PFE013: LHC Part 1 - The Basics

Over the last several years, the LHC has been in the news a lot. Enough to hit critical mass in the media. Apparently, when it comes to science that no one understands, this means that it's okay to write stories based on a bizarre theory someone came up with, write about it as though it's widely accepted, and then include a sentence at the end explaining that it hasn't been proven yet.

Before I talk about these things, I think an understanding of how such a monstrous machine works is helpful to keeping up with a large portion of physics in the news.

A particle accelerator may be used for a variety of different things. Accelerators like the LHC, the Tevatron, or SLAC are used to study basic physics. But accelerators like this account for a very small percentage of all accelerators. There are accelerators for manufacturing electronics, medical research, and medical treatment. Most of this post will focus on the higher energy physics based accelerators, but it all applies to medical, manufacturing accelerators too.

But we have all seen particle accelerators in our everyday lives. A battery is a device that accelerates electrons. It is doing essentially the same thing as the LHC! Just on a scale about nine trillion times smaller. So an accelerator is any mechanism that creates a stream of particles going very quickly (or, more usefully, with more energy).

Particle accelerators can be classified into two main types: circular accelerators, and linear accelerators. Each with its own advantages and disadvantages.

Circular accelerators have three main parts: magnets, rf-cavities, and detectors. Since the particles that are accelerated are charged, magnets are used to bend them in a circle. In fact, there are typically two beams of particles moving in opposite directions. A simple relation can be used to show that how strong the magnets need to be increases as the speed and energy of the particles increases and decreases as the size of the circle increases. Since more new physics can be seen at higher energies, and the limiting factor is often the size of the magnets, these machines can end up being as large as 17 miles around.

The next important part is the rf-cavities. The first thing to know is that magnets can't be used to make particles go faster, they can only change their direction. To get the particles going this fast, you need something else to accelerate them. And the methods used are similar to how microwaves work. The best way to imagine how an rf-cavity works is to think of surfing. The cavity creates waves of energy moving through a chamber, and, if the particles enter the cavity at just the right point on the wave, it will be pushed through the cavity and will get a touch more energy. The major advantage of circular accelerators is that  one rf-cavity can be used many times to accelerate a particle. So particles can gain as much energy as we want, up to infinity, right? Sadly, no. As the particles are bent around the circle, energy is lost. The more energy the particles have and the sharper the curve, the more energy is lost. So eventually the amount of energy lost will equal the amount of energy the cavity can add and the particle has reached its maximum energy.

The final part is the detector. There are a number of monitoring devices to keep track of where everything is. Now they use all kinds of fancy equipment, but a story passed down to me from the early days of accelerators was that to check if the particles were in the pipe, they would stick their head in and actually look. The particles would create a blue light inside their eyeball and they would know that the machine was working properly. The main detectors are where the particles collide. At these points on the ring, the magnets bend the two beams into each other and a bunch of massive collisions (hopefully) happen. Particles are sprayed out in all directions and huge detector measures what happens to all of them, before the next particles collide, an instant later. Then, computer software figures out what happened at the collision point.

A linear accelerator operates in largely the same fashion as a circular accelerator. As it turns out, the energy lost as particles are bent around in a circle is much more for some particles than others (it goes by m^-4 for those interested). So for these sorts of particles (typically electrons) it is more efficient to line a bunch of rf-cavities and either smash two such beams or hit a stationary target. This takes more rf-cavities, but you don't need huge magnets to bend it in a circle and energy isn't lost from doing so.

I should emphasize that as much as I have covered here is only a small portion of the actual mechanics of particle accelerators. There are a number of topics that I glossed over (or simply ignored), so please ask to expand on anything that's confusing or unclear.

That's accelerators.