Ordered Ion Beams in Cryring

Some years ago, it was found at GSI, Darmstadt, that the momentum spread of electron-cooled beams of highly charged ions dropped abruptly to very low values when the particle number decreased to below about 5000 ions. This has been interpreted as an ordering of the ions, such that they line up after one another without being able to pass their neighbours. Such transitions have now also been observed at CRYRING, and it has been seen that the drop in momentum spread is accompanied by a similar drop in Schottky-noise power. Using a model for the particle distribution in a beam with string-like order, characterized by a certain minimum distance between the ions, theoretical calculations of Schottky-noise power have been performed. It is found that the model can reproduce the reduction in noise power, and the good agreement with the experimental data gives evidence for spatial ordering of the ions.