An Overview of the Results from the 30,000 Hr Life Test of Deep Space 1 Flight Spare Ion Engine

Abstract

The extended life test of the Deep Space 1 (DS1) spare flight ion thruster (FT2) was voluntarily terminated on June 26th 2003. Although the engine had not yet reached its end of life at the conclusion of the test, the decision to terminate and begin the post-test destructive analyses was made to benefit near term ion engine development programs. During its 5-year run, the thruster operated for a total of 30,352 hours, processed 235.1 kg of Xenon propellant, and acquired several thousand hours of operation at each of the four independent throttled conditions investigated. The objectives of the test were to characterize previously observed failure modes, identify unknown failure modes, and quantify thruster performance as a function of engine wear and throttle level. Several performance variations and degradation processes were observed and monitored during the course of the test. Degradation processes included erosion of the discharge cathode keeper, ion optics grid sputter erosion, and deposition of material in the neutralizer cathode at low power that later cleared with the return to full power operation. Performance degradation was limited to reduction in measured thrust at the full power point for the final 1000 hours of operation, most likely due to electronbackstreaming. Post-test inspection of the engine was initiated immediately following the test termination, to ascertain causes of the wear, and to look for any previously unknown wear processes. The ion engine consists of various internal systems, and the post-test analysis effort has been divided into separate analysis efforts of the ion optics system, discharge and neutralizer cathode assemblies, and the discharge chamber. Post-test inspection of the ion optics system revealed significant sputter erosion of the accelerator grid and measurable erosion of the screen grid. Post-test inspection results include the presence of through pits in the accelerator grid webbing, but no formation of rogue holes. Inspection of the discharge cathode indicates significant cathode orifice plate sputter erosion, as a result of exposure to the discharge plasma following removal of the keeper, but no erosion of or deposition near the orifice itself. Inspection of the neutralizer revealed no erosion to the keeper, orifice plate, or heater, and an orifice free of the deposits that were previously observed during the minimum power test segment. Post-test inspection indicates the discharge chamber experienced no measurable sputter erosion, although a substantial number of loose molybdenum coated carbon flakes were present on the sputter containment mesh. It is believed that the bulk of the flakes are due to back-sputtered beam target material, subsequently coated by sputter eroded grid material, a facility induced effect. A summary of the BOL and EOL performance, results of the system level inspection, an implication of findings to the ultimate service life capability of the 30-cm technology are presented.