The ions are accelerated through two fine grids with roughly a 1300 V difference between them for 2.3 kW operation, with a thrust of 20-92 mN¨ millinewtons, a specific impulse of 1950-31000 N·s/kg and a total impulse capability of 2.65 x106 Ns. In 1996, the prototype engine endured 8000 hours of continuous operation in a vacuum chamber that simulates conditions of outer space. The results of the prototyping were used to define the design of flight hardware that was built for Deep Space 1 probe. One of the challenges was developing a compact and light weight power processing unit that converts power from the solar arrays into the kwtages needed by the engine.