  We show that strong coupling between ions in an ultracold neutral plasma is
limited by electron screening. While electron screening reduces the
quasi-equilibrium ion temperature, it also reduces the ion-ion electrical
potential energy. The net result is that the ratio of nearest-neighbor
potential energy to kinetic energy in quasi-equilibrium is constant and limited
to approximately 2 unless the electrons are heated by some external source. We
support these conclusions by reporting new measurements of the ion velocity
distribution in an ultracold neutral calcium plasma. These results match
previously reported simulations of Yukawa systems. Theoretical considerations
are used to determine the screened nearest-neighbor potential energy in the
plasma.
