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, providing 13.6 TeV energy in
head-on collisions.
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. |
Remember that the force a constant magnetic field exerts on a moving charge q is given by q(v ×B), so it is perpendicular to v and cannot do work... cannot change the kinetic energy of the charged particle.
 Since the force (red) is perpendicular to the velocity (black), a charged particle in a constant magnetic field moves in a circle with constant speed. The magnetic force is a centripetal force in such a case. The radius of the circle depends on the speed of the particle. The acceleration a = v2/r. |
 The operating principle of the cyclotron. A magnetic field keeps the charged particles moving in a circle, while a potential difference is maintained between the two separate pieces of the accelerator, varying with time such that as the particles cross the gap, the electric field is always accelerating them. |
 The simple operating principle of the Van de Graaff accelerator. A constant potential difference of millions of volts is maintained between the ion source and the outside world. |
 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. In the lab system, almost all the energy goes into the recoil of the initially stationary target particle. |
 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.
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Many sea animals ride the wake of
boats... the wake, a shock wave created because the speed of the
boat is greater than the speed of surface wave propagation,
travels with the speed of the boat, so a sea animal that stays
in the wake is carried along by the boat with no effort.
Physicists have wondered since the 1970s if it was possible to
send a sharp laser pulse through a plasma, creating a wake in
the plasma that moves at almost the speed of light, into which
one could somehow inject charged particles, which would then be
brought to near the speed of light within a very short distance.
Despite many years of effort, it is only very recently that any
progress has been made in this quest! Don't get your hopes
up, because while such accelerators could be just a few feet
long instead of miles long, they require the most powerful
lasers in existence to create the wake!
