Synchrotron

Here, the synchrotron is the circular track, off which the beamlines branch.

A synchrotron is a type of particle accelerator where particles travel around many times in a circle. It uses a magnetic field to turn the particles in the circle and an electric field to speed up the particles. The components are carefully matched up with the travelling particle beam so that the circle stays the same size while the particles go faster. Mark Oliphant invented the proton synchrotron.[1] Vladimir Veksler was the first to publish the idea. Edwin McMillan constructed the first electron synchrotron.

Characteristics

The synchrotron is an improvement over the cyclotron where the particles travel in a spiral pattern. A cyclotron uses a constant magnetic field and a constant-frequency applied electric field. (One of these is varied in the synchrocyclotron). Both of these fields are varied in the synchrotron to change the path from a spiral to a circle. By carefully making the fields bigger as the particles gain energy, the width of the circular path can be kept the same as the machine accelerates the particles. This allows the vacuum chamber for the particles to be a large thin circular pipe torus (donut shape). It is easier to use some straight sections between the bending magnets and some bent sections within the magnets giving the torus the shape of a round-cornered polygon. A path that acts like a very large circle can be constructed using simple straight and curved pipe segments, unlike the disc-shaped chamber of the cyclotron type devices. The shape also requires the use of multiple magnets to bend the particle beams. Straight sections are required at spacings around a ring for both radiofrequency cavities, and in third generation setups space is allowed for insertion of energy extraction devices such as wigglers and undulators. Most synchrotrons use two types of magnets: dipole magnets to bend the particle beam and quadrupole magnets to focus the beam.

The maximum energy that a cyclic accelerator can impart is typically limited by the strength of the magnetic field(s) and the minimum radius (maximum curvature) of the particle path. So, over time, physicist have built accelerators with bigger magnets and larger circles to reach higher particle energy levels.

The interior of the Australian Synchrotron facility. Dominating the image is the storage ring, showing the optical diagnostic beamline at front right. In the middle of the storage ring is the booster synchrotron and linac

In a cyclotron, the maximum radius is quite limited as the particles start at the center and spiral outward. So, the entire path must be a self-supporting disc-shaped vacuum chamber. Since the radius is limited, the power of the machine becomes limited by the strength of the magnetic field. In the case of an ordinary electromagnet the field strength is limited by the saturation of the core (when all magnetic domains are lined up the same way, the field cannot not be further increased to any practical extent). The arrangement of the single pair of magnets the full width of the device also limits the economic size of the device.

Synchrotrons overcome these limits, using a narrow beam pipe which can be surrounded by much smaller and more tightly focusing magnets. The ability of this device to accelerate particles is limited by the fact that the particles must be charged to be accelerated at all, but charged particles under acceleration emit photons (light particles), which makes them lose energy. The limiting beam energy is reached when the energy lost to the lateral (bending) acceleration required to maintain the beam path in a circle equals the energy added each cycle. More powerful accelerators are built by using large radius paths and by using more numerous and more powerful microwave cavities to accelerate the particle beam between corners. Lighter particles (such as electrons) lose a larger fraction of their energy when turning. Practically speaking, the energy of electron/positron accelerators is limited by this radiation loss, while it does not play a significant role in the dynamics of proton or ion accelerators. The energy of those is limited strictly by the strength of magnets and by the cost.

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