Hall effect is known as the appearance of an electric field by separation of charges, inside a conductor through which a current flows in the presence of a magnetic field1 with a component perpendicular to the movement of the charges. This electric field (Hall field) is perpendicular to the movement of the charges and to the perpendicular component of the applied magnetic field. It is named after its first modeler, the American physicist Edwin Herbert Hall (1855-1938).
Plasma is considered the fourth state of matter, and is formed by an ionized gas in which free electrons are found, that is, electrons that are not bound to any atom and ions. These ions are atoms or molecules with positive net charge due to the loss of one or more electrons. The electrical conductivity of the plasma is very high, and this is due to the presence of free electrical charges (ions and electrons), which are affected when they are inside an electromagnetic field.
The Hall effect consists of the appearance of an electric field in a conductor, through which a current flows, when it is crossed perpendicularly by the lines of a magnetic field. This electric field is perpendicular to the plane formed by the electric current and the lines of the magnetic field. This effect was discovered in 1879 by the American physicist Edwin Herbert Hall.
Thanks to consuming 100 million times less propellant or fuel than conventional chemical rockets, a Hall propeller is an attractive candidate to explore Mars, asteroids and even the edge of the solar system. By saving fuel, the propeller would allow much more space to be left in the spacecraft than would be left by a chemical propulsion, and that extra free space could be used with a large load of useful things for the mission. However, the current life of the Hall propellers, which is around 10,000 hours of operation, is too short for most space exploration, which requires at least 50,000 hours of operation.
Hall thrusters are electric rocket motors that use a very high speed plasma jet (in the order of 72,000 kilometers per hour, or 45,000 miles per hour) to push the ship forward. Its principle of operation is based on the creation of a low-pressure and almost neutral plasma discharge in a cross-field magnetic field and electric field configuration. The propellant gas, usually xenon, is ionized by electrons trapped in the magnetic field.
At the beginning of the 1960s, Hall effect ion engines were designed both in the URS and in the United States. The Russian scientist A. I. Morozov perfected the model of stationary plasma engine (SPT). Since the 70s, the Russians have sent more than 100 Hall effect ion motors into space on board their satellites.
In the Hall effect ion motors, an electrostatic field is used to accelerate the ions that are part of the gas flow that leaves the motor cavity. This electrostatic field is formed by the action of powerful electromagnets on the plasma flow (Hall effect). In this type of engine a high density gas such as xenon is used as a propellant. The gas is introduced into the chamber through holes located in the annular anode.
The electrons that are in the chamber affected by the magnetic field and the Hall effect are driven by it, rotating around the force lines of the magnetic field and describing an annular vortex. This annular flow of electrons has two effects. On the one hand, on colliding with the atoms of xenon gas ionizes them. On the other hand, they form a virtual cathode that accelerates the flow of ionized propellant.
At the outlet of the chamber an electron injector neutralizes the flow of plasma to prevent it, due to the positive charge it would have without the electrons, back again attracted by the negative charge of the motor body.
The main drawback of the Hall propellers is that the wall materials of the discharge channel greatly determine the discharge properties, and as a consequence, the level of performance and the operating time.
The materials of the wall intervene in the properties of the plasma mainly through a secondary emission of electrons, a phenomenon where high energy ions hit the surface of the channel wall and induce the emission of secondary electrons. In addition, the erosion of the walls of the discharge cavity due to the bombardment of the high-energy ions shortens the useful life of the propellant.
