Demonstration and characterization of a low power magnetically shielded Hall thruster (Past)

Ryan Conversano Dan Goebel Richard Hofer Ioannis Mikellides Ira Katz Richard Wirz

This research aims to demonstrate the applicability of magnetic shielding to low power Hall thrusters as a means to significantly improve operational lifetime. The key life-limiting factors of conventional Hall thrusters, including ion-bombardment sputter erosion of the discharge channel and high-energy electron power deposition to the channel walls, are well understood on all thruster scales. As thruster power is reduced to the sub-500 W nominal power regime, the increasing surface-to-volume ratio of the discharge channel and decreased thruster component sizes promotes increased plasma-wall interactions and susceptibility to overheating, thereby reducing thruster operational lifetime and performance. Although methods for compensating for these issues have been investigated, unshielded miniature Hall thrusters are generally limited to sub-45% anode efficiencies and maximum lifetimes on the order of 1,000 h. A magnetically shielded field topology aims to maintain low electron temperatures along the channel surfaces and a near-constant plasma potential near that of the discharge voltage across the discharge channel and along its full axial length. These features result in a reeducation of the kinetic energy of ions on trajectories to impact the channel surfaces to below the sputtering threshold, thereby preventing ion-bombardment erosion of the discharge channel. Improved confinement of high-energy electrons is another byproduct of the field structure, aiding in the reduction of electron power deposition to the channel. Magnetic shielding has been shown to dramatically reduce plasma-wall interactions on 4-6 kW Hall thrusters, resulting in significant increases in projected operational lifetimes with minimal effects to thruster performance.

In an effort to explore the scalability of magnetic shielding to low power devices, two magnetically shielded miniature (MaSMi) Hall thrusters were designed and fabricated. The performance of the first thruster, called the MaSMi 40, was characterized at an operating condition of 275 V and 325 W. A peak thrust of approximately 13 mN with a specific impulse of approximately 1,100 s at an anode efficiency of approximately 22% were measured at the nominal operating point. Observations of the near exit plasma discharge during operation and the discharge channel after operation suggested that the outer channel wall of the thruster was well shielded while the inner channel wall appeared to be weakly shielded. Further analysis determined that the MaSMi-40 was unsuccessful in generating a symmetric magnetically shielded field topology; however, the shortcomings of the magnetic circuit design were well understood.

The second design iteration in the development of a low power magnetically shielded Hall thruster was the MaSMi-60. Magnetic field measurements confirmed that a symmetric and fully shielded magnetic field topology was generated by this device across a wide range of possible operating conditions. At operating powers of less than 150 W to nearly 750 W, the key performance metrics of the MaSMi-60 included a measured thrust ranging from approximately 8 mN to over 33 mN with anode specific impulses ranging from under 800 s to nearly 1400 s at anode efficiencies of up to approximately 29%. Visual observations of the discharge plasma and channel walls during and after thruster operation offered strong evidence of magnetic shielding while the discharge was ignited. The plasma properties that define magnetic shielding were then confirmed using experimentally validated computational simulations of the MaSMi 60's plasma discharge.