D. R. DeWitt

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.

Three-dimensional ion beam-profile monitor for storage rings

We have developed a non-destructive three-dimensional beam-profile monitor for storage rings. It allows the measurement of the complete profile of stored ion beams by simultaneously detecting electrons and ions created through ionisation of the residual gas. The horizontal coordinate of each point in the profile is determined by detecting the ionised electrons, guided in a strong magnetic field, on a two-dimensional position sensitive detector. The magnetic field assures precision in the determination of the absolute horizontal position of the ion beam and prevents the detection of background electrons originating outside the monitors active volume. Simultaneously, the vertical coordinate of the beam distribution is obtained by measuring the time difference between the detection of the electrons and the recoil ions in coincidence. The time of flight spectrum shows the different components of the residual gas and allows an intrinsic calibration of the physical parameters. We present results of a test performed with an 8 MeV/u D+ beam at the storage ring CRYRING in Stockholm.

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.

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.

Recombination studies of highly charged ions in the cooler ring CRYRING

Cooler-storage ring facilities offer unique experimental possibilities for studies of electron-ion recombination processes at low relative energies by employing the electron cooler as a target. Through the use of an adiabatically expanded electron beam in the cooler of CRYRING, collisions down to 10−4 eV relative energies were measured with highly charged ions stored in the ring at around 15 MeV/amu energies. Examples of recombination measurements for C4+ and Ar15+ ions at an energy resolution of as low as 10−2 eV FWHM are presented and compared with calculations of the energies and resonance strength for dielectronic recombination using multiconfiguration Breit-Pauli approximations. They agree in spectral shape and absolute cross section but show some deviation for lower n in the Rydberg series.