THE SYNCHROTRON

Some of the problems of the cyclotron were overcome in the synchrotron. This type of accelerator is the basis of most large machines and is called a synchrotron because the amplitude of the magnetic field and the frequency of the accelerating voltage must be synchronised. A simplified diagram of a synchrotron is shown in the diagram.

Particles are shot into the ring at A from a linear accelerator and are bent into the ring by a series of magnets (M). Every orbit they pass through an accelerating gap G and their energy is increased. The radius of the evacuated tube is fixed and so the magnetic field must be steadily increased as the energy of the particles increases to keep the radius of their orbit constant. There are some straight sections with many magnets to deflect the particles into a virtually circular course.

The Proton Synchrotron at CERN in Geneva has an orbit diameter of 172m, deflecting magnets of 1.4T and accelerates protons to 28 GeV. Each proton pulse contains about 10¹¹ protons and during the acceleration the protons travel some 80 000 km (50 000 miles)!

Protons from the synchrotron are shot into the next of CERN's giant machines - the SPS - at 10 GeV. The SPS (Super Proton Synchrotron) began operation in 1976 and is used to accelerate protons to 400 GeV. Each pulse contains 10¹³ protons and lasts for between 2 and 24 microseconds. The machine has a diameter of 2.2 km with six long straight sections and over seven hundred bending magnets, each one just over 6 m long. In the acceleration to 400 GeV, which lasts some 2.7s, the protons orbit the machine about 150 000 times covering a distance of over a million kilometres. The pulses are accelerated by about 2.5 MeV each turn as they pass through two cavities 10m long containing 56 drift tubes.

As their path is bent by the magnets the charged particles emit electromagnetic radiation ("synchrotron radiation") – measuring the velocity of this radiation gives good evidence for the constancy of c – relativity.

Most modern accelerators are synchrotrons as they can be built much larger and hence give much greater energies. The magnetic field need only cover the vacuum tube, not the whole area of the ring as in a cyclotron.

In the synchrotron the frequency has to be varied as the particle velocity increases since the orbit radius remains constant (Frequency = 1/T = v/2πR, f is directly proportional to v)

Analogies
Linear accelerator: a ball rolling along a straight track with people giving it a hit with a bat as it passes them at regular time intervals (the people will stand further and further apart)



Cyclotron (Figure 2): a ball is suspended by a string from a pole. A person gives it a hit so that it moves in a circle and then another hit every time it comes past. The ball gets faster and faster and swings outwards into a larger circle.

Synchrotron(Figure 3): similar to the above but the ball is fixed to the vertical by a string to keep the radius constant. It is given a hit every time it passes and so speeds up, the tension in the string increases as the ball gets faster.

THE LARGE ELECTRON - POSITRON COLLIDER - LEP

This giant machine is the largest in use at CERN at present. The accelerating ring has a circumference of nearly 27 km and lies buried in a 3.8 m diameter tunnel that crosses the France - Switzerland border at the foot of the Jura mountains. (Image CERN copyright)

Work was begun on 13th September 1983 and the first electron - positron collision occurred on 13^th August 1989, less than six years later. The particles are kept in orbit by 3304 bending magnets, each 6m long and giving a field of 0.135T.

There are more than 1012 particles in the collider at any one time and these are separated into four bunches each a few centimetres in length. One important property of LEP is that it is a colliding beams machine. Electrons travelling round the accelerator in one direction and positrons in the other. Two particles of the same mass hitting each other head on each with velocity v will have much more energy available for breaking up the particles than if one was stationary and the other hit it at 2v.

Powerful magnetic lenses focus these two beams so that they are less than 0.001 mm across. The positrons and electrons collide in the middle of one of four huge detectors. These are called ALEPH, DELPHI (DEtector with Lepton, Photon and Hadron Identification), L3 and OPAL (Omni Purpose Apparatus for LEP) and the energy released in a small volume in LEP when the electron- positron beams collide gives an energy density similar to that existing some 10^-10 s after the Big Bang! This high energy will enable the scientists at LEP to "turn the clock back" all those millions of years to study what was happening when the universe was very young.

Large Hadron Collider (LHC) at CERN

The LEP machine has now been modified and is used to collide two beams of protons and first became operational on 10th September 2008. Unfortunately nine days later there was a problem due to a gas explosion which damaged over fifty of the superconducting magnets. Just over a year later the LHC was operational again and the first proton-proton collisions were observed. Now, nearly three years later, over five billion interesting collisions have been recorded by each of two major experiments. Of these, 400 may give evidence for the Higgs boson, confirmation of whose existence was announced in July 2012.

The protons in each beam are injected into the machine with an energy of 450 GeV and accelerated to 7 TeV (7x10¹² eV) by accelerating electrodes each with a field of 5 MVm^-1. The energy available on collision is a massive 14 TeV and it is hoped to achieve this by around 2015.

FERMILAB AND THE TEVATRON

Before the construction of the accelerator at CERN the most powerful machine in the world for accelerating protons was the 1 TeV synchrotron at the Fermi National Accelerator Centre in Illinois in the USA. This machine, 6.4 km in circumference had already accelerated protons to 800 GeV when it was shut down in September 2011

THE SUPER- CONDUCTING SUPER COLLIDER (SSC)

This will be the largest accelerator in the world when completed, at the present time it is still under construction in Ellis County, Texas in the USA. In late 1992 the first part of the giant 85 km long tunnel that will house the accelerator ring had been started. When completed it was hoped that SSC will accelerate protons to 20 TeV using two collider beams to give a total energy of 40TeV (1TeV = 10¹²eV) and be able to explore distances down to 10^-18m. It is likely to cost some $4 billion. In fact the project has been halted and it now seems unlikely that it will ever be finished.

With all high-energy accelerators the masses of the accelerated particles changes due to Einstein's theory of relativity. A particle moving at 90% of the speed of light is about 2.4 times heavier than a similar one at rest and therefore is harder to accelerate. The giant machines have to allow for this effect.

Colliding beams accelerators

In some accelerators the high energy particles collide with a stationary particle such as a nucleus. Some o the energy is used to break up the nucleus but some is "wasted" as kinetic energy in recoil. In the colliding beams accelerators two beams (say of positrons and electrons) are sent round a synchrotron in opposite directions. When they meet they collide with each other and all the energy is available for the annihilation of the particles.

You can compare this with a collision between a car and a wall and two cars coming towards each other from opposite directions – the second case would give the much more serious impact!

Accelerator data

Serpukhov	proton-antiproton 3000 GeV colliding beams
CERN LEP	e+ e- 60 GeV
CERN LHC	proton antiproton 7 TeV
Stanford	e+ e- 50 GeV
DESY	e- p 26-820 GeV
Tokyo	e+ e- 30 GeV