Highly charged ions combine extremely strong electromagnetic fields and a simple electronic structure which makes them an ideal testing ground for the electromagnetic sector of the standard model and even for the physics beyond.

In the presentation, I will focus on most recent results of precision experiments at the ion storage and cooler ring facilities of GSI/FAIR, with particular emphasis on H-, He-, and Li-like heavy ions, aiming for benchmarking the theory of non-perturbative bound-state QED. Examples include the study of ground-state transitions in H- and He-like uranium and intra-shell transitions in He- and Li-like ions [1,2] studies by means of laser and x-ray spectroscopy.

In addition, the present status of hyperfine-structure investigations performed for H- and Li-like bismuth-209 ions will be discussed where the studies have been extended recently to accelerator produced bismuth-208 isotopes [3].

A further fascinating aspect of the ongoing at the GSI/FAIR storage facilities is the intersection of atomic and nuclear physics where the influence of electrons on nuclear properties can be elucidated. As an example, the HiThor ERC project is aiming on precision studies of the nuclear clock transition in 229Th89+. For this project the heavy ion trapping facility HITRAP is of utmost importance, which is currently under commissioning at the ESR storage ring, providing a new platform for precision studies of fundamental interactions. A brief overview of the unique physics program of HITRAP will be given.

[1] R. Lötzsch et al., Nature 625, 673 (2024)

[2] Ph. Pfäfflein et al., Phys. Rev. Lett. 134, 153001 (2025)

[3] M. Horst et al., Nature Physics 21, 1 (2025)