Crystalline Ion Beams in the RF Quadrupole Storage Ring PALLAS

We report on the crystallization of laser cooled Mg ion beams circulating in the table-top rf quadrupole storage ring PALLAS at a velocity of 2800 m/s (beam energy 1 eV). A sudden collapse of the transverse beam size and the low velocity spread clearly indicate the phase transition to a onedimensional linear string of ions. This crystalline beam shows exceptional stability, surviving for more than 3000 revolutions without cooling. Close to the phase transition, the spatial beam profile of non-crystalline beams was found to exhibit an unexpected two component Gaussian distribution