Laser-cooled magnesium ion crystals for sympathetic cooling of highly charged ions in a penning trap

We have investigated laser-cooled magnesium ion crystals with ion numbers ranging from 103 to 105 in the SpecTrap 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, the ion crystals display shell structures depending on experimental parameters, which we have visualized by use of a CCD camera. Singly and multiply charged ion species have been sympathetically cooled after injection into the laser-cooled magnesium ion crystals. The presence of the indirectly cooled ion species was observed via the centrifugal separation of the different ion species. These investigations are 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.