A Coulomb crystal (also Ion Coulomb crystal) is a collection of trapped ions confined in a crystal-like structure at low temperature. The structures represent an equilibrium between the repulsive Coulomb interaction between ions and the electric and magnetic fields used to confine the ions. Depending on the confinement techniques and parameters, as well as the number of ions in the trap, these can be 1-, 2- or 3-dimensional, with typical spacing between ions of ~10μm, which is significantly larger than typical solid-state crystal structures. Outside of ion traps, Coulomb crystals also occur naturally in celestial objects such as neutron stars.