Alex Mathers

Magnetic shielding of the channel walls in a Hall plasma accelerator

In a qualification life test of a Hall thruster it was found that the erosion of the acceleration channel practically stopped after ∼5600 h. Numerical simulations using a two-dimensional axisymmetric plasma solver with a magnetic field-aligned mesh reveal that when the channel receded from its early-in-life to its steady-state configuration the following changes occurred near the wall: (1) reduction of the electric field parallel to the wall that prohibited ions from acquiring significant impact kinetic energy before entering the sheath, (2) reduction of the potential fall in the sheath that further diminished the total energy ions gained before striking the material, and (3) reduction of the ion number density that decreased the flux of ions to the wall. All these changes, found to have been induced by the magnetic field, constituted collectively an effective shielding of the walls from any significant ion bombardment. Thus, we term this process in Hall thrusters “magnetic shielding.”

Demonstration of 10,400 Hours of Operation on a 4.5 kW Qualification Model Hall Thruster

Between 2007 and 2009, Aerojet and Lockheed Martin Space Systems Company (LMSSC) successfully extended the demonstrated operating duration of the qualification model BPT-4000 4.5 kW Hall thruster beyond 10,400 hrs. A total of 452 kg of xenon were consumed during the entire qualification marking the most throughput ever demonstrated on a Hall thruster. The BPT-4000 Hall thruster is part of a 4.5 kW Hall Thruster Propulsion System (HTPS) developed jointly by Aerojet and LMSSC. The system is slated for initial launch in the summer of 2010 on the first Advanced EHF spacecraft. The testing demonstrated the thruster’s ability to provide more than 8.7 MN-s of total impulse and 7,316 ignition cycles. At the conclusion of testing, the thruster showed no signs of degradation in performance and all health indicators were stable. Most significantly, there was no measurable insulator ring erosion from 5,600 hrs to 10,400 hrs indicating that the thruster had reached a “zero” erosion configuration. This result demonstrates that Hall thrusters can, if designed properly, achieve lifetimes comparable to ion thrusters. It also eliminates one of the perceived barriers to the use of Hall thrusters for applications such as asteroid fly-by, cargo transfer, and satellite servicing missions that require significant throughput.

Performance and Environmental Test Results of the High Voltage Hall Accelerator Engineering Development Unit

NASA Science Mission Directorates In-Space Propulsion Technology Program is sponsoring the development of a 3.5 kW-class engineering development unit Hall thruster for implementation in NASA science and exploration missions. NASA Glenn and Aerojet are developing a high fidelity high voltage Hall accelerator that can achieve specific impulse magnitudes greater than 2,700 seconds and xenon throughput capability in excess of 300 kilograms. Performance, plume mappings, thermal characterization, and vibration tests of the high voltage Hall accelerator engineering development unit have been performed. Performance test results indicated that at 3.9 kW the thruster achieved a total thrust efficiency and specific impulse of 58%, and 2,700 sec, respectively. Thermal characterization tests indicated that the thruster component temperatures were within the prescribed material maximum operating temperature limits during full power thruster operation. Finally, thruster vibration tests indicated that the thruster survived the 3-axes qualification full-level random vibration test series. Pre and post-vibration test performance mappings indicated almost identical thruster performance. Finally, an update on the development progress of a power processing unit and a xenon feed system is provided.

Overview of the Development of a Low Cost High Voltage Hall Accelerator Propulsion System for NASA Science Missions

NASA’s Science Mission Directorate In-Space Propulsion Technology Program is funding NASA Glenn Research Center (GRC) to develop a high specific impulse, long-life, low-cost high voltage Hall accelerator (HiVHAc) engineering model (EM) Hall effect thruster. NASA GRC and Aerojet have completed the fabrication and extensive testing of a HiVHAc EM thruster that incorporates a discharge channel replacement mechanism as a means of achieving long-life. HiVHAc EM performance characterization indicated that the design met and exceeded desired performance levels. A new throttle table that includes high thrust-to-power operation has improved the thruster’s performance for some NASA science missions. However, testing also revealed that thermal, magnetic circuit saturation, and channel replacement mechanism issues and challenges exist. As a result, NASA GRC and Aerojet initiated and completed design changes to the EM thruster to alleviate encountered issues and challenges. In addition, the HiVHAc project is leveraging power processing unit (PPU) developments by Aerojet and by NASA’s Small Business Initiative Research Program. This includes evaluating performance of a wide-output range brassboard PPU that can process input voltages between 80 and 160 volts and is capable of output voltages between 200 and 700 V. Finally, the HiVHAc project has leveraged xenon feed system development by the Science Mission Directorate’s In Space Propulsion Technology Program. The HiVHAc project and Air Force Research Laboratory are funding the development of the next generation of light-weight, low-power consumption, and small-footprint xenon feed system. The unit, designated xenon flow control module, is manufactured by VACCO and will be delivered to NASA GRC in September 2011.

Development of a Modular Hall Thruster Power Converter

Aerojet has completed the development and qualification of a 4.5 kW Hall thruster system to serve GEO satellite applications for station keeping and orbit raising as well as for use as primary propulsion on NASA missions. This 4.5 kW Hall thruster system is currently in use on the Air Force’s first Advanced Extremely High Frequency (AEHF) satellite. The system includes the BPT-4000 Hall thruster, Power Processing unit (PPU), Xenon Flow Controller (XFC) and associated electrical harnessing. Hall thruster technology is applicable to a wide range of missions that will employ a variety of power systems and as such Aerojet and the Jet Propulsion Laboratory (JPL) have identified the need to develop a scalable power converter that can be tailored to a given application. The envisioned PPU architecture will be able to function over a wide input voltage range (~2:1), provide efficient throttling over a wide output range (100s of Watts – 10s of kilowatts), and employ a robust topology capable of meeting stringent radiation, derating, and component quality requirements. As an added benefit, a modular power converter will reduce the development time and cost for the next generation of PPUs that will be needed to serve applications requiring variable input voltages, higher output power, and a wider power and voltage throttling range. The modular power converters are capable of accepting unregulated 70-140 V input voltages with an output voltage range of 150-400 V when operated in parallel or 150-800 V when operated in series. A full bridge, phase-shifted, zero voltage switching topology was selected as the most suitable for satisfying the wide input and output range. Two converters were fabricated to demonstrate the individual capability of the module as well as the stacked capability of modules working in series or parallel configurations. Converter power efficiency was measured over the full input and output voltage range. Peak efficiencies meeting the target of 94% were measured. Testing under parallel operation of the converters demonstrated current sharing while series operation demonstrated the full 800 V output capability. Demonstration of these power modules retires the major risk associated with converting Aerojet’s existing regulated PPU technology to an expanded architecture compatible with NASA missions and a variety of commercial platforms.

Development Status of the HiVHAC Hall Thruster

The High Voltage Hall Accelerator (HiVHAC) development program task, funded by the NASA Science Mission Directorates In-Space Propulsion Technology Program, is advancing the current state-of-the-art Hall thruste rs performance, life and cost. Its goal is to meet and/or exceed the requirements of Discovery class missions by developing a thruster that operates over a range of input powers from 0.3 to 3.5 kW, that can operate at specific impulses from 1,000 to 2,800 s, and that can proces s 300 kg of propellant while operating at full power. The HiVHAC program is currently building upon a history of design, analysis, fabrication, and test experience gained from a seri es of experimental investigations over the last several years. The latest design is the NASA- 103M.XL (eXtended Life) Hall thruster. This thruster incorporates design features enabling a projected lifetime in excess of 15,000 hours at a power level of 3.5 kW. A laboratory model of the NASA-103M.XL has demonstrated > 4,000 hours of wear testing at a dis charge voltage of 700 V. The next task of the HiVHAC program is to evolve this design into an engineering model thruster capable of demonstrating TRL-6 readiness. This paper will focus on the development status of the engineering model design.

High voltage hall accelerator propulsion system development for NASA science missions

NASA Science Mission Directorates In-Space Propulsion Technology Program is sponsoring the development of a 3.8 kW-class engineering development unit Hall thruster for implementation in NASA science and exploration missions. NASA Glenn Research Center and Aerojet are developing a high fidelity high voltage Hall accelerator (HiVHAc) thruster that can achieve specific impulse magnitudes greater than 2,700 seconds and xenon throughput capability in excess of 300 kilograms. Performance, plume mappings, thermal characterization, and vibration tests of the HiVHAc engineering development unit thruster have been performed. In addition, the HiVHAc project is also pursuing the development of a power processing unit (PPU) and xenon feed system for integration with the HiVHAc engineering development unit thruster. Colorado Power Electronics and NASA Glenn Research Center have tested a brassboard PPU for more than 1,500 hours in a vacuum environment, and a new brassboard and engineering model PPU units are under development. VACCO Industries developed a xenon flow control module which has undergone qualification testing and will be integrated with the HiVHAc thruster extended duration tests. Finally, recent mission studies have shown that the HiVHAc propulsion system has sufficient performance for four Discovery- and two New Frontiers-class NASA design reference missions.