1. Improved Hall thruster performance via tailored external magnetic field Lyon B. King Department of Mechanical Engineering Michigan Technological University

2. Tailored external magnetic fields Background and Motivation Much interest of late in high thrust-to-power Cathode coupling losses are significant at low discharge voltage Cathode position has been shown to affect plume divergence, coupling voltage, and thruster efficiency Center-mount cathodes are superior to external, but impractical for small thrusters

3. Tailored external magnetic fields Background and Motivation Typical HET Magnetic Field Prior research has focused primarily on the internal B- field Examination of the external field reveals a separatrix Recent research at Tech suggests the separatrix plays an important role in the coupling process

4. Tailored external magnetic fields Background and Motivation Typical HET Magnetic Field Prior research has focused primarily on the internal B- field Examination of the external field reveals a separatrix Recent research at Tech suggests the separatrix plays an important role in the coupling process Typical cathode locations

5. Tailored external magnetic fields Background and Motivation Performance vs. Cathode Position V d = 250 V, Mass Flow = 2.553 mg/s Kr (equiv. 4 mg/s Xe), I mag = 2.5 A

6. Tailored external magnetic fields Background and Motivation Plasma Parameters T e (eV) r = 200 mm r = 120 mm r = 100 mm r = 80 mm r = 60 mm r = 50 mm

7. Tailored external magnetic fields Background and Motivation Plasma Parameters n e (x 10 17 m - 3 ) r = 200 mm r = 120 mm r = 100 mm r = 80 mm r = 60 mm r = 50 mm

8. Tailored external magnetic fields Background and Motivation Problem for Small (0.250 – 2-kW) Thrusters Performance better with cathode inside separatrix Separatrix too close to ion beam for external cathode Cathode will sputter, decreasing lifetime Thruster too small for center mount cathode

9. Tailored external magnetic fields Background and Motivation Original Outer Pole Extended Outer Pole Solution: Move the Separatrix The Extended Outer Pole U.S. Patent application Number 61/118,306

10. Tailored external magnetic fields Background and Motivation B-Field Comparison Original Outer Pole At z=30 mm, r=60 mm Extended Outer Pole At z=30 mm, r=78 mm

11. Tailored external magnetic fields Background and Motivation V d = 250 V, Mass Flow = 2.553 mg/s Kr (equiv. 4 mg/s Xe)‏ Performance Comparison

12. Tailored external magnetic fields Understand physics of electron mobility across the external magnetic field / separatrix Determine how performance gains seen in preliminary work extend to flight-like conditions Explore tailored magnetic fields in search for “optimal” configuration(s) Objective of Research

13. Tailored external magnetic fields Performance studies with Xe at range of operating conditions Numerical modeling of magnetic fields to explore means of controlling separatrix position Continued probe maps of plasma parameters Numerical modeling of cathode / plume coupling Technical Approach

14. Tailored external magnetic fields Design criteria for low-power Hall thrusters having improved efficiency over SOA Better understanding of cathode coupling voltage and loss mechanisms Possible implications for clustered Hall thrusters Anticipated Results