Dielectronic recombination (DR) is a fundamental electron-ion collision process, and the precision determination of the absolute rate coefficients of electron-ion recombination is demanded to understand the astrophysical and other natural as well as manmade plasmas. The DR experiments of highly charged ion (HCI) by employing the electron-ion merged beams technique have been developed for more than two decades at heavy ion storage rings, i.e., the Test Storage Ring at MPIK in Heidelberg [1], the Experimental Storage Ring at GSI in Darmstadt [2], Germany, and also the CRYRING at MSL in Stockholm, Sweden [3]. The research topics of DR experiments are very broad, covering from atomic structure, relativistic effect and strong field QED, to the interface of atomic and nuclear physics.

The Cooler Storage Rings (CSR) at HIRFL were installed in 2007 at Institute of Modern Physics (IMP), in Lanzhou, China. The installations of electron coolers at the main Cooler Storage Ring (CSRm, EC35) and the experimental Cooler Storage Ring (CSRe, EC300) provide ideal research platforms for DR experiments of highly charged stable ions and radioisotopes. Until now, the high precision electron-ion recombination experiments of multi-electron ions ⁴⁰Ar^{12+,13+,14+,15+}, ⁴⁰Ca^14+,16+,17+, Kr^{19+,24+,25+,30+} as well as Ni¹⁹⁺ have been performed at the CSRm and CSRe [4-8]. In order to improve the DR experimental energy resolution and increase the detuning energy range between the electron beam and the ion beam, an electron cooler and a separated ultra-cold electron target will be installed at SRing @HIAF, which will provide a unique opportunity for DR spectroscopy of heavy ion beams [9]. The mainly research topics of DR experiments at the CSRe and HIAF will be focused on: a) astrophysical related precision DR spectroscopy; b) Precision spectroscopy of highly charged ions, H-like, He-like, Li-like; c) DR spectroscopy of radioactive nuclides far away from the β stable line.

Fig. 1. DR is a resonant process, in which a free electron is captured into a bound state of an ion, and simultaneously an initially bound electron is excited through a resonant interaction, forming a doubly excited intermediate state. Subsequently, this doubly excited state decays via photon emission resulting in the stabilization of the recombined ion due to the excitation energy below its ionization threshold, this is called radiative stabilization.

[1] S. Schippers, Nucl. Instr. Meth. B 350 (2015) 61–65.

[2] C. Brandau, C. Kozhuharov, in: V. Shevelko, H. Tawara (Eds.), Atomic Processes in Basic and Applied Physics, Springer, Berlin Heidelberg, Berlin, Heidelberg, 2012, pp. 283–306.

[3] R. Schuch, S. Böhm, J. Phys.: Conf. Ser. 88 (2007) 012002.

[4] Z. K. Huang et al., Astrophys. J. Suppl. Ser. 235 (2018) 2

[5] S.X. Wang et al., The Astrophysical Journal 862 (2018)134

[6] S. X. Wang et al., Astron. Astrophys. 627 (2019) A171

[7] W. Q. Wen et al., The Astrophysical Journal 905 (2020) 36

[8] Z. K. Huang, et al., Physical Review A 102 (2020) 062823

[9] X. Ma et al., Nucl. Instr. Meth. B 408 (2017) 169–173