Electron cooling forces for highly charged ions in the ESR

In the ESR storage ring fully stripped ions up to uranium have been stored and cooled at beam energies between 50 and 370 MeV/u. Longitudinal cooling forces for heavy ions from C6+ to U92+ at various energies have been measured with the ESR electron cooler. Two experimental methods have been established which allow the determination of the cooling force for relative ion velocities from zero to 107 m/s. At low velocities the equilibrium between cooling and longitudinal heating with rf noise is used. For high relative velocities the cooling force is deduced from the momentum drift of the ion beam after a rapid change of the electron energy. Measurements for several ion species are presented which show a pronounced deviation from the expected q2 scaling in the regime of the magnetic cooling force. Further investigations have been performed to study the influence of magnetic field strength and beam energy on the longitudinal cooling force.