Design of a Laboratory Hall Thruster with Magnetically Shielded Channel Walls, Phase II: Experiments

Abstract

Magnetic shielding in Hall thrusters significantly reduces the transport of high-energy ions to the channel walls such that erosion is effectively eliminated. The physics of magnetic shielding were validated through laboratory experiments demonstrating essentially erosionless, high-performance Hall thruster operation for the first time. The magnetic field near the walls of a high-performance Hall thruster was modified to enable magnetic shielding while maintaining the magnetic field topology away from the walls necessary to retain efficient operation. Sixteen diagnostics were deployed to assess the performance, thermal, stability, and wear characteristics of the thruster in its original and modified configurations. At 300 V, 6 kW the total efficiency decreased from 63.5% to 62.4% while specific impulse increased from 1950 s to 2000 s. Wall temperatures in the last 30% of the channel were reduced by 12-16%. The amplitude of discharge current oscillations increased 25% with otherwise no noticeable change in the stability of the discharge. Plasma measurements at the walls validate our understanding of magnetic shielding as derived from the theory. The plasma potential was maintained very near the anode potential, the electron temperature was reduced by a factor of two to three, and the ion current density was reduced by at least a factor of two. Measurements of the carbon backsputter rate, wall geometry, and direct measurement of plasma properties at the wall indicate the wall erosion rate was reduced by 1000X relative to the unshielded thruster and by 100X relative to unshielded Hall thrusters late in life. These changes effectively eliminate wall erosion as a life limitation or failure mode in Hall thrusters and make them, for all intents and purposes, immortal.