How "Atom Smashers" Work

"A particle accelerator experiment is like determining the structure of a television
by looking at the pieces after it has been dropped from the Empire State Building." - unknown

Side view of a collision of two gold beams in the Relativistic Heavy Ion Collider
Photo courtesy Brookhaven National Laboratory

 

A very simply particle accelerator that every household uses is a battery. There is a potential difference, or voltage, between its terminals, which produces an electric field. A charged particle in this field will feel a force and be accelerated. In electric wires, the electrons undergo too many collisions with the wire’s atomic lattice to gain any speed and the energy is transferred to the conductor as heat. Now if you take this idea and think big you'll start to take the steps in building a large scale particle accelerator.

 

There are 5 main stages/parts to a particle accelerator:

 

Getting the Particles That Will Be Accelerated

As long as a particle has charge, (protons, electrons, positrons, ions, and the nuclei of heavy atoms such as gold) it can be accelerated in a particle accelerator. There are many ways to get and isolate these particles. If you take any filament that is heated by an electrical current flowing through it releases a few electrons into the space around it. When a strong electric field is applied, more electrons are pulled out of the hot filament. The electric field accelerates the electrons toward the beginning of the accelerator's structure.

electron gun at SLAC

Another way of getting electrons is by having a polarized laser light knock electrons off the surface of a semiconductor and again an electric field accelerates them towards the accelerator structure.

If you want positrons you can fire an electron beam at tungsten. In the collision, electron-positron pairs are made. The positrons can be accelerated by reversing the directions of the electric and magnetic fields within the accelerator.

There are many similar ways of producing protons, ions, and such.

 

Accelerating the Particle

The idea behind the accelerating part of a particle accelerator is that: like charges repel and opposites attract.

Two plates, each with a small hole in their centers, the first negatively charged, and the second positively charged. If you place an electron between these plates it will be accelerated towards the positive plate. Now when the electron is just about to reach the positive plate the charge on it is changed to negative the electron will feel a repulsive force. But since the electron has momentum it will pass trough the hole in the plate and now be repelled forward, thus accelerating more. Now if after this second plate there was a third positively charged plate, there would be even more of a force acting to accelerate the electron forward.

Now to make this a practical particle accelerator and make sure the electron doesn't bump into anything you place these plates in a vacuum, and since charges have to flow to make the plates positive and negative, the chamber and plates are made of copper.

 

If you push down on the surface of the water you create waves which move water that you have not even touch. It is with this same idea that we can change the charge on the plates. By negatively charging the first plate all the electrons on the second plate are repelled, leaving behind a positively charged plate. The electrons in the copper pipe are now attracted to this positive charge and move to the third plate, causing it to be negatively charged, and the whole above process starts again.

Now to get to the wave idea. If you now switch the charge on the first plate, it will ripple down and you will switch the charges on all the plates. This allows for the constant repel and attract system, described above, that accelerates the particles. The switching of the charge on the first plate and the ripple affect causes electromagnetic standing waves in the cavities. This is done by the Klystrons.

Now since the particle is getting faster the spacing between each plate gets successively larger so that the timing is right with the switching of the charge on the plates. Otherwise you would have the particle getting attracted in the wrong direction and would be slowing it down instead of accelerating it.

 

Focusing and Bending the Particle

Since you don't want your particles crashing into the charged plates but instead want them going through the holes in the middle of the plates, you have to find a way of doing that with out interfering with their motion. Magnets are the answer. Not your typical refrigerator magnets, but high power electromagnets and super conducting magnets.

You set up four magnetic poles at the corners of a square (see figure). This creates a magnetic field that will push the particle back to the center if it strays up or down, and away from the center if it strays left or right.

Now by setting up 4 more magnetic poles following these rotated 90 degrees you focus the stray left and right particles. Now for the second set of magnets the particles that strayed left or right and got pushed away, will now get pushed back towards the center. With this set up you can focus your particles into one nice thin stream.

 

Now if you have a circular accelerator or need to bend the particle beams to hit a target, all you have to do is adjust the focusing magnets a bit and it will push the particles in whatever direction you want.

 

Cooling the Particle Accelerator

The copper vacuum tubes are all underground so changing temperatures from whether of of no real concern. But the magnets used to keep the particles "in line" give off a lot of heat and so that the whole system doesn't melt there needs to be a way to cool it.

There are many metal tubes running along the copper vacuum chambers. The cooling water is circulated to cooling towers above ground to remove the heat. Any super conducting magnets get cooled with liquid nitrogen or liquid helium.

 

Target/Detection

Depending on your experiment depends on what your target is going to be. Some targets may be thin sheets of metal foil. In other experiments, beams of different charged particles collide with each other inside the detectors.

All collisions have to happen near the detection equipment. There are many types of detectors, and sometimes more than one kind is used at a time. Two types that have been around for a while are bubble chambers and cloud chambers. A bubble chamber contains a liquid gas, such as liquid hydrogen. As the particles released from the collision pass through the chamber, they vaporize some of the liquid, leaving a bubble trail that you can then detect. A cloud chamber detector has a saturated vapor inside the chamber. As an energetic particle passes through the vapor, the vapor is ionized, producing a trail, which again can be detected.

There are hundreds of different kinds of detectors and if you would like some more information please visit, The Particle Detectors BriefBook.

 

 

The type of accelerating process I have described in this site uses standing electromagnetic waves. These are used in both linear and circular accelerators.

Linear (Linac), is a long straight set of accelerators and focusing magnets that end at the target and detection sight.

This type of accelerator can only manage to accelerate particles to 200 MeV. Physicists mainly use them as a primary accelerator that feeds into a circular accelerator. In industry and medicine they are used as powerful X-ray machines.

There is another type of linear accelerator. It is a traveling-wave linear accelerator. This speeds particles through a single long pipe by an electromagnetic wave that travels with the particle. This high-frequency wave is called a traveling wave. As long as the wave speed matches the particles' speed, the particles will continue to gain energy.

Stanford Linear Accelerator Center SLAC

Circular (Cyclotron or Synchrotron), do essentially the same jobs as the linear accelerators. However, instead of using a long linear track, they propel the particles around a circular track many times. At each pass, the magnetic field that bends the particles around the track is strengthened so that the particle beam accelerates with each consecutive pass. When the particles are at their highest or desired energy, a target is placed in the path of the beam, in or near the detectors.

 

CERN's circular accelerator can produce particles of 50GeV. Fermi National Accelerator Laboratory (Fermilab) is home to the Tevatron, a particle accelerator capable of producing 1TeV. This is mainly due to the fact that all the magnets used in the accelerator are super conductive, meaning no loss of energy, so it gets more energy from less power. The accelerator is a storage ring collider accelerator. This means two sets of particles can rotate in opposite directions around the ring, then collide with each other at a desired time. This effectively means a collision of at least 2TeV will occur when two particles collide.

Finally let it be noted that all of the above would not be able to happen without high powered computer systems. Controlling everything from the charge on the plates, to cooling the copper vacuum chambers, to collecting all the data, computers make it possible.

 

 

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