Description: An approach for maximizing Hall thruster electrical system efficiency is to power the thruster directly from a high voltage solar array by a method commonly referred to as direct-drive. These direct drive system designs eliminate the power processing unit (PPU) and therefore have a substantial advantage in terms of overall electrical efficiency and mass savings. This Phase I/II proposal effort is comprised of the development of a Hall thruster direct drive unit (DDU). The DDU will include a method for cathode current sharing such that multiple thrusters can be operated in parallel from a single power source. In Phase I cathode current sharing approaches will be experimentally investigated. Approaches to be evaluated include passive and active methods of cathode current control. Active approaches involves independently controlled voltage sources placed in series with each cathode while passive approaches involve controlling cathode emission using heater and keeper power. In Phase II we will develop a nominal 15kW proto-flight brassboard level DDU and deliver it to NASA for additional characterization testing. The DDU unit will include the balance of PPU per specifications provided by NASA for thruster magnets, cathode heater and keeper etc. operation.

Benefits: NASA has identified 30kW-class SEP systems as a high-value intermediate step toward higher power systems due to broad cross-cutting capability. Current NASA investments include advanced next-generation solar arrays and higher power thrusters using either direct-drive and/or high voltage power processing units. Direct-drive is a method of maximizing Hall thruster electrical system efficiency by powering the thruster directly from a high voltage solar array. Studies suggest significant mass savings may be realized by implementing a direct-drive concept. As a result, NASA established the National Direct-Drive Testbed to develop direct-drive capability. The possibility for using Hall thrusters for lunar and Mars missions has also been well investigated. Hall thrusters are a good choice for Mars cargo missions and other studies have found Hall thrusters to be viable options for supporting lunar and Mars exploration. Another NASA study indicates that clustered Hall thrusters would be well sized for manned Mars missions.

A nominal 15kW Hall thruster system with the capability to throttle down to 5kW should find broad applications on DoD and commercial ComSats. Hall thrusters could enhance many high power DoD and commercial missions such as satellite servicing, orbit maintenance, orbit raising and lowering, inclination changes, and repositioning. The system could also find near term application on an all-electric upper stage derived from Busek's ESPA orbit maneuvering system (OMS), a free flying spacecraft based on the ESPA ring that is being developed in cooperation with United Launch Alliance (ULA). A low power system presently utilizes four BHT-1500 Xe Hall effect thrusters and capable of delivering up to five ESPA class (180kg) spacecraft to multiple orbits. The high power (30kW) version would be used for transportation of propellant to a LaGrange positioned fuel depot.