In a Van de Graaff generator, the velocity of an electrically charged particle is increased by exposing that particle to an electric field. The velocity of a proton, for example, may go from zero to 100,000 mi per second (160,000 km per second) as the particle feels a strong force of repulsion from the positive charge on the generator dome. Linear accelerators (linacs) operate on the same general principle except that a particle is exposed to a series of electrical fields, each of which increases the velocity of the particle.
A typical linac consists of a few hundred or a few thousand cylindrical metal tubes arranged one in front of another. The tubes are electrically charged so that each carries a charge opposite that of the tube on either side of it. Tubes 1, 3, 5, 7, 9, etc., might, for example, be charged positively, and tubes 2, 4, 6, 7, 10, etc., charged negatively.
Imagine that a negatively charged electron is introduced into a linac just in front of the first tube. In the circumstances described above, the electron is attracted by and accelerated toward the first tube. The electron passes toward and then into that tube. Once inside the tube, the electron no longer feels any force of attraction or repulsion and merely drifts through the tube until it reaches the opposite end. It is because of this behavior that the cylindrical tubes in a linac are generally referred to as drift tubes.
At the moment that the electron leaves the first drift tube, the charge on all drift tubes is reversed. Plates 1, 3, 5, 7, 9, etc. are now negatively charged, and plates 2, 4, 6, 8, 10, etc. are positively charged. The electron exiting the first tube now finds itself repelled by the tube it has just left and attracted to the second tube. These forces of attraction and repulsion provide a kind of "kick" that accelerates the electron in a forward direction. It passes through the space between tubes 1 and 2 and into tube 2. Once again, the electron drifts through this tube until it exits at the opposite end.
The electrical charge on all drift tubes reverses, and the electron is repelled by the second tube and attracted to the third tube. The added energy it receives is manifested in a greater velocity. As a result, the electron is moving faster in the third tube than in the second and can cover a greater distance in the same amount of time. To make sure that the electron exits a tube at just the right moment, the tubes must be of different lengths. Each one is slightly longer than the one before it.
The largest linac in the world is the Stanford Linear Accelerator, located at the Stanford Linear Accelerator Center (SLAC) in Stanford, California. An underground tunnel 2 mi (3 km) in length passes beneath U.S. highway 101 and holds 82,650 drift tubes along with the magnetic, electrical, and auxiliary equipment needed for the machine's operation. Electrons accelerated in the SLAC linac leave the end of the machine traveling at nearly the speed of light with a maximum energy of about 32 GeV (gigaelectron volts).
The term electron volt (ev) is the standard unit of energy measurement in accelerators. It is defined as the energy lost or gained by an electron as it passes through a potential difference of one volt. Most accelerators operate in the megaelectron volt (million electron volt; MeV), gigaelectron volt (billion electron volt; GeV), or teraelectron volt (trillion electron volt; TeV) range.
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