A Staff Report from the Straight Dope Science Advisory Board

How does a particle accelerator work?

January 24, 2002

Dear Straight Dope:

How does a particle accelerator work? I know it has something to do with huge magnets, but how can they make two atoms line up correctly and hit each other at such incredible velocities? Also how do they ensure that there are no other atoms besides the ones the are colliding inside the track?

Jeez, if you're going to slam only two particles together at a time, you're going to be the absolute worst accelerator physicist ever. A particle accelerator isn't the delicate instrument you imagine--it's more akin to a fire hose. The idea is to aim billions and billions of particles at billions and billions of other particles, cross your fingers, and hope for the best. The more particles you have in the beam, and the more tightly bundled the particles are, the better chance you have that you'll get a collision that produces exciting new particles.

The first step in operating a particle accelerator is coming up with some particles to accelerate. You can get electrons from a device called an electron gun, which boils electrons off a filament. An electron gun is basically the same device you have in the back of your TV or your computer monitor. You can get protons by ionizing (that is, stripping the electrons off of) hydrogen gas, whose atoms consist of one proton plus one electron. To get other types particles, you need to smash your accelerated protons or electrons into something else in order to produce other particles, then use some fancy particle selection techniques to separate the wheat from the chaff, as it were. But we're getting ahead of ourselves.

The main tool for accelerating particles is powerful electric fields. (We'll get to the powerful magnets in a bit.) Let's say you're accelerating electrons (which are negatively charged). You turn on a powerful electric field, and the electrons rush to positive potential. As the electrons get there, you turn off the first electric field and turn on another one downstream, and the electrons rush there. You keep doing this until your electrons are rushing like blazes and you've achieved the electron speed you desire. Of course the process is a little more sophisticated than I've described. Instead of turning on and off the electric fields, you modulate your electric fields sinusoidally, such that the negative electric potential is always just behind where your electrons are, and the positive electric potential always just in front. The electrons sort of surf down the beam line on the electric field modulation wave.

Where do magnets come in? They have two purposes: to steer the beam and to focus it. To steer the beam you set up a dipole magnetic field (a dipole field is a regular uniform magnetic field; you can create one with permanent magnets, but usually you use an electromagnet). When charged particles cross a magnetic field, they experience a force perpendicular to both the magnetic field and to the particle's direction of motion (F = qv x B, for you scientist-types). Thus, if you have an up-down magnetic field, and the particles are moving forward, the particle will turn to the left or right, depending on its charge. 

Obviously you need to steer your beam so that it collides with something, but there are other difficulties as well. To accelerate your particles, you need a lot of electric fields and magnets, and a lot of room for your particles to build up speed. Stanford's linear accelerator is two miles long! That means two miles' worth of magnets, two miles' worth of beam pipe, two miles' worth of vacuum, electricity, tunnel, radiation shielding, etc. It's really huge. And impressive. If you get a chance, take a tour of the Stanford Linear Accelerator Center--they'll let you stand in the access gallery and you can look up and down the two-mile-long facility.

However, most people don't have two miles to donate to particle acceleration (and you'd need a lot more than two miles to accelerate protons, which are 2000 times heavier than electrons). Typically, you reuse your accelerator components by moving your particles around a ring accelerator, where they can pass by the same electric fields thousands of times. To steer your particles around the ring you'll need powerful magnets. The main disadvantage of a ring accelerator is that every time you steer a charged particle, it gives off energy in the form of synchrotron radiation (powerful X-rays). You have to compensate for this energy loss with more and bigger electric fields, which cost a lot to operate, and when you have X-rays being sprayed off all around your accelerator, you've got a shielding nightmare.

The other way magnets come into play is in focusing the beam. You want your particles to be in as tight a beam as possible, particularly if you're trying to slam them into another beam. A higher concentration of particles means a higher probability that a couple of them will smash directly into each other. In addition, if you try to cram a bunch of electrons together, they will start to repel each other, and naturally make the beam larger. For focusing you use quadrapole magnets. These focusing magnets have specialized magnetic fields, such that if a particle is straying to the right, the magnet steers it a little to the left, but if the particle is straying to the left, it gets steered a little to the right.

Finally you've got to steer the beam so it smashes into something. The two smashing methods are fixed target and colliding beams. For fixed target, you slam a beam of particles into some stationary chunk of matter. The advantages of the fixed target method are: (a) it's easier to aim the beam at a large non-moving target, (b) lots and lots of protons are available in chunks of matter, and (c) most all the products of the particle interaction will be moving in a very definite forward direction, so you can easily build detectors in the forward region to detect them. The disadvantages are that chunks of matter pretty much only provide protons and neutrons to smash into--the electrons aren't concentrated enough to be consequential--and there are certainly no anti-protons or positrons (anti-electrons). Also, a target which is just sitting there does not bring any kinetic energy into the equation. Since you are typically trying to convert energy into massive particles (via E = mc2), more energy is better.

That brings us to the other particle smashing method, colliding beams. For colliding beams, you typically smash particles into their antiparticles. (The HERA accelerator at DESY in Germany is unique in that it collides electrons or positrons with protons, for some very interesting physics effects.) For example, to create positrons (anti-electrons), you first accelerate some regular electrons, smash them into a fixed target, and collect any positrons resulting from this interaction. This generally takes a few minutes. Then you start accelerating the positrons and electrons. In a linear accelerator, you accelerate the positrons in a bunch just behind the electrons. (They surf the same electric field waves, but they surf up instead of down, if you get my drift.) In a ring accelerator, you're golden because the same accelerator elements that steer electrons clockwise automatically steer positrons counter clockwise. The advantage of colliding beams is you get lots of energy from both beams, so you can create lots of exciting particles. One disadvantage is that collisions occur in the center of mass, so particle debris gets sprayed all over. That means you have to build a detector that completely surrounds the collision site (typically called a 4p detector), which is expensive. The other disadvantage is that it is very difficult to steer two beams directly into each other. Beams are typically the diameter of a human hair, and the length of a needle. You try smashing together two beams that size going 99.999% the speed of light!

Fortunately, there are talented accelerator physicists everywhere, or at any rate at these fine accelerators: Cornell Electron Storage Ring (CESR), Stanford Linear Accelerator Center (SLAC), Deutsches Elektronen-Synchrotron (DESY), Fermi National Accelerator Laboratory (Fermilab), and European Organization for Nuclear Research (CERN).

For more about particle accelerators I recommend The Particle Adventure. You gotta love a site that shows electrons surfing the electric field modulation wave on tiny surfboards!

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Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.

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