Van de Graaff generator
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 electricity at low current levels. It was invented by American physicist Robert J. Van de Graaff in 1929.
The potential difference achieved by modern Van de Graaff generators can be as much as 5 megavolts. An inexpensive tabletop version can produce on the order of 100 kV and can store enough energy to produce visible electric sparks. Small Van de Graaff machines are produced for entertainment, and for physics education to teach electrostatics; larger ones are displayed in some science museums.
The Van de Graaff generator was originally developed as a particle accelerator for physics research, as its high potential can be used to accelerate subatomic particles to great speeds in an evacuated accelerator tube. It was the most powerful type of accelerator until the cyclotron was developed in the early 1930s. Because electrostatic energy is easily controlled, Van de Graaff generators are still used as accelerators to generate energetic particle beams for nuclear research and nuclear medicine.
The voltage produced by an open-air Van de Graaff machine is limited by arcing and corona discharge to about 5 megavolts. Van de Graaff research accelerators are enclosed in a pressurized tank of insulating gas; these can achieve potentials as large as about 25 MV.
History
Background
The concept of an electrostatic generator in which charge is mechanically transported in small amounts into the interior of a high-voltage electrode originated with the Kelvin water dropper, invented in 1867 by William Thomson, in which charged drops of water fall into a bucket with the same polarity charge, adding to the charge.In a machine of this type, 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. John Gray also invented a belt machine about 1890. 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.
Initial development
Robert Jemison Van de Graaff came to the generator because of his desire to study "individual particles." After graduating from the University of Alabama in 1922 with a degree in mechanical engineering, he grew dissatisfied with thermodynamics and its statistical treatment of particle. At the Sorbonne in 1924, he attended Marie Curie's lectures on radioactivity and witnessed Louis de Broglie's doctoral defense presenting wave-particle duality—an experience he later described as decisive in turning him toward nuclear physics. As a Rhodes Scholar at Oxford from 1925 to 1928, he read Ernest Rutherford's 1927 Royal Society address calling for particle beams that would transcend natural radioactive sources in energy, and began considering how charge might be transported mechanically into a high-voltage terminal.At Princeton in 1929, working under Karl T. Compton, Van de Graaff built his first model from a tin can, a silk ribbon, and a small motor. It achieved 80,000 volts. By September 1931, a twin-sphere machine constructed for roughly $100 reached 1.5 million volts—double any previous direct-current source. The demonstration attracted broad media coverage and the attention of other physicists. That month, Merle Tuve of the Carnegie Institution's Department of Terrestrial Magnetism borrowed the apparatus, strapping it to his automobile for the drive to Washington. Fitted with an acceleration tube, the DTM machine enabled the first Van de Graaff nuclear physics experiments by late 1932.
Van de Graaff followed Compton to MIT in 1931 and, with funding from the Research Corporation and facilities donated by Edward H. R. Green, constructed a far larger generator at the Round Hill research station. Housed in a former aircraft hangar, it featured two 4.6-meter aluminum spheres on 6.7-meter columns and achieved 5.1 million volts differential. Operating in open air meant contending with what the team called the "pigeon effect," arcing caused by accumulated droppings on the spheres. Meanwhile, Raymond Herb at the University of Wisconsin pursued an alternative approach: enclosed, pressurized machines. His 1934 accelerator reached 1 million volts in a compact tank using air mixed with carbon tetrachloride vapor. This pressure-insulation method, rather than sheer scale, would define subsequent development. Van de Graaff applied for a second patent in December 1931, which was assigned to Research Corporation.
Higher energy machines
In 1937, the Westinghouse Electric company built a machine, the Westinghouse Atom Smasher 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 partially 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.
By the 1970s, as much as 14 MV could be achieved at the terminal of a tandem that used a tank of high-pressure sulfur hexafluoride gas to prevent sparking by trapping electrons. This allowed the generation of heavy ion beams of several tens of MeV, sufficient to study light-ion direct nuclear reactions. The greatest potential sustained by a Van de Graaff accelerator is 25.5 MV, achieved by the tandem in the Holifield Radioactive Ion Beam Facility in 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 a much greater velocity than a belt, and both the voltage and currents attainable are much greater than with a conventional Van de Graaff generator. The 14 UD Heavy Ion Accelerator at the Australian National University houses a 15 MV pelletron. Its chains are more than 20 m long and can travel faster than.
The Nuclear Structure Facility 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 m 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 using 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.
Description
A simple Van de Graaff generator consists of a belt of rubber moving over two rollers of differing material, one of which is surrounded by a hollow metal sphere. A comb-shaped metal electrode with sharp points, is positioned near each roller. The upper comb is connected to the sphere, and the lower one to ground. When a motor is used to drive the belt, the triboelectric effect causes the transfer of electrons from the dissimilar materials of the belt and the two rollers. In the example shown, 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 induces a very high electric field near the points of the nearby comb. At the points of the comb, the field becomes strong enough to ionize air molecules. The electrons from the air molecules are attracted to the outside of the belt, while the positive ions go to the comb. At the comb they are neutralized by electrons from the metal, thus leaving the comb and the attached outer shell with fewer net electrons and a net positive charge. By Gauss's law, the excess positive charge is accumulated on the outer surface of the outer shell, leaving no electric field inside the shell. Continuing to drive the belt causes further electrostatic induction, which can build up large amounts of charge on the shell. Charge will continue to accumulate until the rate of charge leaving the sphere equals the rate at which new charge is being carried into the sphere by the belt.
Outside the terminal sphere, a high electric field results from the high voltage on the sphere, which would prevent the addition of further charge from the outside. However, since electrically charged conductors do not have any electric field inside, charges can be added continuously from the inside without needing to overcome the full potential of the outer shell.
File:Spark by Van de Graaff generator.jpg|thumb|250px|right|Spark made by the Van de Graaff generator at The Museum of Science in Boston, Massachusetts
The larger the sphere and the farther it is from ground, the higher its peak potential. The sign of the charge can be controlled by the selection of materials for the belt and rollers. Higher potentials on the sphere can also be achieved by using a voltage source to charge the belt directly, rather than relying solely on the triboelectric effect.
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 air, or other dielectric gas, surrounding it. 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 maximal achievable potential is roughly equal to the sphere radius R multiplied by the electric field Emax at which corona discharges begin to form within the surrounding gas. For air at standard temperature and pressure the breakdown field is about. Therefore, a polished spherical electrode in diameter could be expected to develop a maximal voltage of about. This explains why Van de Graaff generators are often made with the largest possible diameter.