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 this presentation, the focus will be on most recent precision experiments at the ion storage rings of GSI, with particular emphasis on He-like uranium, the simplest multi-electron system at high Z where the interplay between correlation, relativity and QED is a particular challenge for theory. One may note that until now practically no precise experimental data for He-like heavy ions has been available (beyond nuclear charge of 60). More specifically, Bragg spectrometer-based studies of the 3P2-3S1 intrashell transitions will be discussed [1], as well as measurements of the Kα ground-state transitions based on the application of metallic magnetic micro-calorimeters [2]. Complementary studies will be addressed in addition, where elastic scattering of high-energy photons is subject of the research [3]. The goal of these studies is to provide detailed insights into the higher-order QED process of Delbrück scattering, where energic photons are elastically scattered by the virtual electron-positron pairs in the extreme electro-magnetic fields of heavy nuclei [4].

[1] R. Lötzsch et al. submitted for publication

[2] P. Pfäfflein et al., Atoms 2023, 11(1), 5 (2023); and to be published

[3] W. Middents et al., Physical Review A 107, 012805 (2023)

[4] J. Sommerfeldt et al., Phys. Rev. Lett. 131, 061601 (2023)