The combination of heavy ion storage rings and advanced lasers provides a novel research platform for atomic physics and nuclear physics of highly charged ions (HCI) [1]. By utilizing the relativistic Doppler shift effect, unique laser cooling and laser spectroscopy experiments with relativistic HCI beams could be performed at heavy ion storage rings. Laser cooling of ion beams at storage rings to extreme phase-space density will hopefully lead to the phase transition from gaseous to ordering state even to crystalline beams [2]. Although these activities already started in the early nineties, however, because the wavelengths of the electronic dipole transitions in HCI are very short and available laser wavelengths can therefore not resonantly interact with heavier ions in higher charge states, only few ion species have been laser-cooled at storage rings [3]. Very recently, laser cooling of lithium-like ¹⁶O⁵⁺ ion beams with a relativistic energy of 275.7 MeV/u was achieved for the first time at the CSRe at the Institute of Modern Physics (IMP) in Lanzhou, China [4]. In order to cool the O⁵⁺ ion of the 2s_1/2-2p_1/2 optical transition energy as high as 11.95 eV by only one counter-propagating cw laser system with a wavelength of 220 nm, a moderate bunching of the ion beam was applied to provide a bucket force to counteract the velocity-dependent laser force and thus create a stable cooling point in momentum space. To our knowledge, the ¹⁶O⁵⁺ ions are of the highest charge state and at the highest energy that have been ever cooled by laser cooling.

Precision laser spectroscopy and laser cooling experiments are very similar in the way they are being performed and require similar setups and settings at storage rings. In addition to laser cooling, the precision laser spectroscopy of hyperfine splitting of H-like and Li-like bismuth ions have been performed recently to investigate the strong field QED at storage ring ESR [5]. Since the High Intensity heavy ion Accelerator Facility (HIAF) in China [6] and the Facility for Antiproton and Ion Research (FAIR) in Germany [7] will come into operation in ~2025. By exploiting such extreme relativistic velocities, the XUV or even X-ray transition energies of HCI can be Doppler-shifted to the laser accessible region, allowing for laser cooling and precision laser spectroscopy experiments with HCI at the future facilities using present day laser systems [8]. There are three topics for laser cooling and precision laser spectroscopy of HCI at HIAF, including: a) Laser cooling of relativistic heavy ion beams to investigate the properties of ultracold ion beams; b) Precision laser spectroscopy of fine structure and hyperfine structure of highly charged ions to investigate QED effects, relativistic effects and also electron-electron correlation effects; c) Collinear laser spectroscopy of highly charged radioactive ion beams to investigate nuclear properties by measuring the isotope shifts and hyperfine splitting.

Fig. 2. Schematic view of laser cooling of relativistic O⁵⁺ ion beams at the CSRe.

[1] U. Schramm, D. Habs, Prog. Part. Nucl. Phys. 53, 583–677 (2004)

[2] M. Bussmann, ICFA Beam Dynamics Newsletter. 65 (2014)

[3] D. Winters et al., Scr. T166(2015), 014048 (2015)

[4] W. Q. Wen et al., Hyperfine Interactions 240 (2019) 45

[5] J. Ullmann et al., Nat. Commun. 8, 15484 (2017)

[6] X. Ma et al. Nucl. Instrum. Methods B. 408, 169–173 (2017)

[7] H. Backe, Hyperfine Interact. 171(1–3), 93–107 (2006)

[8] H. B. Wang et al., X‐Ray Spectrometry 49,138–142 (2020)