Collisions in high-energy ion beams reduce the beam intensity and can be remedied by extra focusing devices or the use of low-density beams. However, physicists predicted 20 years ago that in a sufficiently cool beam, the ions would not collide because their Coulomb repulsion would outweigh their kinetic energy.

Such 'crystallization' has been achieved before in ion traps - in which the ions are stationary - but it is more difficult in a circulating beam because of the motion of the ions and interactions between the beam and the storage ring. These problems affect both large storage rings - such as the Relativistic Heavy Ion Collider at Brookhaven - and smaller ones.

Schramm and co-workers injected magnesium ions into their 0.36-metre circumference storage ring, PALLAS - the Paul laser cooling acceleration system. The beam was laser-cooled and its fluorescence monitored. The team found that, at a certain laser wavelength, the diameter of the beam fell and the fluorescence peaked sharply. This pinpoints the transition to the crystalline state, during which the range of ion velocities drops by 75%.

The fluorescence measurements showed that the ring contained around 18 000 ions, and the temperature of the beam fell from 30 to 0.4 kelvin as the crystalline state emerged. In this new phase, the ions reach a speed of 2800 metres per second - corresponding to a beam energy of 1 electron volt - and resemble a one-dimensional thread. The beam can perform over 3000 revolutions of their storage ring without further cooling

According to Schramm, the technique could be used for a wide range of experiments. "Crystalline ion beams could aid inertial confinement fusion - which mimics stellar nuclear reactions - while precise experiments with relativistic beams could test special relativity", he says.