Electron cooling of high-Z ion beams parallel to a guide magnetic field

The cooling of high-Z ion beams through collisions with electrons whose temperature parallel to a guide magnetic field is considerably lower than their perpendicular temperature is considered. For initial electron temperatures, magnetic fields, and charged particle densities---as, for instance, are apparently attainable in the experimental storage ring (ESR) at GSI Darmstadt---electrons tend to be trapped in the vicinity of their nearest ion neighbor. This results in an energy exchange with the ions that is qualitatively different from conventional models, where electron cooling is described in terms of small angle collisions or within the linearized dielectric response theory. Such models are justified for situations where the potential energy of interaction is small compared to the relevant kinetic energy; e.g., for light ions. For the case of high-Z ions, however, it is the trapping process itself that drives the cooling. Using a variety of parameterizations of the electron shielding of the ions, it is found that the resulting steady-state ion temperature parallel to the magnetic field is less than a factor of ten higher than the original parallel electron temperature