VAN DE GRAAFF

VAN DE GRAAFF:generator is an electrostatic high voltage generator which can supply a few million volt.it was designed by robert j.van de graaff.

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 physicistRobert J. Van de Graaff in 1929.^[1] 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.

Working of the generator is based on two principles:

1. Discharging action of sharp points, ie., electric discharge takes place in air or gases readily, at pointed conductors.
2. If the charged conductor is brought in to internal contact with a hollow conductor, all of its charge transfers to the surface of the hollow conductor no matter how high the potential of the latter may be.

Construction:
 

It consists of a large metal sphere mounted on high insulating supports. An endless belt b, made of insulating material such as rubber, passes over the vertical pulleys P1 and P2. The pulley P2 is at the centre of the metal sphere and the pulley P1 is vertically below P2. The belt is run by an electric motor M. B1and B2 are two metal brushes called collecting combs.

The positive terminal of a high tension source (HT) is connected to the comb B1. Due to the process called action of points, charges are accumulated at the pointed ends of the comb, the field increases and ionizes the air near them. The positive charges in air are repelled and get deposited on the belt due to corona discharge. The charges are carried by the belt upwards as it moves. When the positively charged portion of the belt comes in front of the brush B2, by the same process of action of points and corona discharge occurs and the metal sphere acquires positive charges. The positive charges are uniformly distributed over the surface of the sphere. Due to the action of points by the negative charges carried by the gas in front of the comb B2, the positive charge of the belt is neutralized. The uncharged portion of the belt returns down collects the positive charge from B1 which in turn is collected by B2. The charge transfer process is repeated. As more and more positive charges are imparted to the sphere, its positive potential goes on rising until a surface maximum is reached. If the potential goes beyond this, insulation property of air breaks down and the sphere gets discharged. The breakdown of air takes place in an enclosed steel chamber filled with nitrogen at high pressure.

History

The Westinghouse Atom Smasher, the 5 MeV Van de Graaff generator built in 1937 by the Westinghouse Electric company inForest Hills, Pennsylvania

This Van de Graaff generator of the first Hungarian linear particle accelerator achieved 700 kV in 1951 and 1000 kV in 1952

A Van de Graaff particle accelerator in a pressurized tank at Pierre and Marie Curie University, Paris

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. John Gray also invented a belt machine around 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.

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 1933, Van de Graaff built a 40-foot (12-m) model at MIT's Round Hill facility, the use of which was donated by Colonel Edward H. R. Green.

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.

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 moreGr Van de Gaaff 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, 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.