Linear Accelerator
The first particle accelerators to be built were linear accelerators, or linacs. In a linac, particles are introduced at one end of the machine, accelerated by a changing electrical field, and then caused to collide with a target at the opposite end of the machine. The first machine of this type was invented by John Cockcroft and E. T. S. Walton at the Cavendish Laboratory in the late 1920s. Cockcroft and Walton developed a method for producing very high voltages in an evacuated glass tube. Protons--hydrogen atoms stripped of their electrons--were introduced at one end of the tube. They were accelerated to very high velocities by the large potential difference within the tube. As the protons traveled from one end of the tube to the opposite end, they reached energies of several hundred thousand electron-volts. An electron-volt is the unit used to measure the energy of particles in a particle accelerator. One electron-volt is the amount of energy attained by an electron as it travels through a potential difference of one volt. The energy achieved by particle accelerators is measured in keV (kiloelectron, or thousand-electron, volts), MeV (megaelectron, or million-electron, volts), GeV (gigaelectron, or billion-electron, volts), or TeV (teraelectron, or trillion-electron, volts). The Cockcroft-Walton accelerator produced particles with energies of about 600 keV. At about the same time that Cockcroft and Walton were building their accelerator American physicist, Robert Van de Graaff was constructing a somewhat similar machine at Princeton University. The Van de Graaff accelerator consists of a hollow metallic sphere on top of an insulating column which forms a large capacitor together with a conductor at the the bottom of the column.. A moving belt picks up electrons at the bottom of the column and carries them into the metallic sphere where they accumulate, producing a voltage difference between the sphere and the bottom of the column. A source of protons in the hollow sphere is injected into the column where they are accelerated by the electric field created by the voltage difference between the top and the bottom of the column. The highly energetic protons thus produced can be used to bombard nuclei in a target. The most powerful Van de Graaff generator can accelerate particles to energies of about 25 MeV. Both the Cockcroft-Walton and Van de Graaff generators are examples of one-stage accelerators. The term one-stage means that particles are accelerated just once after they are introduced into the machine and before they strike a target.
As early as 1928 German physicist, Rolf Wideröe realized that particle accelerators could be made more powerful by accelerating particles a number of times. He suggested that particles be directed through a series of tubes in each of which they would experience an additional force. The cumulative effect of all these "pushes" would be to give particles much more energy than they could attain in a one-stage machine. Wideröe designed a machine that consisted of a series of hollow metal cylinders. The first, third, fifth, and every odd-numbered cylinder was connected with one pole of an electric source, say the negative pole. The second, fourth, sixth, and every even-numbered cylinder was connected to the opposite pole, in this case, the positive pole. When a particle is introduced at one end of the machine, the charges are arranged so that it will be attracted to the first tube. At the moment the particle reaches the center of the first tube, the charges on all tubes are reversed. The particle is now repelled by the first tube and attracted by the second tube. The effect is to give the particle an extra burst of energy. The particle experiences an electric force only when it is in the space between cylinders, not while it is inside a cylinder. While it is within a cylinder, it simply drifts, which explains the name drift tubes given to the cylinders. The drift tubes are progressively longer, going from source to target. Each time a particle is accelerated, it spends a little more time within the drift tube. Making the drift tubes longer each time allows the particle to stay in step with the changing electric current. In 1946, Luis Alvarez, at the University of California, suggested an improvement on this basic model. In the Alvarez design, drift tubes are no longer connected to an external source of power. Instead, they are surrounded by a changing electrical and magnetic field. Bundles of particles move through the machine like a surfer traveling on a water wave. This design makes possible the construction of linacs with higher energies. The most powerful proton linac in use today is located at the Los Alamos National Laboratory in New Mexico. It is about one half mile (0.75 km) long and accelerates protons to energies of about 800 MeV. The largest linac in the world is located at Stanford University in California. The Stanford linac is two miles (3.2 km) long and accelerates particles to energies of about 30 GeV.
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