DRAFT
DRAFT
ACCELARATORS IN PARTICLE PHYSICS
TEACHER NOTES
Accelerators are instruments/machines used to accelerate charged particles. These machines range in size from devices
that fit on a table to ones that are many km (miles) long. An example of an accelerator that is used in all most every
home in the United States is the TV (not plasma or LCD). In the TV electrons are accelerated through the vacuum tube,
strikes the phosphorous coated picture tube which then emits light producing the images which we see. All accelerators
have two things in common; they utilize both electric and magnetic fields.
In general particle accelerators are divided into two types based on their geometry, circular (synchrotrons) and linear
(linac). Two of the larger synchrotrons are located at CERN in Geneva Switzerland and Fermi lab near Chicago. The
largest linear accelerator is located at Stanford University in Menlo Park California.
CERN (16 mile circumference)
Fermi lab (4 mile circumference)
SLAC Stanford Linear Accelerator (consist of both linear and circular)
TV-linear accelerator
The WORKINGS OF A LINAC
It is very easy to accelerate a charged particle, for example if one has an electron and wants to accelerate it all that is
needed is a charged plate (potential difference). The charged plate could be either negative or positive depending on
where the plate is placed. Or better yet one could use both charged plates at the same time and cause the electron to
be pushed and pulled!
_
_
+
e-
+
_
+
_
+
The question is what happens to the electron when it reaches the positive plate, it sticks. This is not what the
accelerator physicists desire to happen, they would like the electron to continue. In order to do this drift tubes are
utilized. The principle is that the electron accelerates between the tubes and then “drifts” or maintains a constant
velocity while in the tube. This is because the copper tube acts as a Faraday cage which shields the electric field. While
the electron is in the drift tube the polarity changes so when it emerges it will be repelled, thereby being accelerated.
The picture below shows an arrangement of drift tubes used in a typical linac. Notice that the tubes get longer as one
moves down the linac, the question is why? The answer is that the particle is accelerating; therefore it travels a greater
distance in the time needed to change the polarity, so by increasing the length of the drift tube the polarity has time to
change.
The picture below shows the drift tubes used in the linac at Fermi lab National Laboratory.
The workings of a Synchrotron
Synchrotrons have two main elements, magnets and RF cavities. The magnets are used to steer and focus the
particles. The force on a charged particle in a magnetic field is given by the Lorentz Force (𝐹 = 𝑞𝑣 𝑥 𝐵) where
q is the electric charge, B is the magnetic field and v is the velocity. The magnetic force is perpendicular to
both the velocity of the particle and the direction of the magnetic field lines. Motion in a uniform magnetic
field is a circle as illustrated below.
Particles in synchrotrons are bent in a circular path by dipole magnets and focused by quadrupole magnets
which have two north poles and two south poles.
Illustration of a quadrupole magnet
A quadrupole magnetic in the antimatter facility located at CERN
The RF cavities are responsible for accelerating the particle utilizing alternating electric fields (similar to the acceleration
process used in the linac). These cavities are interspersed throughout the ring providing a “push” every time the
particles pass.
It is important to remember that magnets are incapable of changing the energy of a particle therefore its use in the
accelerator is to steer the particle. It is the electric fields established by the RF cavities that actually accelerate the
particle. Also this is a simplification of the process involved in accelerating these particles and one should realize that
this is indeed a very complicated and challenging task!
Links for additional information
http://hands-on-cern.physto.se/hoc_v21en/index.html
o Links to a site developed at CERN provides a good overview for particle accelerators.
http://particleadventure.org/frameless/accel.html
o An excellent site explaining how accelerators work and much more.
http://ed.fnal.gov/projects/labyrinth/games/warpspeed/linac/activity.html
o A game which allows students to play with drift tubes.
http://www-bd.fnal.gov/public/
o An easily understandable explanation of Fermi labs accelerator chain.
http://www2.slac.stanford.edu/vvc/accelerator.html
o An accelerator tutorial provided by SLAC.
http://science.howstuffworks.com/atom-smasher2.htm
o An excellent presentation/explanation on accelerators.
Student activities
1. Divide the class into groups giving each group a topic to research and then have them present their findings to
the class. Provide the list of websites included in the teacher notes. Possible topics/questions include: (1) what
is the role of magnets in particle accelerators? (2) What are drift tubes and how do they work? (3)How are
particles accelerated? (4) Why are particles accelerated? (5) What are some uses of particle accelerators?
(6) What are some challenges associated with operating a particle accelerator?
2. Have students go the website with the drift tube game and have a contest to see which group can achieve the
highest energy. Website is provided in the teacher explanation.
3. Have students construct a scale model of a chain particle accelerator (such as Fermi lab), being sure to include
explanations for each step in the process.