Advances in high power, photovoltaic technology has enabled the possibility of reasonably sized, high specific power, high power, solar arrays. At high specific powers, power levels ranging from 50 to several hundred kW are feasible. Coupled with gridded ion thruster technology, this power technology can be mission enabling for a wide range of missions ranging from ambitious near Earth NASA missions to those missions involving other customers as well such as DOD and commercial satellite interests. Indeed the HEFT clearly identified the need for high power electric. The appeal of the ion thrusters for such applications stems from their overall high efficiency, typically >70% and long life. In response to the need for a single, high powered engine to fill the gulf between the 7 kW NEXT system and a notional 25 kW engine, a Phase I activity to build a 25 kW, 50 cm ion thruster discharge chamber was completed with a laboratory model fabricated. The proposed Phase II effort aims to mature the laboratory model into a proto-engineering model ion thruster. The proposed effort involves the evolution of the discharge chamber to a high performance thruster by performance testing and characterization via simulated and full beam extraction testing. Through such testing the design will be optimized leading ultimately to the proposed design, build and preliminary checkout of a proto-engineering model thruster, thereby advancing the TRL level to 4-5 range. Deliverables include the thruster, a design package, and a performance data document.