-- EstherBabetteMenz - 2021-04-23

Dielectronic Recombination Experiments at CRYRING@ESR

This page is intended to give you an overview of techniques and applications used for dielectronic recombination experiments at the CRYRING@ESR heavy ion storage ring.

This includes general information, manuals and instructions for shift crews.

This page is currently under construction.

Information for Shift Crews

The first duty of the shift crew is to monitor experiment conditions and document any changes. For this purpose every experiment has its own elog (you can create your own account). Updates on machine operation can be found in the OLOG (access only possible with a user account from inside the GSI network).

In case of problems with CRYRING (e.g. no beam or reduced intensity, vacuum problems, etc.) the first point of contact is the main control room (or "HKR" in German; phone: 2222). They will either try to fix the problem themselves or contact the experts.

For problems with the data acquisition system or similar experiment-related matters please contact the people in charge of the experiment (relevant phone numbers can usually be found on the whiteboard in the local control room).

Further information regarding specific topics:

• important phone numbers
• instructions for aborted shifts
• data acquisition system
• particle detectors
• detector drive manual
• vacuum system
• CRYRING nomenclature (i.e. "What do all these funny alphanumeric names mean?")

Overview

CRYRING@ESR

Prior to its move from the Manne Siegbahn Laboratory in Stockholm to GSI in Darmstadt, CRYRING and its ElectronCooler facilitated many dielectronic recombination (DR) experiments which greatly benefitted from its excellent vacuum conditions and ultra-cold electron beam [1]. At GSI the commissioning of CRYRING@ESR is gradually coming to an end and we are planning to build on this tradition and capitalize on the availability of a wider range of ion species from the upstream accelerator facility. DR is an important process in both astrophysical and laboratory plasmas, accurate knowledge about the cross sections, rates and emission spectra resulting from DR is therefore important for understanding the physics of astrophysical objects like stars and nebulae and for controlling plasmas in fusion experiments. Another motivation for DR experiments is the possibility to measure shifts in electronic energy levels caused by QED effects and nuclear influences such as hyperfine splitting and isotopy effects, as the process probes transitions between levels of very deeply bound electrons [2],[3].

Dielectronic Recombination

Dielectronic recombination (DR) is a form of electron-ion collision that leads to recombination without photon emission. As the name suggests, DR involves two electrons: a free electron e₁ captured by the ion from the continuum and a bound electron e₂. In contrast to radiative recombination the kinetic energy of the free electron and the binding energy that is released by its capture are not emitted as a photon but instead excite a bound electron. Because a quantized amount of energy is needed for the excitation to a higher electronic state the process of dielectronic recombination is a resonant one. As the free electron is usually captured into an unoccupied shell with high quantum number n the result is an ion or atom in a doubly excited state. The electron capture forms the first step in what is essentially a two-step process. In the second step the excited ion either relaxes via photon emission, thereby completing the process of recombination, or it is auto-ionised when the initial bound electron e₂ descends back to its previous state. The time-reversed process is the Auger-Meitner effect [3] .

Electron Cooling

Electron cooling is employed at CRYRING to significantly reduce the circulating ion beam's momentum spread. Inside the ElectronCooler the `hot' ion beam is merged with a `cool' (i.e. of low transverse and longitudinal momentum spread) electron beam with the same average velocity. Ions that deviate from the beam velocity carry out Coulomb collisions with electrons thereby gaining or losing momentum. The result is a cooled ion beam which approximates the mean velocity and correspondingly the momentum distribution to those of the electron beam.

In addition to cooling the beam the electron cooler may also be used as an electron target for recombination experiments with a well-defined energy of the captured electrons relative to the ions. For this the velocity of the electron beam is systematically scanned by varying the acceleration voltage which is split into a basic voltage U₀ and a variable voltage U_k that is used for recombination experiments. This leads to a variation of the electrons' relative kinetic energy in the stored ions' centre-of-mass frame. Ions that capture electrons and thereby change their charge-to-mass ratio are separated from the main beam at the next dipole magnet and can be detected using single-particle detectors. This information in combination with the velocity of the electron beam allows probing of the resonant DR process.

Experimental Setup

There are currently two detector setups downstream from the electron cooler. A YAP:Ce scintillation detector directly behind the dipole magnet is used for lighter ions and low charge states while a detector channel electron multiplier (CEM) further down the straight section can observe highly charged heavy ions

References

[1] Schippers et al., Nucl. Instrum. Methods 350 (61-65), 2015

[2] G. W. F. Drake (Ed.), Handbook of Atomic, Molecular and Optical Physics, Berlin, Heidelberg: Springer, 2006

[3] M. Lestinsky and Y. Litvinov and Th. Stöhlker, Physics Book: CRYRING@ESR, Eur. Phys. J. Special Topics 255.5, 2016

[4] H.F. Beyer, H.-J. Kluge and V.P. Shevelko, X-Ray Radiation of Highly Charged Ions, Berlin, Heidelberg: Springer, 1997