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Hall Effect Thruster
The Hall effect is the production of Hall voltage(a voltage difference) across an electrical conductor that is transverse to an electrical current in that conductor and a magnetic field that is perpendicular to the current.
This effect was discovered by Edward Herbert Hall in 1879. He made the discovery while he was working on his doctoral degree. His measurements of the tiny effect produced in the apparatus he used was accomplished 18 years before the discovery of the electron.
The Hall effect is due to the nature of current in a conductor. Current consists of the movement of many small carriers(electrons, holes or both). Moving charges experience the Lorentz force when a magnetic field is present that is perpendicular to their motion. In the absence of the magnetic field, the charges follow ‘line of sight’ paths between collisions with impurities. On the other hand, when a perpendicular magnetic field is present, their paths are curved and moving charges accumulate on one face of the material. Equal and opposite charges are exposed on the other face, where there is a scarcity of mobile charges. The result is an asymmetric distribution of charge density across the element that is perpendicular to the ‘line of sight’ path and the applied magnetic field. The separation of charge establishes an electric field that opposes the migration of further charge, and a steady electrical charge builds up while the current is flowing. The Hall effect offered the first proof that electric currents in metals are carried by moving electrons. The effect is very useful in measuring either the carrier density or the magnetic field.
There are a few practical applications of the theory and effect today. ‘Hall probes’ are often used to measure magnetic fields, or inspect tubing or pipelines. These devices produce a very low signal level and require amplification. The old vacuum tube amplifiers were too expensive, power consuming, and unreliable for everyday applications, but with the development of low cost integrated circuits that the Hall effect sensor became suitable for mass application. Many devices contain both the sensor and a high gain integrated circuit amplifier.
Another application is the Hall effect thruster(HET). An HET is a relatively low power device that is used to propel some craft once they get into orbit. In this propulsion system atoms are ionized and accelerated by an electric field. A radial magnetic field on the thruster is used to trap electrons. A large potential is established between the end of the thruster where neutral propellant is fed and the area where the electrons are produced. Electrons trapped in the magnetic field cannot fall down the potential and are extremely energetic which allows them to ionize neutral atoms. Neutral propellant is pumped into the chamber and is ionized by the trapped electrons after which positive ions and electrons are ejected from the thruster as a quasi-neutral plasma, producing the needed thrust. How’s that for Star War’s technology available today?
We have written many articles about the Hall Effect for Universe Today. Here’s an article about SMART-1′s ion engine, and here’s an article about the ion engine.
If you’d like more info on the Hall Effect, check out the Glenn Research Center Homepage, and here’s a link to the Glenn Research Center Gallery for recent images.
Sources:
Wikipedia
Hyperphysics
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