Setup of a Penning trap for precision laser spectroscopy at HITRAP

Ion traps have been established as a powerful tool for ion cooling and laser spectroscopy experiments since a long time ago. SpecTrap, one of the precision experiments associated to the HITRAP facility at GSI, is implementing a Penning trap for studies of large bunches of externally produced highly charged ions. The extremely strong electric and magnetic fields that exist around the nuclei of heavy elements drastically change their electronic properties, such as energy level spacings and radiative lifetimes. The electrons can therefore serve as sensitive probes for nuclear properties such as size, magnetic moment and spatial distribution of charge and magnetization. The energies of forbidden fine and hyperfine structure transitions in such ions strongly depend on the nuclear charge and shift from the microwave domain into the optical domain. Thus, they become accessible for laser spectroscopy and its potentially high accuracy. A number of such measurements has been performed in storage rings and electron beam ion traps and yielded results with relative accuracies in the 103 to 104 region. This work presents the construction and commissioning of a Penning trap used for capturing highly charged ions in-flight and cooling them nearly to rest, thus reducing Doppler broadening and increasing the possible accuracy. As an important step-stone towards that goal, singly charged Mg ions were produced in an electron impact ion source, transported to the trap and laser cooled to sub-K temperatures. Indications of Coulomb crystals formation are reported. Non-destructive detection of the stored ions was performed both electronically, with the attached cryogenic resonant circuits, and optically, by observing the fluorescence directly from the trap centre. The designed electronic components can also be used for resistive cooling, which becomes particularly efficient for highly charged ions. It was demonstrated that large ion clouds can be created in the trap, formed by accumulating many consecutive ion source shots. Ions were stored with lifetimes of the order of 100 s and cooled to temperatures lower than 60 mK. The same technique can be used in future experiments for mixing laser-cooled Mg+ with highly charged ions delivered by HITRAP. Thus, sympathetic cooling of highly charged ions to sub-K temperatures will become possible. This is sufficient for reducing the Doppler width of a transition in a heavy ion to several MHz and enables laser spectroscopy experiments with a relative accuracy up to 108. vii