Development status, key technologies, and prospects of neutralizer-free ion thrusters

Abstract

<p indent="0mm">The ion thruster is one of the most widely used electric thrusters and is famous for its high specific impulse and efficiency, but there are some shortcomings of the conventional ion thruster, such as the complex structure and the difficulty of miniaturization. One of the most attractive solutions to make up for these shortcomings is the neutralizer-free ion thruster which can reduce the volume and mass of the ion propulsion system and improve the system reliability by canceling the neutralizer system. In this review, the principles, strengths, and weaknesses of the conventional ion thruster are summarized, which leads to the concept of the neutralizer-free ion thruster. Combined with the operations of ion thrusters in real space missions, the advantages of removing neutralizer systems are analyzed. According to the neutralization methods, the neutralizer-free ion thruster can be divided into two types which are the electronegative ion thruster and RF (radio-frequency) ion thruster based on the self-bias effect. At present, the neutralizer-free ion thruster is mainly developed in Europe, Japan, and the United States of America. Researchers in China also perform some numerical and experimental studies on the neutralizer-free ion thruster, but there is still no report on the relatively mature prototype model. Lastly, based on the application requirements of the space propulsion and characteristics of the neutralizer-free ion thruster, the key technologies of the neutralizer-free ion thruster are discussed: (1) For the sake of prolonging the service life and improving the beam performance, the choice of propellant in neutralizer-free ion thrusters, especially the electronegative ion thruster where the propellant should be electronegative, needs more consideration than conventional ion thrusters. (2) The voltages applied on the grid system are generally AC voltages. Thus, to extract the collimated ion beams with high neutralization degree, the determination of the frequency and amplitude of grid voltages should consider many factors, such as the propellant type, insulation design, and plasma properties. (3) The adjustment of the working mode becomes more complex in the neutralizer-free ion thruster, since it is more difficult to adjust the AC grid voltage than conventional DC grid voltages and the relationship between the AC grid voltage and thruster performance also needs further investigations. (4) The magnetic filter plays an important role in the electronegative ion thruster, but it should be optimized because it increases the volume and weight of the thruster and weakens the advantages of the neutralizer-free ion thruster. (5) The ion-ion plasma in the electronegative ion thruster and the sheath near the grid with AC voltages make the plasma characteristics in the neutralizer-free ion thruster quite different from the conventional ion thruster. It is critical to perform more studies on the plasma physics of the neutralizer-free ion thruster to better design the thruster and adjust its performance. Accordingly, it’s concluded that the investigations of the choice of the propellant, design of the magnetic filter and grid system, plasma physics, and flexible regulation method of the grid voltage should be focused on. Moreover, to further understand the performance of the neutralizer-free ion thruster, new methods for diagnosing this new type of thruster need to be developed.