applied sciences  
Article  
Preliminary Design Tool for Medium-Low-Power Gridded Ion Thrusters [PERSON] 1,*\({}^{\text{\textregistered}}\), [PERSON] 2\({}^{\text{\textregistered}}\) and [PERSON] 2\({}^{\text{\textregistered}}\)  
first planetary defense test mission [3; 4; 5]. The DART spacecraft exploits the NEXT (NASA's evolutionary xenon thruster) gridded ion thruster, and recent studies have been performed on this thruster's life [6]. Another line of research proposes to direct the plume particle flow of an electric thruster to space debris or asteroids, to change their flight path, and studies on the GIT's beam are being carried out [7]. Other studies regarding the active space debris removal in LEO through small satellites powered by GITs can be found in Ref. [8]. Gridded ion thrusters (GITs) belong to the category of electrostatic ion engines and are composed of three main components: hollow cathodes, grids, and a discharge chamber in which plasma is generated. In an ion thruster, a propellant is injected from the propellant tank into an anode potential discharge chamber. Xenon is the typical propellant used in GITs; it is inert, easily ionized, has high storage density, and has high atomic mass, allowing one to generate a good level of thrust, but other choices such as iodine have been investigated and tested as alternatives. Iodine creates more thrust than xenon at similar operating power; it is cheaper and more abundant, but it is toxic and corrosive [9]. Furthermore, recent studies have been carried out on the use of argon as a propellant, which is much less expensive than xenon [10]. A hollow cathode emits high-energy electrons, which impact the propellant atoms that lose one or two electrons, generating some positively charged ions. A gas, with no overall electric charge, called plasma, consisting of positive ions and negative electrons, is produced. To improve the efficiency of this ion-generation process, different types of magnetic confinement are employed at the anode wall in order to increase the electron path length and the residence time of electrons in the chamber before the loss to the anode wall (which has high positive potential due to the voltage applied by the discharge power supply), raising the probability of propellant ionization by the injected electrons. The accurate design of the magnetic field is necessary to increase the efficiency, keeping an appropriate electron loss to the anode to produce stable discharge. In the past, empirical studies have tried to find the optimal design of the magnetic field to confine electrons and ions in ion thrusters. The results of these studies showed that the ring cusp magnetic field generated by permanent magnets was the best solution for electrons and ions confinement [11; 12]. As a consequence, this configuration has become the most employed in thruster design [13]. The positively charged ions generated in the chamber are then accelerated by a system of two or three charged grids, which are made of thousands aligned holes and expelled by the chamber at very high speeds (up to 40 km/s), producing thrust. The first grid (screen grid) has an high positive potential, and the second grid (acceleration grid) has a negative potential. The third grid (deceleration grid) has a higher potential than the second grid and is used to improve the thruster life, shielding the acceleration grid from ion bombardment, but it has disadvantages such as the increase in constructive complexity of the thruster; for this reason, the third grid is not used in some thrusters. The geometry of the grids is a critical performance driver in ion-thruster design; improving the grid geometry could significantly increase the performance of GITs in terms of specific impulse and erosion rate. Recent studies, with the aim of optimizing the grid design, have been carried out [14]. Finally, the ion beam is neutralized thanks to a neutralizer--in general, an external smaller hollow cathode that produces electrons to make the total charge of the beam neutral.