Ion traps and storage rings provide unique experimental conditions for the study of the properties of stable or radioactive, singly or highly charged ions. Storage of the confined particles for extended periods of times and cooling are the key issues for extremely high accuracy and sensitivity. At many accelerators such facilities are in operation or planned for experiments in atomic and nuclear physics.
1 WHY TRAPPING, WHY COOLING?
Storage of precious particles is a matter of economy. Since much effort is put into the production of radioactive isotopes, highly charged ions or exotic particles, it makes much sense to confine those precious particles for repeated interactions with other particles like photons, electrons, atoms, ions, etc. Furthermore, the particles can be accumulated in the storage devices by stacking. In this way, the luminosity for reaction experiments or the signal-tobackground ratio for laser spectroscopy investigations can be enhanced very efficiently. Because of their sensitivity to the charge-to-mass ratio, ion traps and storage rings have inherent mass spectroscopic capabilities. Therefore, these devices can be used as mass separator or mass spectrometer, or for manipulation or preparation of ionic species. Examples are polarization of atoms or nuclei, preparation of specific charge states like bare or hydrogen-like ions, or separation of ground and isomeric nuclear states. The long storage time in combination with the feature to store -and in this way to observe -the mother as well as the daughter nuclei allows one to study nuclear decays in a new manner.
Cooling of particles is another very general key issue for high-accuracy experiments. If the velocity of the stored particle is reduced to nearly rest in space in an ion trap or, alternatively, if the velocity spread of particles circulating in a storage ring is diminished, the resolving power as well as the sensitivity are increased due to, for example, reduced line broadening by Doppler effect or due to smaller amplitudes of the motion and, therefore, less important magnetic-field inhomogeneities. New phenomena can be observed in this way as demonstrated by the observation of crystallization of an ion cloud or Bose-Einstein condensation in the case of neutral atoms.
The long interaction time made possible by trapping and cooling gives rise to ultimate sensitivity (spectroscopy with a single stored particle and single-particle detection) and ultimate accuracy (approaching the limit set by the Heisenberg uncertainty principle). Some examples will be discussed briefly below.