Laser cooling of stored high-velocity ions by means of the spontaneous force

Abstract

A longitudinal laser cooling of ion beams at about 5% of the velocity of light has been performed at the Heidelberg Test Storage Ring with various cooling schemes employing the spontaneous force. For a 7.29-MeV $^{9}\mathrm{Be}^{+}$ beam with an initial longitudinal temperature of 2700 K, the main characteristics of laser cooling in a storage ring are discussed. When undamped, the transverse betatron oscillations of the coasting ions limit the longitudinal temperature after laser cooling to typically 1 K. After damping the transverse motion by precooling the ions with an electron cooler, longitudinal temperatures of below 30 mK have been obtained in the subsequent laser cooling. In this case, the longitudinal ion-beam temperature can be understood as an equilibrium of the laser cooling rate with the heating rate due to intrabeam scattering. Moreover, single binary Coulomb collisions between the (still transversely hot) ions can cause such longitudinal velocity changes that ions are lost out of the critical capture range of the laser cooling force. In these two ways, intrabeam scattering imposes a substantial limit on the temperature or number of laser cooled ions in a storage ring. In our experiments, this process presently limits the ratio between the density-dependent Coulomb energy and the longitudinal thermal energy spread to a value on the order of 1, where liquid rather than gaseous behavior of the ion beam is expected to set in.