1. I2 that described the Thruster/Power Processor Element for which GRC was responsible. Ground tests were conducted to validate subsystem life and performance while in-space measurements were taken to assess interactions between the ion propulsion system, the spacecraft, and the space plasma. Results of engineering model development tests, environmental tests, wear tests, and in-space operation are synopsized. The flight hardware requirements and the basic designs of the thruster, PPU, and the DCIU are described along with design drivers related to spacecraft integration and the space environment.
2. NASA's research and technology programs in the 1988 to 1993 period led to the development of mature laboratory-class, 30-cm diameter xenon ion thrusters.' The NSTAR ground test program advanced the thruster design to engineering model level where the thruster weight was reduced in a flight-like design. Four EMTs were built and they were dedicated to thruster development, to integration with the PPUs, and to weartests to demonstrate the performance and service life needed for a variety of proposed demanding missions. NASA GRC built two'BBPPUs and HED built one, all of which were designed to operate over a wide range of bus voltages and produce a 4.6:l power throttling capability while providing a specific mass of less than 5 kg/kW and an efficiency in excess of 0.90 at fullpower'*. One of the important ground test program goals was to demonstrate successful integration of the DCIU, BBPPU, and the EMT. The designs of the thruster and BBPPU were validated through a series of development and wear tests. A subset of these tests is identified in Table I. Two wear-tests (NPT116 and DTl5") and two major life demonstration tests (LDT" and ELTIY) were the focal points of the ground test program. The first two tests employed EMTs and laboratory power supplies while an EMT and BBPPUs were initially used for the LDT. The on-going ELT is using the flight spare thruster, fabricated by HED, which had undergone acceptance level tests.
3. Based on the experience gained in the ground test program, the in-space segment of the NSTAR Thruster/Power Processor Element involved the HED build of a Pathfinder (precursor) thruster, two each of flight ion thrusters (one was the retrofitted Pathfinder Thruster), PPUs, DCIUs, and cable harnesses. The flight ion thruster design evolved from the second EMT that had completed the LDT and the flight thruster design was driven primarily by the structural and thermal requirements. The flight PPU design, driven by high efficiency requirements and solar array bus voltages from 80 V to 160 V, had slightly different requirements than the initial BBPPU whose development started in 1993. The DCIU provides for initiation of thruster operation, throttling, xenon flow control, data, and recovery from fault conditions. The flight thruster, PPU, and DCIU were integrated and underwent acceptance tests prior to delivery to the DSI facility where the PPU. DCIU, and Pathfinder Thruster were acceptance tested at the spacecraft level. During the course of the DSI mission, a diagnostics package using a quartz crystal microbalance, solar absorptance ' measurements, plasma probes, magnetometers, and a plasma wave antenna is documenting NSTAR system contamination, communications impacts, and electromagnetic interferepce.*' DS I spaceclaft systems are providing propulsion subsystem information related to thrust, thrust-vector, and compatibility with spacecraft systems. The EMTs and BBPPUs were built and functionally tested from 1993 through 1997. Random vibration, thermal-vacuum, plumes, and communicat/on impact tests were also conducted over that period. Wear tests and life tests were ongoing throughout the program. The NSTAR flight hardware delivery to the DSI integration and test program was completed in June 1998.
4. Table 2 is a brief descript Ion of the evolution of the EMT design. Design changes were incorporated as a result of wear measuremeni's made after the 2000-hour test. Additional thruster nlodifications were made to accommodate random vibration and thermal vacuum requirements. The first changes were made after significant screen grid (positive grid) and main hollow cathode assembly wear was experienced during the 2000-hour test. For subsequent tests, the screen grid was referenced to cathode potential, rather than allowed to float, to control the potential drop in the vicinity of the grid. A cathode keeper electrode was added to mitigate cathode orifice plate erosion and discharge chamber surface treatments were added to retain sputtered material and control the size of flakes that might spa]]."**' EMT4 wa!. retrofitted to include most of the important features that were expected to be incorporated into the fligh'. thrusters. EMT4's major attributes include an all-titanium discharge chamber, wire-mesh throughout tte discharge chamber, a discharge keeper electrode,md grit-blasted components to increase radiation heat trailsfer.
5. Breadboard Power Processor: