Observation of ion Coulomb crystals in a cryogenic linear octupole rf ion trap

Ion Coulomb crystals of laser-cooled Ca+ ions have been observed in a cryogenic linear octupole rf ion trap for the first time. Due to the almost field-free central region, large ion crystals containing over 10(4) ions of millimeter size were successfully produced. The shapes of the crystals can be manipulated by changing the static voltages applied to the octupole rods and the endcap electrodes. Quasi-two-dimensional Coulomb crystals consisting of a small number of ions were also observed with single-ion sensitivity. When a large number of ions is crystallized in the octupole trap, a new kind crystal image with a dip along the axial direction was obtained. The results show that a linear octupole trap and multipole traps are available for producing large, cold, and novel ion Coulomb crystals. The planar ring-type crystal which has not been observed before may lead to future interesting applications.