Beam profile measurements have been carried out to study the influence of the length of the ion source outlet, the source pressure, and the source magnetic field on the properties of an ion beam. From a variation of the outlet length conditions can be deduced for optimum design with respect to gas efficiency and profile shape. The profile width is found to decrease with increasing source pressure and decreasing magnetic field. This effect is attributed to a corresponding change in the energy distribution of the ions. Profile distortions occurring at high source pressure are likely to be due to ion scattering in the extraction region.