Magnesium Ion Crystals at SpecTrap/HITRAP

We have investigated laser-cooled magnesium ions stored in a Penning trap. The ions are produced externally and are dynamically captured in the trap. We have combined buffer-gas cooling and laser cooling, thus reducing the ion temperatures from Mega-Kelvin to milli-Kelvin on the timescale of seconds. At this temperature, the ions adopt crystalline structures. For ion numbers of the order of a few thousand, these so-called ’mesoscopic’ ion crystals display shell structures depending on experimental parameters, which we have visualized by use of a CCD camera. We have investigated the fluorescence signal depending on laser parameters and characterized the crystal structures. This is part of the sympathetic cooling of highly-charged ions as a next step in the framework of the SpecTrap experiment at the HITRAP facility at GSI/FAIR.