Particle Collider
The original concept behind the construction of particle accelerators was to build machines that could accelerate protons, electrons, and other small particles to very high energies. These particles would then be allowed to collide with stationary targets outside the machine. These collisions would provide information about the fundamental structure of matter.
The earliest linear accelerators and cyclotrons were all built on this design, and they were very successful in producing new data on the composition of matter. But they all possessed one major handicap.
When high energy particles collide with a stationary target, only a relatively small fraction of their energy is available to take part in the reaction. By far the greatest fraction of their energy is used simply to move pieces of the target around. When a proton moving with one TeV (teraelectron, or trillion-electron, volts) of energy strikes a stationary proton, for example, only 41 Gev (gigaelectron, or billion-electron, volts), or 4.1% is actually available for an interaction between the protons.
One solution to this problem was envisioned by Donald Kerst (1911-) at the University of Illinois in 1956. Kerst suggested that two particle beams be accelerated in opposite directions at the same time. For example, a synchrotron might consist of two rings arranged next to each other. Particles might be accelerated in a clockwise direction in one ring and in a counter-clockwise direction in the other ring. At some particular moment, the two particle beams could be allowed to collide with each other. When that happens, all the energy possessed by both beams could be utilized in particle interactions. None would be wasted in lost motion, as is the case with stationary targets. The collision of two one-TeV proton beams, for example, would release two TeV of energy for interactions.
The first successful demonstration of this principle was achieved in 1963 at the Italian National Laboratory in Frascati. A 200-MeV beam of electrons was allowed to collide with a 200-MeV beam of positrons (positive electrons).
The most powerful particle accelerators today all make use of the colliding-beam principle. Such accelerators also make use of a storage ring . A storage ring looks like any other synchrotron ring. But the magnetic field around the storage ring is controlled so that particles within it are maintained at constant energies for long periods of time--sometimes hours--until they are needed. When an experiment is about to be carried out, the particles are released from the storage ring and allowed to collide with a second beam traveling in the opposite direction.
In 1972, the linear accelerator at Stanford University was modified to make possible colliding-beam experiments. Particles leaving the 3.2 kilometer-long accelerator are divided into two beams. The particles in one beam are converted to positrons and then allowed to collide with electrons from the other beam. This arrangement (known as SPEAR) produces the most energetic electron-positron collisions available anywhere in the world.
The largest and most powerful circular colliders in existence are located at the Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois, at the Centre Européen pour la Recherche Nucléair (CERN) in Geneva, Switzerland, and at the Deutsches Electronen-Synchrotron (DESY) in Hamburg, Germany. The Fermilab accelerator is also known as the tevatron because of its ability to accelerate particles to energies of one-TeV. All three machines have been used to make important discoveries of new particles.
For many years, scientists have been eager to build an even larger particle accelerator. All the major discoveries that are possible with one TeV or lower energy particles appear to have been made. New frontiers will be crossed, scientists believe, only when a larger machine is available. For theoretical reasons, one with an energy output of about 20-TeV is especially desirable.
The most serious technical problem involved in building such a machine is the size of magnets needed to control particle beams. By one estimate, the magnets needed for a 20 TeV machine would require four billion watts of power, more than the combined output of the three largest nuclear power plants in the United States.
Fortunately, an alternative solution for this problem is available. In 1911, the Dutch physicist, Heike Kamerlingh Onnes (1853-1926), discovered the phenomenon of superconductivity, the tendency of some materials to lose their resistance to the flow of electrical current at very low temperatures. Magnets made of superconducting materials can be operated at about one percent the cost of conventional magnets. An accelerator that used superconducting magnets appeared to be a realistic possibility. Thus was born the idea of the superconducting super collider (SSC).
For more than a decade, planning for the SSC has gone on under the auspices of the Universities Research Association, a group of 56 universities in 26 states and the province of Ontario. In June 1987, President Ronald Reagan (1911-) announced his support for the construction of the SSC. A year later, he approved a site near Waxahachie, Texas, for the location of the machine.
The SSC will consist of a storage ring with a circumference of 51.54 mi (82.94 km) buried 18 ft (6 m) below the earth's surface. The ring will consist of a concrete tunnel 10 ft (3 m) in diameter. Particles will travel in two metal tubes a few centimeters in diameter and 28 in (70 cm) apart. Protons will be accelerated to energies of 20 TeV, resulting in a release of 40 TeV when two opposing beams collide with each other. Before entering the main ring, protons will be accelerated by four smaller machines, a linear accelerator and three smaller synchrotrons.
Plans for construction of the SSC were surrounded by vigorous controversy. Critics claimed that the projected cost of the machine (originally, $4.5 billion) was more than the nation should spend on basic research which will provide few practical benefits. Many scientists worried that the project would drain money away from other research priorities. One Congressman called the scheme a "quark-barrel project," after one of the subatomic particles that would be studied with the SSC.
In spite of these objections, construction of the SSC began in the early 1990s. The project has not gone smoothly, however. Prototype magnets have not worked as well as anticipated, requiring a rethinking of many aspects of the SSC's design. This has driven up the estimated cost of the SSC at least fifty percent, and the United States government has had to interest other nations in supporting the project. These obstacles once again brought the machine into the center of political debate, and President Bill Clinton (1946-) has cancelled funding for the project.
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