Van de Graaff generator
- For the band with a similar name, see Van der Graaf Generator.
A Van de Graaff generator is a machine which uses a moving belt to accumulate very high charges on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can be up to 5 megavolts. Applications for high voltage generators exist with high voltage X-ray tubes, sterilization of food, and nuclear physics experiments. The Van de Graaf generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.
A simple Van de Graaff generator consists of a belt of silk running over two pulleys, one of which is surrounded by a hollow metal sphere. Two electrodes, E1 and E2, in the form of sharply pointed cones, are positioned respectively near to the bottom of the pulley and inside the sphere. E2 is connected to the sphere, and E1 is giving a high potential with respect to earth; a positive potential in this example.
This high voltage ionises the air at the tip of E1, repelling positive charges onto the belt and they are carried up inside the sphere. This positive charge induces a negative charge to the electrode E2 and a positive charge to the sphere (to which E2 is connected). The high potential difference ionises the air inside the sphere and negative charges are repelled on to the belt, discharging it. As a result of the Faraday's ice pail or Faraday cage effect, positive charge on E2 migrates to the sphere regardless of the sphere's existing voltage. As the belt continues to move, a constant current travels via the belt, and the sphere charges further positive, until the rate of leakage equals the rate at which charge is induced. The larger the sphere and the farther it is from ground, the higher will be its final potential, with the potential being roughly the sphere diameter multiplied by the value for e-field breakdown for the surrounding gas.
The other method for building Van de Graaff generators is to use the triboelectric effect. The two rollers for the belt are made of different materials, far from each other on the triboelectric series. When the belt comes into contact with one and is then separated, charge is transferred from the roller to the belt, and the roller becomes charged. When the belt comes into contact with the other roller and is then separated, charge is transferred from the belt to the roller, and that roller develops an opposite charge. The e-fields from the rollers then induce a corona discharge at the tip of the pointed electrodes. The electrodes then "spray" a charge onto the belt which is opposite in polarity to the charge on the rollers. The remaining operation is otherwise the same as the voltage-injecting version above. This form is easier to build for science fair or homemade projects, since it doesn't require a dangerous high voltage source. The trade-off is that it cannot build up as high a voltage as the other type, and operation is difficult under the humid conditions which prevent triboelectric effects from occurring.
Since a Van de Graaff generator can supply the same small current at almost any level of electrical potential, it is an example of a nearly-ideal current source.
The Van de Graaff generator was developed, starting in 1929, by MIT physicist Robert J. Van de Graaff. The first model was demonstrated in October 1929. The first machine used a silk ribbon bought at a five and dime store as the charge transport belt. In 1931 a version capable of producing 1,000,000 volts was described in a patent disclosure. This version had two 60 cm diameter charge accumulation spheres mounted on Pyrex glass columns 180 cm high; the apparatus cost only $90.
Van de Graaff applied for a patent in December 1931, which was assigned to MIT in exchange for a share of net income. The patent was later granted.
A more recent development is the tandem Van de Graaff accelerator, containing one or more Van de Graaff generators, in which negatively charged ions are accelerated through one potential difference before being stripped of two or more electrons, inside a high voltage terminal, and accelerated again.
By the 1970s, up to 14 million volts could be achieved at the terminal of a tandem that used a tank of high pressure sulfur hexafluoride (SF6) gas to prevent sparking by trapping electrons. This allowed the generation of heavy ion beams of several tens of megaelectronvolts, sufficient to study light ion direct nuclear reactions.
A further development is the pelletron, where the rubber or fabric belt is replaced by a chain of short conductive rods connected by insulating links. The gas-insulated column is replaced by a vacuum chamber, and the air-ionizing electrodes are replaced by a grounded roller and inductive charging electrode. The chain can be operated at much higher velocity than a belt, the vacuum withstands much higher potentials without sparking, and both the voltage and currents attainable are much higher than with a conventional Van de Graaff machine.
A common misspelling of the name is Van der Graaf (with an R and a single F). See also Van der Graaf Generator (band).
Van de Graaff generators on display
One of the largest Van de Graaff generators in the world, built by Dr. Van de Graaff himself, is now on permanent display at Boston's Museum of Science. With two conjoined 15 foot aluminum spheres standing on columns many feet tall, this generator can often reach 2 million volts on a cool, crisp New England day. Shows using the Van de Graaff generator and several Tesla coils are conducted several times each day.
- Article "Van de Graaff's Generator", in "Electrical Engineering Handbook", Richard C. Dorf (ed)., CRC Press, Boca Raton, Florida USA, 1993 ISBN 0849301858
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- US2922905 -- "Apparatus For Reducing Electron Loading In Positive-Ion Accelerators"
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