰ p(p), mainly because of the unusually small variation of Q vs. momentum. The bunched p "test" beam is lost due to head-tail instabilities, and vertical coasting beam modes of the p stack lead to an intolerable emittance growth. Based on estimated AA transverse impedances, a fairly simple feedback system was built. Particular features are (i) low power requirements (10 W); (ii) housing the electronics outside the tunnel; (iii) a "double damper" arrangement enabling the system to tackle simultaneously the p stack and the counter-rotating p test beam; (iv) the system's use for controlled beam blow-up to determine machilne apertures. Whereas the measured growth rates match the resistive wall impedance, significantly higher damping rates are required to eliminate all coasting beam modes, for yet unknown reasons.