Sheath expansion into a cylindrical bore is studied for radii larger than the ion‐matrix overlap radius in the context of plasma‐based ion implantation. The temporal dependence of the sheath width, sheath speed, and ion impact energy and the dependence of the maximum and average ion impact energy on the radius of the bore are computed. It is found that the sheath speed increases as the radii decreases, and that it is, in all cases, greater than the planar sheath speed. After the sheath has reached the axis, the potential drop across the bore decays rapidly. Consequently, the ion impact energy first increases and then decreases in time. Both the maximum and average impact energy increase with radius. ©1996 American Institute of Physics.