Figure 1. Plasma potential profiles downstream of two BHT-200 thrusters.
Data were obtained at 5 mm intervals in each direction using the emissive probe. (From ref. 1.)
Figure 2. Clustered P5 Thrusters Sharing a Centered Cathode (see upper diagram) - Discharge current as a function of thruster centerline distance at an anode flow rate of 5.25 mg/s at a backpressure of 5.4x10-6
Torr-Xe. (From ref. 2.)
As worldwide telecommunication demands increase, so too do the satellites that support these growing networks (e.g. in the U.S., 30% of TV service subscribers used
satellite service (70% used cable) in 2009 versus just 6% in 1999 [from tba.org]).
The development of comparably higher power Hall thrusters is following suit.
The large solar arrays donned by modern telecommunication satellites, the high power-density (< 1 kg/kWe, kg/kW-electrically-generated) nuclear We to MWe power sources of future NASA
missions, along with increased satellite maneuverability demands from the Air Force, altogether represent the growing demand for an effective and efficient means
of electrically propelling these large spacecraft. The excellent performance
characteristics of modern Hall thrusters places them as a leading electrical spacecraft
propulsion candidate for the growing base of applications thus discussed. Yet, the
ground based testing of high power Hall thrusters has limits imposed by the difficulty in
maintaining a space-simulating high-vacuum pressure (1x10-5 torr or lower) at the high
mass-flowrates required to maintain the high power discharges (see ref. 2). Thus, interest has
grown in the use of multiple Hall thrusters clustered together as a means of achieving
greater overall thrust while still retaining the excellent performance characteristics of a
single Hall thruster; one with moderate enough power to have undergone complete
ground-based testing and development with coexisting vacuum chamber facilities.
In general, a cluster of thrusters totaling a given high-power level (e.g. 200 kW)
will result in lower thrust efficiency, higher system dry-mass, and shorter lifetime than a
single (monolithic) larger thruster of equivalent power. Yet, the advantages of a N-thruster
clustered approach include: N-1 system redundancy, N-step fully optimized
throttlability, and cheaper system development. Indeed, the recent JAXA (Japan
Aerospace Exploration Agency) asteroid explorer Hayabusa, launched with a 4-thruster
cluster of ion engines, has managed to complete its asteroid sample-return
mission—beleaguered by multiple EP engine failures—thanks to clustered
redundancy. (From ref. 4)
Work with clustered Hall thrusters at PEPL has spanned three major projects.
First, conducted in 2002-2004, a plume study with a cluster of 4 low-power 200-W Busek BHT-200 Hall thrusters demonstrated slight beam focusing with otherwise linearly superpositioned plasma properties (see ref. 1 for more details).
Second, conducted in 2004-2005, a facility study with a cluster of two medium power 5-kW PEPL P5 Hall thrusters demonstrated anomalous thrust measurements can occur at high vacuum chamber back-pressures (see ref. 2 for more details).
And third, conducted in 2005-2008, several studies with a cluster of 4 low-power 600-W Hall thrusters investigated various clustered thruster dynamics and demonstrated increased thrust with close clustered thruster spacing (see ref. 3-4 for more details).
Figure 3. BHT-600 cluster running in LVTF at 600 Watts (300 V, 2.01 A) X 4 thrusters.
Thrusters mounted on PEPL's milliNewton thrust stand
Inset photos, of Busek BHT-600 cluster front and rear prior to acceptance testing (U.S. quarter, 25 mm diameter, indicates scale). (From ref. 4.)
Selected Relevant Publications
Beal, B. E., "Clustering of Hall Effect Thrusters for High-Power Electric Propulsion Applications,"
Ph.D. Dissertation, University of Michigan, 2004.
Walker, M. L. R., "Effects of Facility Backpressure on the Performance and Plume of a Hall Thruster,"
Ph.D. Dissertation, University of Michigan, 2005.
Lobbia, R. B. and Gallimore, A. D., "Performance Measurements from a Cluster of Four Hall Thrusters,"
IEPC-2007-177, 30th International Electric Propulsion Conference, Florence, Italy, Sept. 17-20, 2007.
Lobbia, R. B., "A Time-resolved Investigation of the Hall Thruster Breathing Mode,"
Ph.D. Dissertation, University of Michigan, 2010.