W. Zong

Energy and charge dependence of the rate of electron - ion recombination in cold magnetized plasmas

We have measured electron - ion recombination rates for bare ions of , , , and in a storage ring. For the multi-charged ions an unexpected energy dependence was found, showing a strong increase of the measured rates over the calculated radiative recombination rate for electron beam detuning energies below the electron beam transverse temperature. The measured enhanced rates increase approximately as with the charge state Z. A comparison of these rates with theoretical predictions for collisional - radiative recombination in the cold magnetized electron plasma, in particular three-body recombination including radiative de-excitation of electrons in Rydberg levels, is made.

DENSITY DEPENDENCE OF RECOMBINATION IN THE ELECTRON COOLER OF CRYRING

The electron-ion recombination rates of Ne10+ and D+ have been measured as a function of relative energy and electron density. We found a strong enhancement of e-Ne10+ recombination over expected radiative recombination rates below 1 meV relative energies, reaching a factor of 4 close to zero relative energy. Remarkably, the measured rate coefficients decrease as a function of electron density for both systems. Studies of recombination of D+ indicate that this density dependence may be due to temperature variations of the electron beam with the electron density.

Spectroscopy of highly doubly-excited states of helium through dielectronic recombination

We report a spectroscopic determination of dielectronic recombination resonances with hydrogen-like helium. An energy resolution of 0.3 eV FWHM was achieved through the use of a cooled ion beam in the CRYRING storage ring. First theoretical results, including correlation to all orders, for the Delta n=1 and Delta n=2 resonances are presented. Unexpectedly, it was found that the Delta n=2 resonances are dominated by certain 3F0 doubly-excited states, in contrast to the results for Delta n=1 transitions.

Low-energy recombination of

Low-energy recombination of has been measured at the storage ring CRYRING with a high-energy precision of 15 meV. Using different electron currents the approach for correcting the variation of the ion energy due to the drag force was checked. A multi-configuration Breit-Pauli perturbation calculation using the program AUTOSTRUCTURE for the dielectronic recombination rates has been implemented. The cut-off of Rydberg states by field ionization in the analysing magnet was modified by taking radiative cascade into account. Comparison of this calculation with the measurement shows a good agreement in resonance strength but a small discrepancy in resonant energy. At very low energy, the non-resonant recombination rate is found to be a factor of 2 higher than the estimated radiative recombination rate. No indication of an electron density dependence is found.

Laser-Induced Recombination of D(+)

Laser induced recombination into the L shell (n = 2) of deuterium was investigated at the heavy-ion cooler storage ring CRYRING for two values of the electron density. The data shows the expected enhancement of radiative recombination, induced by the intense laser field applied, over the spontaneous case. Like in previous measurements, a strong intensity below the photoionization threshold was observed. Qualitatively, this gain is described by an average field induced threshold shift model. However, when reducing the electron density by roughly a factor of two, the effective field increased by an order of magnitude. This surprising result indicates that the precise origin of the observed fields is not simply related to the known field sources.

DIELECTRONIC RECOMBINATION OF LI-LIKE NEON AND ARGON

We have measured dielectronic recombination rates and energies for δn=0 transitions of Ne7+ and Ar15+ beams stored in CRYRING. The energy resolution was in the order of 10−2 eV FWHM; the absolute accuracy in the position of the resonances is in the same order. The energy positions of the dielectronic recombination resonances are compared to theoretical calculations of the fine-structure splittings.

Study of radiative recombination into deuterium Rydberg states

Deuterium Rydberg states which were populated by radiative recombination in the electron cooler of CRYRING have been detected using state-selective field ionization in an external electric field. Absolute radiative recombination rate coefficients for Rydberg states between 12<or=n<or=18 are extracted and agree well with theoretical calculations using the Kramers cross section corrected by the Gaunt factor. The influence of the Stark effect on the Rydberg states has to be taken into account to explain the data. It was found that the relative population distribution of Rydberg states as well as the rate coefficient are very sensitive to the transverse electron temperature in the electron cooler.

Dielectronic recombination of Li-like fluorine ions

Some recent new developments in dielectronic recombination (DR) measurements at heavy-ion storage rings are presented. A DR spectrum measured at CRYRING in Stockholm of F6+ is used to illustrate a few points regarding this kind of measurements. From the spectrum a transversal temperature of the adiabatically expanded electron beam of 3.0(5) meV is deduced, which is slightly higher than expected.

Recent dielectronic recombination experiments

New recombination experiments with merged cold beams of electrons and atomic ions have been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt, and CRYRING in Stockholm. A brief overview is given on the recent activities in which the Giessen group was engaged. Topics of this research were dielectronic recombination (DR) of astrophysically relevant ions, recombination of highly charged ions with respect to cooling losses in storage rings, field effects on DR, search for interference effects in photorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments.

A first study of dielectronic recombination with the super-expanded electron beam in CRYRING

A first temperature measurement of the expanded electron beam from a super-conducting magnet in the cooler of CRYRING is reported. It is based on detection of a dielectronic recombination resonance in C2+. For this measurement C3+ was stored in the ring at 3 rm{MeV/amu} energy. A fit to the resonance with free transverse and longitudinal temperatures gave 3.9 × 10-3 eV and 4.5 × 10-5 eV, respectively. The result is also compatible with the values of 10-3 eV for the transverse and 6.7 ×10-5 eV for the longitudinal component, in good agreement with expected values.