Zajfman and colleagues injected around 10 000 argon ions into an 'electrostatic trap' - a tube 40 cm long with ring-shaped electrodes around both ends. These electrodes act as 'mirrors', and their voltages can be tuned to propel the argon ions up and down the tube. The ions travel at slightly different speeds when they are injected into the tube, and this velocity spread becomes more pronounced as they travel back and forth between the mirrors.

At certain voltages, however, the team noticed that the ions all travelled at the same speed, and stayed tightly packed in a cloud. 'Our first reaction was, "that's impossible"', says Zajfman. 'But then we had to scratch our heads and try to understand it'.

Zajfman and colleagues believe that this effect is due to 'Coulomb repulsion', which usually pushes ions apart when they get too close to each other. They suggest that this repulsion makes the ions bounce off one another when they collide within the cloud, and the energetic ions impart energy to the less energetic ions during this process. After a short time, the momentum is distributed evenly throughout the cloud and the ions all travel at the same speed. The overall lifetime of the cloud is determined by the number of neutral atoms that leave the electrostatic trap. These neutral atoms are produced when ions collide with residual gas atoms in the tube.

Zajfman and colleagues are optimistic that their technique could lead to a mass spectrometer with significantly better resolution than existing devices. Mass spectrometry measures the masses of ions by studying their motion as they are injected into an electric field. But a sample of ions only stays together for short time inside the mass spectrometer, and this limits the resolution of the technique.