NASA Solar Technology Application Readiness

The NASA Solar Technology Application Readiness (NSTAR) is a type of spacecraft ion thruster called electrostatic ion thruster. It is a highly efficient low-thrust spacecraft propulsion running on electrical power generated by solar arrays. It uses high-voltage electrodes to accelerate ions with electrostatic forces.
Development and performance.
The purpose of NSTAR program was to develop a xenon-fueled ion propulsion system for deep space missions.
The NSTAR electrostatic ion thruster was developed at NASA's Glenn Research Center and manufactured by Hughes, and Spectrum Astro, Inc. in the early 1990s. The feed system development was a collaborative effort between JPL and Moog Inc.
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, a specific impulse of 19000-30500 N·s/kg (1950-3100 s) 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 voltages needed by the engine.
Performance.
The engine achieves a specific impulse of one to three thousand seconds. This is an order of magnitude higher than traditional space propulsion methods, resulting in a mass savings of approximately half. This leads to much lighter and less expensive launch vehicles. Although the engine produces just 92 millinewtons (0.331 ounce-force) thrust at maximum power (2,100W on DS1 mission), the craft achieved high speed because ion engines thrust continuously for long periods of time.
Applications.
The NSTAR ion thruster was first used on the Deep Space 1 (DS1) spacecraft, launched on 24 October 1998. The Deep Space mission carried out a flyby of asteroid 9969 Braille and Comet Borrelly. It produced 2.3 kW and was the primary propulsion for the probe.
The second interplanetary mission using NSTAR engine was the "Dawn" spacecraft, with three redundant units with a 30 cm diameter each. "Dawn" is the first NASA exploratory mission to use ion propulsion to enter and leave more than one orbit.
NASA engineers state that NSTAR engines, in the 5-kilowatt and 0.04 pound-thrust range, are candidates for propelling spacecraft to Europa, Pluto, and other small bodies in deep space.