Atomic Collisions and Spectroscopy at the Heavy-Ion Storage Ring TSR

The storage ring TSR is used to cool and accumulate a wide range of heavy-ion beams produced at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany. With ionization stages reaching up to 50 (bare nuclei up to Cl17) and beam energies of typically 5-8 MeV/u, storage lifetimes amount to minutes up to hours; repeated cycles of injection and electron cooling (reducing the ion beam diameter Tom centimeters to millimeters in about 2 sec) make it possible to accumulate beam currents up to 1 mA in the ring. With the circulating beam, in particular the interaction of multicharged atomic ions with electrons of variable, well defined energies (10 MeV to 3 keV) is studied, detecting recombination and electron impact ionization. High-resolution spectra of the dielectronic recombination cross section have been obtained, which provide a sensitive test for atomic-structure calculations describing the large number of doubly excited states contributing to the recombination rate. For lithiumlike ions (Si11+, Cl14+) all possible outer- and inner-shell excitations were investigated[1]. Low-energy di- electronic recombination measurements are presently being extended[2] also to more complex open-L-shell ions (F11+-F23+). Very recently the enhancement of dielectronic recombination by ambient electric fields was studied with Cl14=; at well-controlled fields of order 100 V /cm the enhancement of the cross section by up to a factor of 3 in certain energy regions could be followed in detail[3] . In addition to the information on doubly excited levels from dielectronic recombination experiments, spectroscopic results on multicharged ions were also obtained by laser-stimulated recombination[4] , allowing highly excited levels in multicharged ions to be reached by laser transitions from continuum states populated by surrounding free electrons. Moreover, for a number of metastable levels in multicharged ions the natural lifetimes in the millisecond to second range could be determined with high accuracies of about 0.2%, using dielectronic resonances to identify metastable ions and also direct optical observation of vacuum-ultraviolet emission