Beam loss rates with an internal gas target in an electron-cooled storage ring Implications for luminosity optimization

Abstract

Measurements have been made in the Cooler ring at the Indiana University Cyclotron Facility (IUCF) of the mean lifetime of stored and electron-cooled proton beams in the presence of various target gas species at two ring locations differing in dispersion. A broad range of beam energy T b, target atomic number Z t and thickness x t was employed. The loss (removal) cross sectionσ L per target atom, extracted from the lifetime measurement, exhibits a minimum for small x t which is found to scale with ( Z t T b ) 2 . This behavior is expected because forward Coulomb scattering from target nuclei should be the dominant loss mechanism under the conditions of measurement. A marked increase in σ L for larger x t is attributed to emittance growth arising from multiple scattering in competition with the transverse cooling rate. Lattice dispersion at the target increases the contribution to beam heating by electrons in the target atoms and introduces an additional loss mechanism. From the measured σ L and beam lifetime, a time cycle may be selected which gives the optimum time-averaged luminosity for a given experiment. Scaling of σ L allows prediction of the optimum luminosity over the full operating regime of the cooling ring.