Van de Graaff generator
Van de Graaff generator.
|Uses||Accelerating electrons to sterilize food and process materials, accelerating protons for nuclear physics experiments, driving X-ray tubes, etc.|
|Inventor||Robert J. Van de Graaff|
|Related items||Van de Graaff, linear particle accelerator|
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high amounts of electrical potential on a hollow metal globe on the top of the stand. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved in modern Van de Graaff generators can reach 5 megavolts. A tabletop version can produce on the order of 100,000 volts and can store enough energy to produce a visible spark.
A Van de Graaff generator operates by transferring electric charge from a moving belt to a terminal. The high voltages generated by the Van de Graaff generator can be used for accelerating subatomic particles to high speeds, making the generator a useful tool for fundamental physics research.
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A simple Van de Graaff-generator consists of a belt of silk, or a similar flexible dielectric material, running over two metal pulleys, one of which is surrounded by a hollow metal sphere. Two electrodes, (2) and (7), in the form of comb-shaped rows of sharp metal points, are positioned respectively near to the bottom of the lower pulley and inside the sphere, over the upper pulley. Comb (2) is connected to the sphere, and comb (7) to the ground. A high DC potential (with respect to earth) is applied to roller (3); a positive potential in this example.
As the belt passes in front of the lower comb, it receives negative charge that escapes from its points due to the influence of the electric field around the lower pulley, which ionizes the air at the points. As the belt touches the lower roller (6), it transfers some electrons, leaving the roller with a negative charge (if it is insulated from the terminal), which added to the negative charge in the belt generates enough electric field to ionize the air at the points of the upper comb. Electrons then leak from the belt to the upper comb and to the terminal, leaving the belt positively charged as it returns down and the terminal negatively charged. The sphere shields the upper roller and comb from the electric field generated by charges that accumulate at the outer surface of it, causing the discharge and change of polarity of the belt at the upper roller to occur practically as if the terminal were grounded. As the belt continues to move, a constant 'charging current' travels via the belt, and the sphere continues to accumulate negative charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current. The larger the sphere and the farther it is from ground, the higher will be its final potential.
Another method for building Van de Graaff generators is to use the triboelectric effect. The friction between the belt and the rollers, one of them now made of insulating material, or both made with insulating materials at different positions on the triboelectric scale, one above and other below the material of the belt, charges the rollers with opposite polarities. The strong e-field from the rollers then induces a corona discharge at the tips of the pointed comb 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 type of generator is easier to build for science fair or homemade projects, since it does not require a potentially dangerous high-voltage source. The trade-off is that it cannot build up as high a voltage as the other type, that cannot also be easily regulated, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects). Finally, since the position of the rollers can be reversed, the accumulated charge on the hollow metal sphere can either be positive or negative.
A Van de Graaff generator terminal does not need to be sphere-shaped to work, and in fact, the optimum shape is a sphere with an inward curve around the hole where the belt enters. Since electrically charged conductors have no e-field inside, charges can be added continuously. A rounded terminal minimizes the electric field around it, allowing greater potentials to be achieved without ionization of the surrounding air, or other dielectric gas. Outside the sphere, the e-field quickly becomes very strong and applying charges from the outside would soon be prevented by the field.
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 maximum achievable potential is approximately equal to the sphere's radius multiplied by the e-field where corona discharges begin to form within the surrounding gas. For example, a polished spherical electrode 30 cm in diameter immersed in air at STP (which has a breakdown voltage of about 30 kV/cm) could be expected to develop a maximum voltage of about 450 kV.
The fundamental idea for the friction machine as high-voltage supply, using electrostatic influence to charge rotating disk or belt, can be traced back to the 17th century or even before (cf. Friction machines History)
The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de Graaff at Princeton University on a fellowship, with help from colleague Nicholas Burke. The first model was demonstrated in October 1929. The first machine used an ordinary tin can, a small motor, and a silk ribbon bought at a five-and-dime store. Whereupon he went to the head of the physics department requesting a hundred dollars to make an improved version. He did get the money, with some difficulty. By 1931 he could report achieving 1.5 million volts, saying "The machine is simple, inexpensive, and portable. An ordinary lamp socket furnishes the only power needed." According to a patent application, it had two 60-cm-diameter charge-accumulation spheres mounted on borosilicate glass columns 180 cm high; the apparatus cost only $90 in 1931.
Van de Graaff applied for a second 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. An example of a three-stage operation has been built in Oxford Nuclear Laboratory in 1964 of a 10 MV single-ended "injector" and a 6 MV EN tandem.
One of Van de Graaff's accelerators used two charged domes of sufficient size that each of the domes had laboratories inside - one to provide the source of the accelerated beam, and the other to analyze the actual experiment. The power for the equipment inside the domes came from generators that ran off the belt, and several sessions came to a rather gruesome end when a pigeon would try to fly between the two domes, causing them to discharge. (The accelerator was set up in an airplane hangar.)
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. The highest potential sustained by a Van de Graaff accelerator is 25.5 MV, achieved by the tandem at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory.
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, 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, and both the voltage and currents attainable are much higher than with a conventional Van de Graaff generator. The 14 UD Heavy Ion Accelerator at The Australian National University houses a 15-million-volt pelletron. Its chains are more than 20 meters long and can travel faster than 50 km/hr.
The Nuclear Structure Facility (NSF) at Daresbury Laboratory was proposed in the 1970s, commissioned in 1981, and opened for experiments in 1983. It consisted of a tandem Van de Graaff generator operating routinely at 20 MV, housed in a distinctive building 70 metres high. During its lifetime, it accelerated 80 different ion beams for experimental use, ranging from protons to uranium. A particular feature was the ability to accelerate rare isotopic and radioactive beams. Perhaps the most important discovery made on the NSF was that of super-deformed nuclei. These nuclei, when formed from the fusion of lighter elements, rotate very rapidly. The pattern of gamma rays emitted as they slow down provided detailed information about the inner structure of the nucleus. Following financial cutbacks, the NSF closed in 1993.
Van de Graaff generators on display
The largest air-insulated Van de Graaff generator in the world, built by Dr. Van de Graaff in the 1930s, is now on permanent display at Boston's Museum of Science. With two conjoined 4.5-meter (15-foot) aluminium spheres standing on columns 22 feet (6.7 m) tall, this generator can often reach 2 MV (2 million volts). Shows using the Van de Graaff generator and several Tesla coils are conducted two to three times a day. Many science museums, such as the American Museum of Science and Energy, have small-scale Van de Graaff generators on display, and exploit their static-producing qualities to create "lightning" or make people's hair stand up. Van de Graaff generators are also used in schools and in science shows.
Comparison with other high-voltage generators
Other classical electrostatic machines like a Wimshurst machine or a Bonetti machine can easily produce more current than a Van de Graaff generator for experiments with electrostatics, and have positive and negative outputs. The less-insulated structures, however, result in smaller voltages.
The idea of bringing small amounts of charge into the middle of a large metal container where charge can accumulate to large values is central to the operation of the van de Graaff generator. The same physics is at work in the Kelvin water dropper. In that device, separate charged drops of water fall under gravity against same-charge buckets, building up charge. In the van de Graaff, small units of charge on an insulating belt ride into the large sphere using external work provided by a motor.
- U.S. Patent 1,991,236 — "Electrostatic Generator"
- U.S. Patent 2,922,905 — "Apparatus For Reducing Electron Loading In Positive-Ion Accelerators"
- Robert J. Van de Graaff
- Electrostatic induction
- Triboelectric effect
- Static electricity
- High voltage
- Kelvin Water Dropper
- Tesla coil
- Oudin coil
- Wimshurst machine
- Zavisa, John M. "How Van de Graaff Generators Work". HowStuffWorks. Retrieved 2007-12-28.
- Van de Graaff biography
- R. Van de Graaf, Phys. Rev. Vol.38, 1931, p.1919
- Niels Bohr's Times, Abraham Pais, Oxford University Press, 1991, pp.378-379
- Article "Van de Graaff's Generator", in "Electrical Engineering Handbook", (ed)., CRC Press, Boca Raton, Florida USA, 1993 ISBN 0-8493-0185-8
- J. Takacs, Energy Stabilization of Electrostatic Accelerators, John Wiley and Sons, Chichester, 1996
- J S Lilley 1982 Phys. Scr. 25 435-442 doi:10.1088/0031-8949/25/3/001)
- "The Bonetti electrostatic machine". www.coe.ufrj.br. Retrieved 2010-09-14.
|Wikimedia Commons has media related to Van de Graaff generators.|
- UVA Virtual Lab: Van de Graaff Generators University of Virginia
- Interactive Java tutorial - Van de Graaff Generator National High Magnetic Field Laboratory
- The Van de Graaff Accelerator Facility Western Michigan University
- Dr. Van de Graaff's huge machine at Museum of Science
- Van de Graaff Generator Frequently Asked Questions
- "Vivitron English version". Retrieved 2005-12-26. Vivitron 20MV+ generator
- Illustration from Report on Van de Graaff Generator From "Progress Report on the M.I.T. High-Voltage Generator at Round Hill"
- Nikola Tesla, "Possibilities Of Electrostatic Generators DOC". Scientific American, March, 1934. (.doc format)
- Paolo Brenni, The Van de Graaff Generator - An Electrostatic Machine for the 20th Century Bulletin of the Scientific Instrument Society No. 63 (1999)
- Charrier Jacques "Le générateur de Van de Graaff". Faculté des Sciences de Nantes.
- Making VDGs
- Hellborg, Ragnar, ed. Electrostatic Accelerators: Fundamentals and Applications [N.Y., N.Y.: Springer, 2005]. Available on-line at: http://books.google.com/books?id=tc6CEuIV1jEC&pg=PA51&lpg=PA51&dq=electrostatic+accelerator+book&source=web&ots=Qa0DbmiZJt&sig=bLoYaz_VUpBr7-Wv4lk_fLBnUo4#PPP1,M1
- Build your own VDG
-  The Magic House, St. Louis Children's Museum]