Deceleration and Electron Cooling of Highly Charged Ions at HITRAP

The investigation of highly charged ions (HCI) offers the potential to recreate environments that are otherwise inaccessible on Earth. For instance, the magnetic field experienced by electrons in hydrogen-like heavy ions close to the surface of the nucleus compares to that in the vicinity of a pulsar. Consequently, this allows for the most stringent tests of fundamental theories, such as quantum electrodynamics (QED), in these extreme fields. Ultimate precision is typically attained in experiments conducted on cooled and confined ions in an ion trap. However, to date this remains a viable option only for light and medium-heavy, highly charged ions. The production process of large quantities of heavy, highly charged ions, such as hydrogen-like uranium U⁹¹⁺, relies on electron stripping at high kinetic energies. This technique is commonly employed in accelerator facilities, such as the heavy ion research facility GSI Helmholtzzentrum für Schwerionenforschung in Germany. In order to confine heavy HCI in an ion trap for precision experiments, prior deceleration subsequent to the production process is necessary. This constitutes the objective of the HITRAP facility at GSI, where the energy of the ion beam is reduced in a linear decelerator. Subsequently, the ions will be captured in a Penning trap and cooled by the application of electron cooling for further transport to precision experiments. In the context of the work, the linear decelerator was recommissioned following a shutdown period of almost eight years. For this purpose, ³⁶Ar¹⁸⁺ ions were produced by the accelerator complex of GSI, decelerated to 4 MeV/u in the Experimental Storage Ring ESR, and injected into the HITRAP facility. After successful deceleration, the ion beam was transported for the first time further along the low-energy beam line towards the HITRAP Cooling Trap, captured, and confined for short periods of time. Furthermore, the trap setup was commissioned with ⁴⁰Ar¹⁶⁺ ions produced by an electron beam ion trap (EBIT). The simultaneous storage of these ions with a cold plasma of electrons resulted in an considerable reduction of the ions' kinetic energy. This constitutes the first demonstration of electron cooling on highly charged ions in a Penning trap. Together with the first successful storage of accelerator-produced HCI, this marks a tremendous advancement in the commissioning of the HITRAP facility, as a result of which an experiment at HITRAP has been included for the first time in the beamtime schedule of GSI for the upcoming year.