In the 1920s, physicists realized that to probe systems smaller than an atom (roughly 10-10 meters in radius), the fundamental relation for probability wavelength, λ = h/p, required the ability to give electrons and protons a kinetic energy of an MeV or far more using a technology that would allow increases essentially without practical limit. In 1930, two types of accelerators were devised that became the standard accelerators in nuclear and particle physics. One was the cyclotron, invented by Ernest Lawrence, which has become the most common of all particle accelerators at the present time. The other was the Van de Graaff accelerator, which was the mainstay and workhorse of all nuclear physics research from 1945 until roughly 1980. Many other types of particle accelerators have been developed over the years, but they all fall into two general classes, (1) Electrostatic accelerators, which use a static potential difference to accelerate charged particles, and (2) Electromagnetic accelerators, which use rapidly changing electromagnetic fields to pump kinetic energy into charged particles at a steady rate. All very-high-energy accelerators are electromagnetic in operation.
 A typical early 1960s Van de Graaff accelerator. These were about the size of a railway locomotive and accelerated protons to about 12 MeV. We had one exactly like this one, here at UT, when I came on board at the old Center for Nuclear Studies (1960 - 1976). It was given to Texas A&M around 1980. Click on the image for more info. |
 A typical cyclotron used for nuclear physics research. Some of these could accelerate protons up to 20 to 50 MeV. |
The most important advance over the cyclotron was the syncrotron, which used a steadily increasing magnetic field to keep the particles in the same circular path while their kinetic energy and momentum increased, allowing them to be continuously accelerated by the electromagnetic fields. All the largest particle accelerators on earth use the synchrotron principle. The largest such accelerator is the Large Hadron Collider in Europe. The LHC consists of a 27-kilometer ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way. The LHC accelerates protons to a kinetic energy of 6.8 TeV.
Historically, many of the most important accelerators have been linear rather than circular. Probably the most important was SLAC, which could accelerate electrons up to 50 GeV, and is about 3.2 kilometers long. It was this facility that used deep inelastic electron scattering to observe quarks and gluons inside the proton, and also to discover the most massive quarks.
 In addition to using the LHC, researchers here at UT also use RHIC, the Relativistic Heavy Ion Collider, which can accelerate nuclei to kinetic energies of 3.85 GeV per nucleon. It is located at Brookhaven National Laboratory on Long Island. The detector UT researchers use is STAR. Notice the linear accelerator (in red) which is used to create the beam initially. |
 In the 1970s your instructor used LAMPF, a linear accelerator that could produce 800 MeV protons, located at Los Alamos atop one of the many mesas. The accelerator is still running, now known as Los Alamos Neutron Science Center (LANSCE). It is half a mile long. |
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 The operating principle of the cyclotron. |
 The simple operating principle of the Van de Graaff accelerator. |
 The most powerful Van de Graaff accelerator in the world, it accelerates protons to 30 MeV or gold nuclei to 340 MeV. |
Synchrotrons use the same accelerating systems as linear accelerators, but in addition they have a complex magnetic system to bend the beam in a circle, and most importantly, to keep the radius of the circle the same, no matter how the kinetic energy of the particles increases... which is accomplished by increasing the magnetic field bending the beam to compensate for the increased speed of the particles. All high energy accelerators used for particle physics research are synchrotons, and furthermore they create colliding beams; that is, the particle beam is split into two, which travel in opposite directions around the circular path, and are allowed to collide at special points around the ring. This approach is vital because it doubles the energy of the collision, so that an accelerator which can accelerate particles to, say, 5 TeV can bring 10 TeV into the center-of-momentum system of the collision!
 If β is low, the beam is narrower, "squeezed". If β is high, the beam is wide and straight. |
 See example 4-25 in the text. Relativity demands an extreme penalty for doing things in the lab frame. |
 A linear accelerator used for cancer therapy |
 Earnest Lawrence at the controls of a large cyclotron |
 A hospital cyclotron used to prepare short-lived isotopes for medical diagnostics. |
There are currently more than 30,000 accelerators in operation around the world. Almost all of them are cyclotrons or synchrotrons. Both types of accelerators have almost endless technological and medical uses.
