Circular Accelerators
Cyclotron
The cyclotron is a particle accelerator conceived by Ernest O. Lawrence in 1929, and developed, with this colleagues and students at the University of California in the 1930s. (For a short pictorial history, see The Development of the Cyclotron at LBNL.)
A cyclotron consisted of two large dipole magnets designed to produce a semi-circular region of uniform magnetic field, pointing uniformly downward.
These were called Ds because of their D-shape. The two D's were placed back-to-back with their straight sides parallel but slightly separated.
An oscillating voltage was applied to produce an electric field across this gap. Particles injected into the magnetic field region of a D trace out a semicircular path until they reach the gap. The electric field in the gap then accelerates the particles as they pass across it.
The particles now have higher energy so they follow a semi-circular path in the next D with larger radius and so reach the gap again. The electric field frequency must be just right so that the direction of the field has reversed by their time of arrival at the gap. The field in the gap accelerates them and they enter the first D again. Thus the particles gain energy as they spiral around. The trick is that as they speed up, they trace a larger arc and so they always take the same time to reach the gap. This way a constant frequency electric field oscillation continues to always accelerate them across the gap. The limitation on the energy that can be reached in such a device depends on the size of the magnets that form the D's and the strength of their magnetic fields.
Once the synchrotron principle was developed (see below), it was found to be a much cheaper way to achieve high energy particles than the cyclotron and so the original cyclotron method is no longer used.
Synchrotron
A synchrotron (sometimes called a synchro-cyclotron) is a circular accelerator which has an electromagnetic resonant cavity (or perhaps a few placed at regular intervals around the ring) to accelerate the particles.
There are several circular accelerators at Fermi National Accelerator Laboratory. Particles pass through each cavity many times as they circulate around the ring, each time receiving a small acceleration, or increase in energy. When either the energy or the field strength changes so does the radius of the path of the particles.
Thus, as the particles increase in energy the strength of the magnetic field that is used to steer them must be changed with each turn to keep the particles moving in the same ring. The change in magnetic field must be carefully synchronized to the change in energy or the beam will be lost. Hence the name "synchrotron". The range of energies over which particles can be accelerated in a single ring is determined by the range of field strength available with high precision from a particular set of magnets. To reach high energies, physicists sometimes use a sequence of different size synchrotrons, each one feeding the next bigger one. Particles are often pre-accelerated before entering the first ring, using a small linear accelerator or other device.
Synchrotron Radiation
Synchrotron radiation is the name given to the electromagnetic radiation emitted by the charged particles circulating in a synchrotron. It occurs because the charged particles are accelerated (deflected) by the magnetic field from the dipole magnets to make the beam travel around the ring. Any accelerated charged particle produces some electromagnetic radiation.
The wavelength and intensity of the synchrotron radiation depends on the energy
and type of the emitting particle. If all you are interested in is storing a high energy beam, then synchrotron radiation is a problem. The energy lost from the beam by this
radiation effect must be restored by introducing accelerating cavities at one or more places in the ring, to give the particles a kick in energy every time they pass. The
amount and energy of the radiation depends on the speed of the radiating particles and the magnetic field strength. As the particle approaches the speed of light, the effect
increases rapidly. The special relativity factor, gamma (
, is the ratio of the energy
of the particle to its rest mass-energy, mc2. The energy loss for a given electron energy is proportional to ()3.
Dependence on Particle Type
For an 1.5 GeV electron in the SPEAR storage ring, gamma is approximately 3000. For a 50 GeV electron in the SLC arcs, gamma is approximately 100,000. Gamma is the ratio of the energy of the particle to its rest mass-energy, mc2. Thus, because a proton is so much more massive than an electron, a proton with 1 TeV = 1,000 GeV energy has a gamma factor of only 1,000. (1 TeV is the energy produced by the synchrotron at Fermilab). Thus synchrotron radiation is much greater for electrons than for equal energy protons. This is the reason why much higher energy synchrotrons can be built for protons than for electrons.
SSRL
At SPEAR, the synchrotron radiation has wavelengths from ultraviolet to x-ray, just the right scale to use it as a probe of the atomic and molecular scale structure of matter. The Stanford Synchrotron Radiation Lightsource at SLAC is devoted to studies using this powerful tool.
Storage Ring
A storage ring is the same thing as a synchrotron, except that it is designed just to keep the particles circulating at a constant energy for as long as possible, not to increase their energy any further. However, the particles must still pass through at least one accelerating cavity each time they circle the ring, just to compensate for the energy they lose to synchrotron radiation.
Two storage rings have been built at SLAC; SPEAR, a 3 GeV ring completed in the early 70's and PEP a 9 GeV ring completed in the early 80's. SPEAR is now used solely by SSRL while PEP has been rebuilt as a two-ring facility known as the B Factory where results are being accumulated by the BaBar detector and studied by the BaBar collaboration.