Two different types of electrostatic thrusters are used to propel spacecraft: ion thrusters and Hall-effect thrusters. Ion thrusters have the benefit of relatively high exhaust velocities with higher overall thrust efficiencies. Hall-effect thrusters typically offer higher thrust-to-power ratios, but they operate somewhat less efficiently than ion thrusters. To take advantage of both thrusters’ strengths, innovators at NASA's Glenn Research Center have developed what is essentially a dual-thruster propulsion system. This patented electric propulsion device offers the ability to switch a spacecraft's propulsion system to ion-thruster mode or Hall-thruster mode, depending on whether the principal need is efficiency or thrust power. In addition, Glenn's novel technology allows the dual thruster to operate in burst mode, with one thruster acting as an ion thruster and one as a Hall thruster. This capability can be valuable to facilitate smooth transitions between operating modes or to raise total thrust level (for short periods) beyond what either mode can achieve on its own.
Ion thrusters and Hall-effect thrusters both generate beams of ions to create thrust. The main difference between the ion thruster and the Hall-effect thruster lies in the extraction system for the ions. The ion thruster uses two or three multi-aperture grids, in which the ions are accelerated due to the difference in potential between the first (screen) and second (accelerator) grids. Like the ion thruster, the Hall-effect thruster creates thrust by ionizing propellant within an electric field; however, instead of a grid, the Hall thruster has an electron plasma at the open end of the thruster. Because of the counter-flowing electron and ion currents in the Hall-effect thruster channel, a greater ion flux can be achieved compared to the ion thruster. For this reason, the Hall thruster yields a higher thrust-to-power ratio, whereas the ion thruster can produce higher exhaust velocities with higher overall thrust efficiencies.
Glenn's design has several noteworthy advantages. Having the neutralizer cathode assembly (NCA) in a central position eliminates the cantilevered-out-board NCA that most conventional ion thrusters use. In addition, the ion optics allow the ion thrusters to be scaled to very high power by permitting very large beam areas with relatively small electrode spans. These flat ion optics electrodes also improve thrust-to-power ratios and efficiencies compared to more conventional, spherically domed electrodes. Finally, the increased anode-surface area for electron collection means that the engine can operate at higher discharge currents, and therefore higher input power levels. All of these advantages add up to an electric propulsion device that combines the efficiencies of an ion thruster with the thrust-to-power capacity of a Hall-effect thruster. Glenn's technological advancement enables spacecraft to travel farther, faster, and more cheaply than with any other propulsion technology.
Potential applications include station-keeping on communications satellites, controlling the orientation and position of orbiting satellites, providing the main propulsion on deep space probes, and use on geosynchronous earth orbit (GEO) communications satellites.