# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 7 Hall Effect Propulsion/Hall Thrusters
### Design Considerations and Technologies  
Recall that, in a typical Hall thruster, magnetic field (\(\mathbf{\tilde{B}}\)) is applied across an accelerating electrical discharge (\(\mathbf{\tilde{E}}\)) allowing to trap the electrons in the _Hall effect_ (\(\mathbf{\tilde{E}}\times\mathbf{\tilde{B}}\)) _direction_. The anode is located at the base of the discharge channel, and also serves as the source of the neutral propellant. An external cathode is present outside the discharge channel and provides electrons that move towards the anode across the radial magnetic field [63; 80]. When the electrons enter the magnetic field, they spiral around the thruster axis in the (\(\mathbf{\tilde{E}}\times\mathbf{\tilde{B}}\)) direction and their interaction with the incoming propellant leads to the ionization of the propellant [63]. Hall thrusters can be classified into two types: magnetic layer and anode layer thrusters. The magnetic layer thrusters have continuous and extended acceleration zones for sufficient ionization and stability. They also have a ceramic wall, and their acceleration channel length is longer than the channel width. On the other hand, Hall thrusters with anode layer have a narrow acceleration zone (length of the discharge channel is shorter compared to the channel width) [81; 82]. The electron temperature of anode layer thrusters is higher than that of magnetic layer thrusters due to the lower electron energy losses [81]. Similar to ion engines, the Hall thrusters make use of heavy elements as propellants, for instance, xenon (Xe), krypton (Kr), iodine (I), bismuth (Bi) and argon (Ar). Of these, xenon has been favored for its lower ionization energy, higher atomic mass and easy storage. However, it is expensive to purchase and to perform ground tests with xenon [83]. Many cheaper alternatives to xenon exist, but further experiments have to be conducted to prove their usefulness [84]. Figure 6: Schematic of a Hall Effect Propulsion System. Lifetime of a Hall thruster is mainly limited by the erosion of the components protecting its magnetic circuitry from discharged plasma (ionized propellant). Once the magnetic poles are exposed over time, further degradation or overheating may occur, affecting the nominal magnetic field and thereby the thruster's performance. In order to determine the lifetime of a Hall thruster, apart from the usual long duration qualification tests, shorter duration experiments are also performed so that the erosion behavior can be characterized enabling the extrapolation of the thruster lifetime [85]. In comparison with gridded ion thrusters where ion beam can be properly controlled, it is more difficult to control the same in Hall thrusters leading to the wall erosion. This issue has troubled Hall thrusters for many decades, ever since they were introduced for larger spacecraft. Wall erosion is mainly caused when the the ions are driven towards the wall material because of elevated parallel component of electric field and the high electron temperature. Recently, a new technique called Magnetic Shielding (MS) was proposed that could potentially eliminate wall erosion in Hall thrusters. It is to be noted that, the magnetic and electric fields that are supposed to be mutually perpendicular are not so when under electron pressure [86]. When the walls are magnetically shielded, the electric field component parallel to the wall is nearly eliminated, resulting in the decrease of ion bombardment on the walls. For instance, when a magnetically shielded Boron Nitrate wall was used at an ion threshold energy of 25 V, the computed wall erosion rate was found to be approximately 600 times lower at the inner wall (wall closer to central line) than when unshielded. The outer wall on the other hand was found to experience zero erosion [86]. Characterizing the performance of newly designed Hall thrusters is often a tedious and expensive process [87]. To save time and cost incurred for characterization, scaling/sizing relations were introduced.