Summary form only given. High-performance 5 to 10 kW class ion propulsion technology has been identified as a key requirement and priority for technology development to support the Exploration of the Solar System (ESS) theme of the Space Science Enterprise. Identified applications include Mars missions, Europa Lander, Saturn Ring Observer, Neptune Orbiter, Comet Nucleus Sample and Return, and Venus Surface Sample and Return. To that end, there is an ongoing, effort at NASA Glenn Research Center (GRC) to extend ion thruster technology beyond the 30 cm, 2.3 kW engines developed under the NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR) program. Two prototype (laboratory model) 40 cm, 5 kW ion thrusters were developed using two different discharge chamber geometries to evaluate magnetic field contours and discharge chamber performance. Differences from the nominal NSTAR-type ion thruster include an increase in discharge chamber diameter from 30 to 40 cm, the use of mild steel instead of non-ferrous Al or Ti for the anode, an increase in the number of rings of magnets forming the ring-cusp magnetic field, and, in the case of the purely conical chamber, a significant change in the overall geometry of the discharge chamber. Final magnetic field configurations including comparisons to numerical modeling and initial thruster performance data are presented. The impact of this data on the design of high-fidelity (engineering model) 40 cm, 10 kW thrusters currently being developed at NASA GRC is also discussed.