Van de Graaff generator
Van de Graaff generator
|Uses||Accelerating electrons to sterilize food and process materials, accelerating protons for nuclear physics experiments, producing energetic X-ray beams in nuclear medicine, physics education, entertainment|
|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 electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct current (DC) electricity at low current levels. 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. Small Van de Graaff machines are produced for entertainment, and in physics education to teach electrostatics; larger ones are displayed in science museums.
The Van de Graaff generator was developed as a particle accelerator in physics research, its high potential is used to accelerate subatomic particles to high speeds in an evacuated tube. It was the most powerful type of accelerator in the 1930s until the cyclotron was developed. Today it is still used as an accelerator to generate energetic particle and x-ray beams in fields such as nuclear medicine. In order to double the voltage, two generators are often used together, one generating positive and the other negative potential; this is called a tandem Van de Graaff accelerator. For example, the Brookhaven National Laboratory Tandem Van de Graaff achieves about 30 million volts of potential difference.
The voltage produced by an open-air Van de Graaff machine is limited by arcing and corona discharge to about 5 megavolts. Most modern industrial machines are enclosed in a pressurized tank of insulating gas; these can achieve potentials up to about 25 megavolts.
A simple Van de Graaff generator consists of a belt of rubber (or a similar flexible dielectric material) running over two rollers of differing material, 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 near the bottom of the lower roller and inside the sphere, over the upper roller. Comb (2) is connected to the sphere, and comb (7) to ground. The method of charging is based on the triboelectric effect, wherein simple contact of dissimilar materials causes the transfer of some electrons from one material to the other. For example (see the diagram), the rubber of the belt will become negatively charged while the acrylic glass of the upper roller will become positively charged. The belt carries away negative charge on its inner surface while the upper roller accumulates positive charge. Next, the strong electric field surrounding the positive upper roller (3) induces a very high electric field near the points of the nearby comb (2). At the points, the field becomes high enough to ionize air molecules, and the electrons are attracted to the outside of the belt while positive ions go to the comb. At the comb (2) they are neutralized by electrons that were on the comb, thus leaving the comb and the attached outer shell (1) with fewer net electrons. By the principle illustrated in the Faraday ice pail experiment, i.e. by Gauss's Law, the excess positive charge is accumulated on the outer surface of the outer shell (1), leaving no field inside the shell. Electrostatic induction by this method continues, building up very large amounts of charge on the shell.
In the example, the lower roller (6) is metal, which picks negative charge off the inner surface of the belt. The lower comb (7) develops a high electric field at its points that also becomes large enough to ionize air molecules. In this case the electrons are attracted to the comb and positive air ions neutralize negative charge on the outer surface of the belt, or become attached to the belt. The exact balance of charges on the up-going versus down-going sides of the belt will depend on the combination of the materials used. In the example, the upward-moving belt must be more positive than the downward-moving belt. As the belt continues to move, a constant 'charging current' travels via the belt, and the sphere continues to accumulate positive 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 peak potential. In the example, the wand with metal sphere (8) is connected to ground, as is the lower comb (7); electrons are drawn up from ground due to the attraction by the positive sphere, and when the electric field is large enough (see below) the air breaks down in the form of an electrical discharge spark (9). Since the material in the belt and rollers can be selected, the accumulated charge on the hollow metal sphere can either be made positive (electron deficient) or negative (excess electrons).
The triboelectric type of generator described above is easier to build for science fair or homemade projects, since it does not require a high-voltage source. Higher potentials can be reached with alternative designs (not discussed here) in which high voltage sources are used at the upper and/or lower positions of the belt to more efficiently transfer charge onto and off the belt.
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. 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 electric field becomes very strong and applying charges directly from the outside would soon be prevented by the field. Since electrically charged conductors have no electric field inside, charges can be added continuously from the inside without raising them to the full potential of the outer shell. 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 (R) multiplied by the electric field (Emax) where corona discharges begin to form within the surrounding gas. For air at STP the breakdown field is about 30 kV/cm. Therefore, a polished spherical electrode 30 cm in diameter could be expected to develop a maximum voltage (R Emax) of about 450 kV. This explains why Van de Graaff generators are often made with the largest possible diameter.
The concept of an electrostatic generator in which charge is mechanically transported in small amounts into the interior of a high voltage electrode goes back to the Kelvin water dropper, invented in 1867 by William Thomson (Lord Kelvin), in which charged drops of water fall into a bucket with the same polarity charge, adding to the charge. In this machine the gravitational force moves the drops against the opposing electrostatic field of the bucket. Kelvin himself first suggested using a belt to carry the charge instead of water. The first electrostatic machine that used an endless belt to transport charge was constructed in 1872 by Augusto Righi. It used an india rubber belt with wire rings along its length as charge carriers, which passed into a spherical metal electrode. The charge was applied to the belt from the grounded lower roller by electrostatic induction using a charged plate. Gray also invented a belt machine. Another more complicated belt machine was invented in 1903 by Juan Burboa A more immediate inspiration for Van de Graaff was a generator W. F. G. Swann was developing in the 1920s in which charge was transported to an electrode by falling metal balls, thus returning to the principle of the Kelvin water dropper.
The reason that the charge extracted from the belt moves to the outside of the sphere electrode even though it already has a high charge of the same polarity is explained by the Faraday ice pail experiment.
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.
In 1937, the Westinghouse Electric company built a 65 feet (20 m) Van de Graaff generator capable of generating 5 MeV in Forest Hills, Pennsylvania. It marked the beginning of nuclear research for civilian applications. It was decommissioned in 1958 and was demolished in 2015.
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.[page needed]
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 kilometres per hour (31 mph).
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. In these generators, however, corona discharge from exposed metal parts at high potentials and poorer insulation result in smaller voltages. In an electrostatic generator, the rate of charge transported (current) to the high voltage electrode is very small, so the maximum voltage is reached when the leakage current from the electrode equals the rate of charge transport. In the Van de Graaff generator, the belt allows the transport of charge into the interior of a large hollow spherical electrode. This is the ideal shape to minimize leakage and corona discharge, so the Van de Graaff generator can produce the highest voltage. This is why the Van de Graaff design has been used for all electrostatic particle accelerators.
- 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
- Westinghouse Atom Smasher
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|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: https://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