Spectroscopy on trapped highly charged ions with soft X-ray synchrotron and FEL radiation

Spectroscopic investigation of highly charged ions (HCI) in the X-ray regime enables precise benchmarking of theoretical calculations including effects of bound-state quantum electrodynamics and nuclear origin. It further aids in extracting properties and dynamics of hot astrophysical plasmas by means of plasma modeling. In the framework of this cumulative thesis, an electron beam ion trap has been combined with bright X-ray light sources, such as synchrotrons and X-ray free-electron lasers (XFEL) to investigate HCIs with X-rays of various properties.\\ In particular, high-precision transition energy measurements of astrophysically relevant diagnostic lines of light lithium-like elements and neon-like iron have been performed with ppm precision, made possible by resolving a key issue in monochromator-based X-ray absorption spectroscopy. These improved transition energy measurements not only allow to test theoretical predictions but also nail down the doppler velocity of astrophysical plasma to few km/s. Furthermore, two techniques for assessing transition oscillator strengths have been developed. A novel synchrotron-based approach using the Hanle effect in the soft X-ray regime for measuring femto- to picosecond lifetimes of allowed transitions in helium-like nitrogen is demonstrated. Second, a pioneering lifetime measurement of femtosecond transitions of helium-like neon and fluorine using an all-X-ray pump-probe technique at an XFEL is presented. Finally, an investigation of well-controlled multi-photon ionization in neon-like krypton producing charge-states well beyond the single-photon ionization limit is presented, which served as a basis for the lifetime measurement.