A model is developed for nanosecond laser ablation of a Cu target, followed by the expansion of the Cu vapor in He background gas at 1 atm. Typical calculation results include the temperature distribution in the target, information about target melting and vaporization, the density profiles of the Cu vapor and the He background gas in front of the target, as well as the velocity and temperature distributions in the plume. Comparison is made with results calculated for expansion in vacuum. It is found that the vapor plume pushes the background gas away from the target, but at the same time, the vapor plume is much more confined in space compared with expansion in vacuum. Indeed, the plume expansion velocity is much lower in the case of a background gas. Also, the plume temperature is much lower, which is attributed, among other things, to the cooling effect of the background gas.