  We study the long-time evolution of the ion temperature in an expanding
ultracold neutral plasma using spatially resolved, laser-induced-fluorescence
spectroscopy. Adiabatic cooling reduces the ion temperature by an order of
magnitude during the plasma expansion, to temperatures as low as 0.2 K. Cooling
is limited by heat exchange between ions and the much hotter electrons. We also
present evidence for an additional heating mechanism and discuss possible
sources. Data are described by a model of the plasma evolution, including the
effects of ion-electron heat exchange. We show that for appropriate initial
conditions, the degree of Coulomb coupling of ions in the plasma increases
during expansion.
