Observation of ordered structures of laser-cooled ions in a quadrupole storage ring

We observed phase transitions and ordered structures of laser-cooled ${\mathrm{Mg}}^{+}$ ions stored in a radio-frequency quadrupole storage ring with a diameter of 115 mm. The ions are confined two dimensionally by a quadrupole field and can move freely along the ring circumference. By cooling the ions with tangentially intersecting laser beams, we observed a linear string of ions aligned along the center line of the quadrupole field. For higher ion densities the linear structure changes into a helical structure. The dynamics of the stored ions was studied.