A new and more flexible ion source assembly has been built for continued study of a duoplasmatron with an expanded plasma front and a cylindrical Pierce-electrode extraction geometry. Studies of conditions necessary for the formation of an expanded plane plasma boundary are discussed, and progress toward the achievement of an aberration-free parallel beam of several hundred mA of protons is reviewed. Recent studies suggest that a duoplasmatron with an expanded plasma front and a diverging Pierce-electrode extraction geometry would produce significantly higher aberration-free beam currents than can be obtained with the present cylindrical geometry. The information necessary for the design of lens optics for matching such a source, or any source which produces a diverging aberration-free beam, to an accelerator or plasma containment device is provided in a convenient and comprehensive form by beam characteristic curves which give beam radius and divergence as functions of perveance at some convenient measuring plane close to the extractor for several fixed values of arc current. Data provided by a constant perveance experiment establish practical limits on obtainable beam current at the chosen design points on the characteristic curves.