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THE SYNCHROTRON
Some of the problems of the cyclotron
were overcome in the synchrotron. This
type of accelerator is the basis of most Particle injection
large machines and is called a
synchrotron because the amplitude of
the magnetic field and the frequency of
electromagnet (M)
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.
G
Accelerating cavity
A
Deflecting electromagnet
M
Emerging beam
Figure 1
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/2R, f is directly proportional to v)
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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)
Synchrotron
Cyclotron
Figure 2
Figure 3
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.
[CERN copyright]
Aerial view of CERN
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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.
FERMILAB AND THE TEVATRON
At present the most powerful machine in the world for accelerating protons is the 1 TeV
synchrotron at the Fermi National Accelerator Centre in Illinois in the USA. This machine, 6.4
km in circumference has already reached 800 GeV.
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 of 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.
Figure 4
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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
CERN LEP
Fermilab
Stanford
DESY
Tokyo
proton - antiproton 3000 GeV colliding beams – energy of each beam
e+ e 60 proton antiproton 270 GeV
proton antiproton 1000 GeV
e+-e 50 GeV
e--p 26 GeV – 820 GeV
e+-e 30 GeV
Large Hadron Collider (LHC) at CERN
The LEP machine has now been modified and is used to collide two beams of protons. 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.
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