Multiply charged ion generation and effects on performance and ion beam characteristics in Hall thruster plasmas

Abstract

Electric propulsion such as a pulsed plasma thruster, an ion thruster, and a Hall thruster, has been widely studied and developed for a variety of space missions including formation flying and micro-spacecraft constellation due to their advantages of providing a highly specific impulse and a reduced propellant mass in comparison with conventional propulsion. The basic concept of electric propulsion was developed in the early 1900s, and one of the electric propulsion, which is a stationary plasma thruster (SPT), was first launched into space in 1971 into space for station keeping on communication satellite. Electric propulsion can be classified into electrothermal, electrostatic, and electromagnetic types based on the operating principle. Electric propulsion has steadily been used for space missions like station keeping on satellites, and more difficult missions such as the Hayabusa asteroid sample return mission. As the demand for simple and advanced missions in space increases, electric propulsion will be an essential device for space applications in the near future. A Hall thruster is an electrostatic type electric propulsion, which generates plasmas by Hall effects in the channel. In general, electrons are supplied from a cathode, and magnetic fields are produced by electromagnets or permanent magnets that retain electrons at the end of the channel so that ionized ions are accelerated by electric fields. Hall thrusters have a simple and compact device structure and show a higher thrust density and a specific impulse at a given power on account of unlimited space charges. These make a Hall thruster more valuable for space applications. There are two types of Hall thrusters: annular type and cylindrical type. The annular type thruster has been studied since the 1960s, but recently, the cylindrical type thruster, which does not have an inner magnetic core and an inner wall, has been developed to make up for the shortcomings of the annular type Hall thruster. In this dissertation, we reported a study on the generation of multiply charged ions and its effects on the thruster performance as well as the ion beam characteristics in Hall thruster plasmas. Ion beam characteristics related to the multiply charged ions are dependent on the geometrical structures. Two types of thrusters, a cylindrical Hall thruster (CHT) and an annular Hall thruster (AHT), are compared under an identical condition including an identical channel diameter, channel depth, and propellant mass flow rate. A high propellant utilization in a CHT is caused by a high ionization rate, which leads to a large fraction of multiply charged ions. Ion currents and utilizations are very different due to the presence of multiply charged ions. Furthermore, multiply charged ions and plume characteristics in Hall thruster plasmas are investigated with regard to the magnetic field configuration. Differences in the plume shapes and differences in the fractions of ions with different charge states are demonstrated in the counter-current and co-current magnetic field configurations. The number of multiply charged ions is much higher, and high-charge-state ions including $Xe^{4+}$ are observed in the co-current configuration. Multiply charged ions have an influence on the thruster performance as well as the ion beam characteristics. In two typical magnetic field configurations of a CHT, ion beam characteristics, such as ion current, propellant efficiency, ion energy distribution, and ion fractions with different charge states, are obviously different. Therefore, these features bring about different thruster performances. Furthermore, large fractions of multiply charged ions and high ionization rates are observed and they have a strong influence on the thruster performance and the ion beam characteristics. The overall relationship between the thruster performances and plume characteristics under different magnetic field configurations is studied in relation to the multiply charged ions. Finally, we analyzed the significant parameters such as the thruster performance, the beam characteristics including geometrical and magnetic configurations, the magnetic mirror effects, the ion energy distribution function, and the multiply charged ion fractions to understand the effects of these characteristics on the generation of multiply charged ions. Among the co-current and counter-current configurations of the CHT and the AHT, the co-current configuration shows the highest fractions of multiply charged ions and high energetic ions and the widest full width at half maximum. In particular, for the CHT, the region where the magnetic fields are broadly distributed has a great influence on the fraction of multiply charged ions, and it is confirmed by altering the channel length of the CHT. The distribution of the magnetic field is related to the electron confinement so that the broad region where magnetic fields are distributed in the CHT could enhance the probability of the stepwise ionization in the channel due to broadly confined electrons.