Crystallisation of ion beams in the rf quadrupole storage ring PALLAS

The crystallisation of laser-cooled Mg ion beams circulating in the table-top rf quadrupole storage ring PALLAS at a velocity of the order of 2800 m/s (beam energy 1 eV) is reviewed. Emphasis is given to the description of the experimental techniques that were required for this first realisation of crystalline beams. The equivalence of the equations of motion for rf electric and magnetic confinement and thus for PALLAS and high-energy storage rings is pointed out. The phase transition is indicated by the detection of a sudden collapse of the transverse beam size, by the low velocity spread, and by the exceptional stability of the crystalline state, persisting for more than3000 revolutions or 10 focusing periods without any cooling.