Prospects for laser spectroscopy of highly charged ions with high-harmonic XUV and soft x-ray sources

We present novel high photon flux XUV and soft x-ray sources based on high harmonic generation (HHG). The sources employ femtosecond fiber lasers, which can be operated at very high (MHz) repetition rate and average power (>100 W). HHG with such lasers results in similar to 10(13) photons s(-1) within a single harmonic line at similar to 40 nm (similar to 30 eV) wavelength, a photon flux comparable to what is typically available at synchrotron beam lines. In addition, resonant enhancement of HHG can result in narrow-band harmonics with high spectral purity-well suited for precision spectroscopy. These novel light sources will enable seminal studies on electronic transitions in highly-charged ions. For example, at the experimental storage ring 2s(1/2)-2p(1/2) transitions in Li-like ions can be excited up to Z= 47 (similar to 100 eV transition energy), which provides unique sensitivity to quantum electro-dynamical effects and nuclear corrections. We estimate fluorescence count rates of the order of tens per second, which would enable studies on short-lived isotopes as well. In combination with the Doppler up-shift available in head-on excitation at future heavy-ion storage rings, such as the high energy storage ring, even multi-keV transitions can potentially be excited. Pump-probe experiments with femtosecond resolution could also be feasible and access the lifetime of short-lived excited states, thus providing novel benchmarks for atomic structure theory.