Effects of magnetic field topography on ion thruster discharge performance

Abstract

Traditional magnetic field design techniques for dc ion thrusters typically focus on closing a sufficiently high maximum closed magnetic contour, Bcc, inside the discharge chamber. In this study, detailed computational analysis of several modified NSTAR thruster 3-ring and 4-ring magnetic field geometries reveals that the magnetic field line shape as well as Bcc determines important aspects of dc ion thruster performance (i.e. propellant efficiency, beam flatness and double ion content). The DC-ION ion thruster model results show that the baseline NSTAR configuration traps the primary electrons on-axis, which leads to the high on-axis plasma density peak and high double ion content observed in experimental measurements. These problems are further exacerbated by simply increasing Bcc and not changing the field line shape. Changing the field line shape to prevent on-axis confinement (while maintaining the NSTAR baseline Bcc) improves thruster performance, improves plasma uniformity and lowers double ion content. For these favorable field line geometries, we observe further improvements to performance with increased Bcc, while maintaining plasma uniformity and low double ion content. These improvements derive from the fact that the field lines guide the high-energy primaries to regions where they are most efficiently used to create ions while a higher Bcc prevents the loss of ions to the anode walls. Therefore, it is recommended that the ion thruster designer first establish a divergent field line shape that ensures favorable beam flatness, low double ion content and reasonable performance; then the designer may adjust the Bcc to attain desirable performance and stability for the target discharge plasma conditions.