Crystalline ion beams

By freezing out the motion between particles in a high-energy storage ring, it should be possible(1-4) to create threads of ions, offering research opportunities beyond the realm of standard accelerator physics. The usual heating due to intra-beam collisions should completely vanish, giving rise to a state of unprecedented brilliance. Despite a continuous improvement of beam cooling techniques, such as electron cooling and laser cooling, the ultimate goal(5) of beam crystallization has not yet been reached in high-energy storage rings. Electron-cooled dilute beams of highly charged ions show liquid-like order(6,7) with unique applications(8). An experiment(5) using laser cooling(9,10) suggested a reduction of intra-beam heating, although the results were ambiguous. Here we demonstrate the crystallization of laser-cooled Mg+ beams circulating in the radiofrequency quadrupole storage ring PALLAS(11,12) at a velocity of 2,800 m s(-1), which corresponds to a beam energy of 1 eV. A sudden collapse of the transverse beam size and the low longitudinal velocity spread clearly indicate the phase transition. The continuous ring-shaped crystalline beam shows exceptional stability, surviving for more than 3,000 revolutions without cooling.