Experimental investigations of plasma parameters and species-dependent ion energy distribution in the plasma exhaust plume of a Hall thruster.

Abstract

For the purpose of understanding and characterizing the behavior of multiply charged propellant ions in a Hall thruster plume, the plasma parameters and ion energy distribution in the SPT-100 plume were studied. The investigations started with characterizing the plasma properties over an extensive volume of the plume. The distinct peak structures of plasma parameter profiles provided a great insight into the behavior of plume particles. In the very-near-field study, the radial profiles of ion current density, electron temperature, and electron number density were measured. The electron population in the plume was controlled by the competing effects of the two phenomena: the electric and magnetic field influences on the electrons which was dominant very near the thruster exit, and the quasineutrality in the plasma which was dominant farther downstream of the thruster exit. In the near- and far-field study, the angular profiles of ion current density, electron temperature, electron number density, and plasma potential were measured. The original annular structure of the ion beam affected the Plume characteristics in the near-field. Both studies implied the existence of multiply charged propellant ions in the plume. To characterize the species-dependent ion parameters, an <bold>ExB</bold> probe was utilized to measure ion energy distributions of each ion species. Each peak in the measured probe traces were curve-fitted with a model based on the kinetic theory of gases to obtain ion energy distribution functions, which provided the exponential factor n of the distribution functions, ion beam energy, ion energy spread, and ion species fraction. The existence of Xe<super>4+</super> ions in the plume was revealed. The beam energy and ion species fractions data gave rise to a simple ionization and acceleration mechanism in the SPT-100 discharge chamber. The angular profiles of ion species fractions implied that the primary production of Xe<super>1+</super> ions was clearly separated from the primary production of Xe<super>2+</super> and Xe<super> 3+</super> ions by a narrow boundary in the discharge chamber. These studies provided the most comprehensive collection of plasma parameters in the SPT-100 plume. Also, the <bold>ExB</bold> technique was the first high-resolution, species-dependent, direct measurements of ion energy distribution in the Hall thruster plume.