Kristi de Grys

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.

Evaluation of a 4.5 kW Commercial Hall Thruster System for NASA Science Missions

The readiness of commercial Hall thruster technology is evaluated for near-term use on competitively-award, cost-capped science missions like the NASA Discovery program. Scientists on these programs continue to place higher demands on mission performance that must trade against the cost and performance of propulsion system options. Solar electric propulsion (SEP) systems can provide enabling or enhancing capabilities to several missions, but the widespread and routine use of SEP will only be realized through aggressive cost and schedule risk reduction efforts. Significant cost and schedule risk reductions can potentially be realized with systems based on commercial Hall thruster technology. The abundance of commercial suppliers in the United States and abroad provides a sustainable base from which Hall thruster systems can be cost-effectively obtained through procurements from existing product lines. A Hall thruster propulsion system standard architecture for NASA science missions is proposed. The BPT-4000 from Aerojet is identified as a candidate for near-term use. Differences in qualification requirements between commercial and science missions are identified and a plan is presented for a low-cost, low-risk delta qualification effort. Mission analysis for Discovery-class reference missions are discussed comparing the relative cost and performance benefits of a BPT-4000 based system to an NSTAR ion thruster based system. The BPT-4000 system seems best suited to destinations located relatively close to the sun, inside approximately 2 AU. On a reference near Earth asteroid sample return mission, the BPT-4000 offers mass performance competitive with or superior to NSTAR at much lower cost. Additionally, it is found that a low-cost, mid-power commercial Hall thruster system may be a viable alternative to aerobraking for some missions. Suggestions for the near- and far-term implementation of commercial Hall thrusters on NASA science missions are discussed.

Life and Operating Range Extension of the BPT-4000 Qualification Model Hall Thruster

Following completion of the 5,600 hour qualification life test of the BPT-4000 4.5 kW Hall Thruster Propulsion System, NASA and Aerojet have undertaken efforts to extend the qualified operating range and lifetime of the thruster to support a wider range of NASA missions. The system was originally designed for orbit raising and stationkeeping applications on military and commercial geostationary satellites. As such, it was designed to operate over a range of power levels from 3 to 4.5 kW. Studies of robotic exploration applications have shown that the cost savings provided by utilizing commercial technology that can operate over a wider range of power levels provides significant mission benefits. The testing reported on here shows that the 4.5 kW thruster as designed has the capability to operate efficiently down to power levels as low as 1 kW. At the time of writing, the BPT- 4000 qualification thruster and cathode have accumulated over 400 hours of operation between 1 kW-2 kW with an additional 600 hours currently planned. The thruster has demonstrated no issues with longer duration operation at low power.