Using a two fluid model, we investigate the effects on the ion impact energy, of varying the shape of a large, negative, finite rise time, voltage pulse to a planar target. Two pulse shapes are tested, one with exponential shutoff, and one with a powered linear shutoff. For pulse lengths under 50 μs, we find that pulse shaping can be used to make moderate changes in the energy distribution of the implanted ions. For pulse lengths over 50 μs, there is a negligible difference in the ion impact energy distribution for the pulse shapes tested. © 1994 American Institute of Physics.