Advanced Hall Electric Propulsion for Future In - Space Transportation

The Hall thruster is an electric propulsion device used for multiple in-space applications including orbit raising, on-orbit maneuvers, and de-orbit functions. These in-space propulsion functions are currently performed by toxic hydrazine monopropellant or hydrazine derivative/nitrogen tetroxide bi-propellant thrusters. The Hall thruster operates nominally in the 1500 s specific impulse regime. It provides greater thrust to power than conventional gridded ion engines, thus reducing trip times and operational life when compared to that technology in Earth orbit applications. The technology in the far term, by adding a second acceleration stage, has shown promise of providing over 4000s Isp, the regime of the gridded ion engine and necessary for deep space applications. The Hall thruster system consists of three parts, the thruster, the power processor, and the propellant system. The technology is operational and commercially available at the 1.5 kW power level and 5kW application is underway. NASA is looking toward 10 kW and eventually 50 kW-class engines for ambitious space transportation applications. The former allows launch vehicle step-down for GEO missions and demanding planetary missions such as Europa Lander, while the latter allows quick all-electric propulsion LEO to GEO transfers and non-nuclear transportation human Mars missions. TECHNOLOGY DESCRIPTION HALL THRUSTER OPERATION The Hall thruster is an electric propulsion device used for orbit raising, on-orbit maneuvers, and de-orbit functions which are currently performed by hydrazine monopropellant or hydrazine derivative/nitrogen tetroxide bi-propellant thrusters. The Hall thruster nominally operates in the 1500 s specific impulse regime. It provides greater thrust to power than the conventional gridded ion engines, thus reducing trip times and requiring lower operational lifetimes when compared to that technology in Earth Orbit applications. The technology in the far term, by adding a second acceleration stage, has shown promise of providing over 4000s Isp, the regime of the gridded ion and necessary for deep space applications. The Hall thruster system consists of three parts, the thruster, the power processor, and the propellant system. A simplified schematic diagram of a Hall thruster is presented in figure. 1 and an overview of the underlying physics is available in Ref. 1. The typical propellant for a Hall thruster is a high molecular weight inert gas such as xenon. A power processor is used to generate an electrical discharge between a cathode and an annular anode through which the majority of propellant is injected. A critical element of the device is the incorporation of a radial magnetic field, which serves to impart a spin to the electrons coming from the cathode and to retard their flow to the anode. The spinning electrons collide with the neutral xenon, ionizing it. The xenon ions are then accelerated from the discharge chamber by the electric potential maintained across the electrodes by the power processor. The velocity of the exiting ions, and hence the specific impulse, is governed by the voltage applied by the discharge power supply and is typically 15,000-16,000 m/s at 300 V. A sample Hall electric thruster is shown in figure 2. HISTORICAL DEVELOPMENT As with the majority of electric propulsion devices, a great of deal of early research and technology development was accomplished in the 1960’s in both the United States and in the Soviet Union. At that time, high erosion rates and low performance led to the cessation of research on Hall thrusters in the US, in favor of the development of gridded ion accelerators. This effort culminated with the successful flight