Definitions

comb-shaped

Van de Graaff generator

[van duh graf]
A Van de Graaff generator is an electrostatic machine which uses a moving belt to accumulate very high electrostatically stable voltages on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can reach 5 megavolts. Applications for these high voltage generators include driving X-ray tubes , accelerating electrons to sterilize food and process materials , and accelerating protons for nuclear physics experiments. The Van de Graaff generator can be thought of as a constant-current source connected in parallel with a capacitor and a very large electrical resistance.

Description

A simple Van de Graaff generator consists of a belt of silk, or a similar flexible dielectric material, running over two 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 pulley and inside the sphere. (2) is connected to the sphere, and a high DC potential (with respect to earth) is applied to (7); a positive potential in this example.

The high voltage ionizes the air at the tip of (7), repelling (spraying) positive charges onto the belt, which then carries them up and inside the sphere. This positive charge induces a negative charge to the electrode (2) and a positive charge to the sphere (to which (2) is connected). 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 final potential.

The other method for building Van de Graaff generators is to use the triboelectric effect. The strong e-field from the rollers then induces 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 type of generator is easier to build for science fair or homemade projects, since it doesn't require a potentially dangerous high voltage source. The trade-off is that it cannot build up as high a voltage as the other type, and operation may become difficult under humid conditions (which can severely reduce triboelectric effects).

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. Van de Graaf generators often have ground wires.

History

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. 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 able to produce 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.

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 Green.

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.

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 spectacular 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 machine.

The Nuclear Structure Facility, or 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 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

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. Shows using the Van de Graaff generator and several Tesla coils are conducted several times each day.

Comparison with other high voltage generators

Other classical electrostatic machines like a triplex Wimshurst Machine or a Bonetti machine can easily produce more current, but the less insulated structures result in smaller voltages.

Patents

  • — "Electrostatic Generator"
  • — "Apparatus For Reducing Electron Loading In Positive-Ion Accelerators"

References

See also

External links

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