Impedance measurements with strongly cooled beams at LEAR

At the CERN Low Energy Antiproton Ring (LEAR), high-density beams are obtained with electron and stochastic cooling. The authors have tried to determine the characteristics of the beam and its environment in a regime where the cooling force is present and where the impedance is space-charge-dominated. Methods used include beam-transfer function (BTF) measurements and Schottky scans. It is shown that, when the beam is strongly cooled, Landau damping is lost. A powerful active feedback is necessary to recover beam stability and increase the possible circulating current. It is noted that transverse BTFs are a convenient tool for adjusting and monitoring the action of the damper. By a combination of longitudinal BTF and Schottky measurements, the momentum distribution of very cold beams can be determined despite the strong signal distortion, and the beam coupling impedance can be estimated. >