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

In the CERN A¿ntiproton A¿ccumulator, 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.