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New designs for the coaxial plasma injector (cablegun) used in the plasma opening switch (POS) of the DECADE pulsed-power facility were tested experimentally. A two-beam Mach-Zehnder interferometer operating in heterodyne mode was used to acquire, in addition to time-varying path-integrated electron number density and effective injection velocity, transverse radial scans for the ejected plasma. Abel inversion of this projected data yielded electron number density at a given downstream position as a function of radial position and time. Modifications to the baseline DECADE cablegun (a copper-core copper-sheath coaxial arrangement, with a Teflon insulator and a 60-degree muzzle cone) were investigated. Replacement of the copper center conductor with tungsten reduced radiative ablation of copper into the plasma and produced higher effective injection velocity with somewhat lower number density and similar radial divergence properties to that of the baseline cablegun. Substitution of the Teflon insulator with polyethylene continued that trend. The effect of cone angle was investigated by means of a simple flat-face arrangement whose plasma exhibited similar divergence to that of the baseline cablegun, although with lower number density, higher velocity, and more pronounced modulation in response to the driver current. The addition of a boron-nitride auxiliary nozzle proved effective at controlling the plasma divergence and concentrating the free electrons on axis while reducing the effects of driver current modulation.