In addition to the CRYRING electron cooler that can be nicely utilized as a cold merged-beams free-electron target for high-resolution collision spectroscopy, for electron-ion collision experiments also a dedicated electron target that operates in crossed beams collision geometry has been built-up and is presently being commissioned at CRYRING@ESR (Figure 1). Electrons of a ribbon-shaped intense electron beam interact with the stored and cooled ion beam at a laboratory-frame collision angle of 90°. Such a "crossed beams" electron-ion collision experiments have been successfully studied for more than 35 years at low-energy single-pass setups, for instance at the University of Giessen. A "crossed-beams" set-up has never been realized at a heavy ion storage ring.

At high beam energies the "crossed-beams" kinematics lead to an energy resolution that is principally lowered compared to the co-propagating beams setup at the cooler. Yet, the resolution is expected to be still more than one order of magnitude higher compared to collision experiments at a gas-jet target. A major asset of a transverse electron target is a collision volume between electrons and ions that is spatially well localized and not surrounded by a guiding solenoidal B-field. The beam overlap region can be easily accessed from many directions which allows one to perform photon and electron spectroscopy with large solid angles.

The benefits and possible experimental scenarios of such a transverse electron target at CRYRING@ESR are manifold. Here, a brief summary is given. More details and elaborated examples are given in the CRYRING physics book and the CRYRING instrumentation TDR.

By design, in a free-electron target the process of kinematic capture ("Non-Radiative Capture", NRC) is absent. Using a gas-jet target, at very low ion energies, NRC becomes the dominant recombination and beam-loss process, and eventually renders experiments with very heavy highly charges ions difficult if not impossible. A free-electron target allows for recombination experiments of highly charged ions such as bare uranium (U⁹²⁺) even at the lowest energies available at CRYRING. Due to negligible Doppler shift and broadening, such low energies are favorable for high-precision spectroscopic studies, such as QED investigations in the strongest electromagnetic fields. Spectroscopy of threshold and resonant process is facilitated due to high-resolution and the possibility to change the relative collision energy in well-defined small steps and independently of the ion energy. The present experiment proposals for the CRYRING@ESR transverse-electron-target facility comprises:

• High-resolution photon-ion coincidence experiments of electron-ion excitation or recombination with large solid angle and determination of the polarization of the emitted x-ray photons.
• Measurements of the processes of resonant and non-resonant electron scattering.
• Experiments of electron-impact ionization of highly charged ions, in particular of the indirect contributions due to excitation-autionization (EA) and resonant ionization mechanisms in the relativistic domain.
• First observation of the process of nuclear excitation by (resonant) electron capture (NEEC), the time-inverse process of internal conversion.