2. Hall Thruster Physics The Hall thruster schematic is shown in Fig. 1. It consist of a coaxial annular cay where plasma is created by passing current between the annular anode on the vity upstream end of an otherwise dielectric cavity and the externally located cathode The propellant enters this plasma cavity via an annular manifold at the anode. a radial magnetic field is applied, either by ring-shaped permanent magnets, or through coils and soft iron yokes. The magnetic field greatly slows down the axial mean velocity of the electrons, which due to the low collisionality prevailing, are forced to execute mostly E xB drift around the annulus, while being radially confined by sheaths on the insulating walls The ions meanwhile, are only weakly affected by the magnetic field, and, if the density is low enough that collisions are rare, are simply accelerated by the electrostatic field to an exit velocity (1) where V is the potential at the place where the ion is created( with respect to the outside potential). Because of the quasi-neutrality faciliated by the presence of the electrons, no space-charge limitation arises in this type of thruster(as opposed to gridded ion thrusters), and the acceleration distance can be several cm, compared to the typical 0. 5-1 mm gap used in ion engines. This flexibility is one of the main advantages of the Hall thruster. It also removes the strong thrust density limitation dictated by the Child-Langmuir law in ion engines To the extent that collisions do occur, but, more importantly because of electron electrons also travel axially across the b field under the influence of the applied axial E field. They are then collected by the upstream anode, and pumped by the power supply to an external cathode. The emitted electrons mainly join the accelerated ions to form a neutralized plasma beam, but, inevitably a fraction also diverts upstream into the accelerator section This fraction is to be minimized(this is the role of the magnetic field) because their acceleration to the anode potential is one of the devices loss mechanisms. On the other hand not all of this energy is lost to the anode because a good part of it is used to produce ionization of the injected neutral The name Hall Thruster"arises form the mechanism by which thrust forces are exerted on the solid parts of the engine. As indicated ions are simply accelerated by the electrostatic e field but since the ions are in a quasineutral plasma throughout an equal and opposite electrostatic force is exerted on the free electrons in tha plasma. In the presence of the radial magnetic field b, however these electrons are not free to accelerate towards the anode; instead, they drift"azimuthally (perpendicular to both e and B)at such a velocity as to generate an equal and opposite magnetic force on themselves. If we denote by x the forward axial direction (Fig. 2) the electrons end up balanced 16. 522, Space Propulsion Lecture 17 Prof. Manuel martinez Page 3 of 72. Hall Thruster Physics The Hall thruster schematic is shown in Fig. 1. It consist of a coaxial annular cavity where plasma is created by passing current between the annular anode on the upstream end of an otherwise dielectric cavity and the externally located cathode. The propellant enters this plasma cavity via an annular manifold at the anode. A radial magnetic field is applied, either by ring-shaped permanent magnets, or through coils and soft iron yokes. The magnetic field greatly slows down the axial mean velocity of the electrons, which, due to the low collisionality prevailing, are forced to execute mostly E × B drift around the annulus, while being radially confined by sheaths on the insulating walls. The ions meanwhile, are only weakly affected by the magnetic field, and, if the density is low enough that collisions are rare, are simply accelerated by the electrostatic field to an exit velocity ci = 2eV mi (1) where V is the potential at the place where the ion is created (with respect to the outside potential). Because of the quasi-neutrality faciliated by the presence of the electrons, no space-charge limitation arises in this type of thruster (as opposed to gridded ion thrusters), and the acceleration distance can be several cm, compared to the typical 0.5 - 1 mm gap used in ion engines. This flexibility is one of the main advantages of the Hall thruster. It also removes the strong thrust density limitation dictated by the Child-Langmuir law in ion engines. To the extent that collisions do occur, but, more importantly, because of electron scattering by a combination of plasma electrostatic fluctuations and wall collisions, electrons also travel axially across the B field under the influence of the applied axial E field. They are then collected by the upstream anode, and pumped by the power supply to an external cathode. The emitted electrons mainly join the accelerated ions to form a neutralized plasma beam, but, inevitably, a fraction also diverts upstream into the accelerator section. This fraction is to be minimized (this is the role of the magnetic field) because their acceleration to the anode potential is one of the device’s loss mechanisms. On the other hand, not all of this energy is lost to the anode, because a good part of it is used to produce ionization of the injected neutral gas. The name “Hall Thruster” arises form the mechanism by which thrust forces are G E field, but since the ions are in a quasineutral plasma throughout, exerted on the solid parts of the engine. As indicated, ions are simply accelerated by the electrostatic an equal and opposite electrostatic force is exerted on the free electrons in that plasma. In the presence of the radial magnetic field G B , however, these electrons are G E and not free to accelerate towards the anode; instead, they “drift” azimuthally (perpendicular to both G B ) at such a velocity as to generate an equal and opposite magnetic force on themselves. If we denote by x the forward axial direction (Fig. 2) the electrons end up balanced as 16.522, Space Propulsion Lecture 17 Prof. Manuel Martinez-Sanchez Page 3 of 7