Simulation study of emittance growth from Coulomb collisions in low-temperature ion beams

Yuri, Yosuke; Okamoto, Hiromi*; Sugimoto, Hiroshi*

The collisional heating process of highly space-charge-dominated coasting ion beams in a storage ring was systematically investigated using the molecular dynamics simulation technique. To evaluate the heating rate over the whole temperature range, the simulation was started from an ultra-low-emittance state where the beam is Coulomb crystallized. When the ring is properly designed, the heating rate is quite low at ultralow temperature because random Coulomb collisions are suppressed. It gradually increases during the melting process of a Coulomb crystal and eventually comes to a peak at relatively low temperature where the beam is in a liquid phase. The dependence of the heating behavior is explored on various machine and beam parameters, such as the line density, betatron tune, kinetic energy, and mass number of ions. An approximate formula is given to estimate the magnitude of emittance at which the collisional heating is maximized.