Introduction - Early History of Accelerators
Particle accelerators are devises that increase the energy of elementary particles, such as electrons, protons, etc., by making them travel faster and faster. This built-up kinetic energy is then used to create very powerful collisions. For example, electrons are made to accelerate so that they move near the speed of light. These energetic electrons are then impinged upon nuclei of atoms. Because of their high energy, these electrons can then penetrate deep into the nucleus. From the way that these electrons get deflected, a great deal of information can get extracted about the target nucleus. In other experiments accelerated particles are made to travel in opposite directions. These two "opposing beams" are then made to have a head-on collision. The huge amount of kinetic energies that these beams have before the collision is converted, through E = m c2, into new forms of matter. In this way new particles are discovered.
Basic physics of accelerators relies on two separate features of electromagnetic interactions. The first of these is that charged particles experience an electric force when they are in the presence of other charged particles. In a typical battery, like the ones used in flashlight for example, a chemical reaction creates charged particles of different polarities at the two ends of the battery. The one marked - (negative) has a large number of electrons, while the one marked + (positive) lacks in equal number of electrons. When the flashlight is turned on the switch completes a (conductive) path between the two poles of the battery. As a result, the electrons at the - pole, which repel one another, can move through this conductive path to the + pole that also attracts them. In doing so they pass through the flashlight's lamp filament that has a relatively large resistance. These electrons give most of their kinetic energy to the atoms of the filament that release this energy as light. Finally they arrive at the positive pole of the battery. In a sense, these electrons travel "down hill" from the - pole to the + pole. This down hill grade is often referred to as an electric potential difference. Electrons "fall" in a potential difference and positive charges rise (like bubbles in water or helium balloons in air). To create steeper grades we need more negative poles; or in the context of electric potentials, we need larger potentials. A typical flashlight battery's potential is 1.5 volts. The small accelerator in our physics department is about 2 MeV. Large particle accelerators accelerate charged particles in potential differences that are thousands of volts larger.
The second physics principle that is used in accelerators is that charged particles bend as they travel in magnetic regions (magnetic fields). The faster they travel the less they bend; but as the region's magnetic field increases in strength even very fast particles are made to bend. In an accelerator electromagnets - coils of wire that carry electric current inside them - are used to bend the path of the accelerating charged particles. Especially designed electromagnets are also used to bring particles that are moving together closer to each other and thus focus the beam of charged particles.
In addition to these basic physics principles particle accelerator designs must also pay attention to Einstein's relativity! As the particles are accelerated these faster and faster traveling entities get more and more massive. This increases the energy that it takes to make them move faster to the point that limits the effectiveness of the accelerators. (It is because of this that colliders become more effective than linacs, see below.) Another interesting physics of accelerators is that charges that accelerate also radiate (electromagnetic radiation). Particles that are moving with similar speeds and accelerations in accelerators then emit X-rays that are often used for other (non-particle physics) purposes. This is an added benefit of accelerators for some researchers, but a real problem for those who are supplying $ to make the accelerators run!
The first particle accelerator that was developed was a cyclotron. For a diagram and description of the cyclotron and synchrotron, please the web site of Stanford Linear Accelerator Lab. But briefly, all accelerators use the same set of principles: they use an electric field to accelerate charged particles, and a magnetic field to bend their path of motion. In the case of a cyclotron, two strong semicircular magnets are used to bend charge particles. These semicircular magnets are separated from each other by a gap, over which an accelerating electric field is applied. This way, the charged particles circle around and in doing so receive repeated acceleration.
Today's accelerators are either linacs or circular. A linac, or a linear accelerator, accelerates charged particles in a straight path. Once the particles reach their full velocities, they are made to collide with each other or with other particles that are also accelerated in the device. The idea is that with the same electric potential and the same magnets a beam of oppositly charged particles could undergo acceleration in the opposite direction. Then when these beams have reached desired kinetic energies they are made to collide head-on. The energy that is rendered in these head-on collisions are then twice as much as a traveling beam is made to collide with a stationary target. As a result, in the colliders the particles do not need to move quite as fast as they would in a linac, for the same energy. This, of course, translates into far less operating costs, i.e. better efficiencies. For a more detailed explanation please see the Stanford University's web site on their linac. Circular accelerators, synchrotrons, are similar in operation to the linacs, but they can continue accelerating a beam over and over again. As a result, these accelerators can achieve higher particle energies. For more details on these accelerators, please visit the site on the web at CERN, the European Organization for Nuclear Physics and European Laboratory for Particle Physics. Please note that CERN is the "birth place" of the wide-world-web (or is it the world-wide-web, or web-world-wide?)! In fact, the web (and e-mail?) came about, for good or bad, because of particle physics.
After many years of hard work experiments in high energy physics have lead theorists to settle on a model that describes the fundamental interactions in nature, called the Standard Model. In this model all known particles interact using three types of forces: gravitational, electro-weak, and hadronic. With these forces and with leptons and quarks the Standard Model can, in theory, explain how all other matter is made.
Has particle physics reached its ultimate goal through this model? Many physicists would answer negatively. In fact new experiments that are in the plans are investigating for the evidence that there are black holes formed in these high energy experiments. Such evidence, if reported, could lead to the unification of forces - in the pursuit of long held expectation that all forces are one and the same single force. Also, as new string theories may suggest, we are not living in four dimensional space-time, as Einstein suggested, but in 11, 12 or 13 dimensions.
For a flavor of efforts in high energy physics research please visit, for example, the site at CERN.
Last Modified Friday, November 2, 2007 malekis@union.edu
