M. Kleinod

Recombination of with free electrons at very low energies

Recombination of ions with free electrons has been investigated in a merged-beams experiment at the UNILAC of GSI in Darmstadt. Special emphasis was put on the recombination at very low energies in the electron-ion centre-of-mass frame. At we found an enormous recombination rate exceeding the theoretical expectations for radiative recombination by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

Recombination of electrons with highly charged heavy ions at very low energies

Recombination of highly charged ions with free electrons is studied in merged-beams experiments at the UNILAC accelerator in Darmstadt and at the heavy-ion storage ring TSR in Heidelberg. Unexpected high recombination rates are observed for a number of ions at very low energiesEcm in the electron-ion center-of-mass frame. In particular, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates of U28+ ions nearEcm=0 eV. The observations point to a general phenomenon in electron ion recombination depending onEcm, on the ion charge state, and possibly also on electron density, electron beam temperature, and strength of external magnetic fields.

Recombination of free electrons with ions

This paper reports first experimental data on radiative recombination of ions with free electrons and discusses a measurement of recombination rates of U28+ ions which appears to contradict known theories.

AN EBIS/T WITH INTEGRATED PENNING TRAP

A special problem in atomic physics research with highly charged ions is to prepare ions with a unique charge state inside of EBIS or EBIT devices. On the other hand, there are great losses resulting from the transport of the ions from the source to an external trap. Therefore we are setting up an EBIS/T with internal Penning trap. This new set-up will be able to study electron–ion interaction with well-defined initial and final charge states, distinguishing between single step successive ionisation and multiple step ionisation of charge states similar to the crossed beams method but for much higher charge states. Another feature of this system is to determine with high precision the ion charge state distribution in the EBIS/T by application of Fourier Transform Ion Cyclotron Resonance (FT-ICR). This method allows the on-line monitoring of the ion distribution and the evolution of the charge state population together with its dependence on the degree of space charge compensation of the electron beam in the EBIS/T. It will be possible to study ion dynamics in compensated space charge potentials. In case of high homogeneity of the magnetic field in the trap region, experiments may be considered to measure directly binding energies of highly-charged ions and other topics of high resolution mass spectroscopy.

Recombination measurements at low energies with Ar16+ and Ar18+ ions in a dense, cold electron target

Recombination of multiply charged ions with electrons at very low relative energies has become a major topic of interest, due to the observation of rates which are enhanced beyond the expectations for radiative recombination. We present results for Ar16+ and Ar18+ ions from systematic measurements along the argon isonuclear sequence using a high density cold electron beam target (ne = 7 × 109 cm-3) at the UNILAC of GSI. The transverse and longitudinal temperatures of the electron beam were determined from DR resonance features observed with metastable Ar16+ (23S) ions. The rate at Erel = 0 for radiative recombination of completely stripped Ar18+ calculated with electron beam temperatures kT∥ = 0.002 eV, kT⊥ = 0.2 eV amounts to α = 10-9 cm3 s-1. This is exceeded by nearly a factor of 10 by the rate measured in experiments with Ar18+ ions.

Radiative recombination of highly charged ions: Enhanced rates at low energies

In a single‐pass merged‐beams experiment employing a dense cold electron target recombination of highly charged ions is studied. Unexpected high recombination rates are observed at low energies Ecm in the electron‐ion center‐of‐mass frame. In particlar, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates at Ecm=0 eV for U28+ and for Au25+ ions. Considerable rate enhancement is also observed for Ar15+. This points to a general phenomenon which has to be interpreted as a consequence of high electron densities, low electron beam temperatures, high ion charge states and presence of strong magnetic fields.

Status report on the Frankfurt EBIS research program

The Frankfurt superconducting EBIS has been run beyond the space-charge neutralization limit making use of the evaporative cooling technique showing a continuous transition between EBIS to EBIT operation. The experience gained on the performance of this more sophisticated source has been employed to develop simpler EBIS sources: The MEDEBIS using a warm solenoid will demonstrate rapid extraction of bare nuclei for single turn injection into a synchrotron for cancer therapy. The PPM-focused EBIS with permanent magnets and the XEBIS without any focusing magnetic field have been developed to demonstrate by its reduced power consumption and size to be more suitable to small laboratories. High emission current densities from a laser heated tip cathode offer the formation of a well-defined high current density beam. Several spectrometers have been built and installed to offer diagnostics for the EBIS operation as well as atomic physics research. Ionization measurements can be performed using the magnetic switchyard, an external electrostatic and magnetic spectrometer allows to study secondary electron emission by highly charged ions as well as to ionize and analyse sputtered atoms.

Recombination of highly charged ions with free electrons

Recombination of highly charged ions and free electrons is studied in interacting‐beam experiments. Beside direct recombination into bound states by radiative capture a variety of resonant recombination phenomena is observed. Resonant recombination produces a highly excited, usually short lived electron‐ion compound which can stabilize by the emission of photons and/or electrons. Depending on this emission, the final charge state of the ion can be one less than the parent charge state, but it can also be higher and thus a net single or multiple ionization of the ion is observed after the initial recombination.

Recombination of Au25+ with free electrons at low energies

The origin of the widely observed enhancement of rates for electron-ion recombination at very low energies is still unknown. We investigated the recombination of Au25+ with free electrons in a merged-beams experiment at the UNILAC accelerator of the GSI in Darmstadt. At Erel= 0 eV we found an enormous enhancement factor of 365 compared to the theory of radiative recombination. An increase of the electron density by a factor of 10 had not much influence on the measured rate coefficient.

Recombination of free electrons with U28+ ions

This paper reports a measurement of radiative recombination rates for U28+ ions and free electrons at energies from 0 to 2 eV. The results appear to contradict known theories.