To meet the requirements of large flagship-type missions, the NASA Glenn Research Center has developed the next generation ion propulsion system. The NASA Evolutionary Xenon Thruster (NEXT) is a 40 cm diameter ion engine, double the beam extraction area of the NSTAR ion engine. The NEXT ion engine development followed the “derating” philosophy used for the NSTAR engine. The NEXT engine consists of a stainless steel semi-conic discharge chamber with a ring-cusp magnetic field geometry. Hollow cathodes are employed for electron emission in the discharge chamber and to neutralize the ion beam. The ion optics are dished grids with the same geometries as NSTAR. The engine was designed to be throttled from 1.1-6.1 kW to adjust for varying available power resulting from solar panel degradation over the life of the mission. The NEXT ion engine has been demonstrated over a throttling range of 1.1-6.9 kW, specific impulse range of 2210-4100 s, with resulting thrust of 50-237 mN. The fourth Laboratory Model NEXT engine, referred to as LM4, was built at the NASA GRC with the intention of conducting detailed mappings of the discharge plasma via electrostatic probes and Laser-Induced Fluorescence (LIF).