Have you ever wondered how nuclear physicists discover subatomic particles like quarks, muons, and leptons? It is not possible for them to use a microscope and take a peak inside the quantum world. Instead, they use giant particle accelerators, instruments that are able to accelerate narrow beams of electrons, protons, and other charged particles close to the speed of light. The very high-energy versions of these accelerators, while massive in size and cost, allow the study of matter at extremely small scales of both time and space and under conditions similar to those just after the Big Bang that created the universe. Research at particle accelerators around the world is constantly pushing the limits of what we can learn about the inner workings of the atom, with several breakthrough discoveries over the last fifty years being rewarded with Nobel Prizes in Physics. How exactly do these particle accelerators work?
Before investigating the accelerators, it is important to explain a bit about electromagnetism. If you have ever played around with a magnet, you know that like charges repel each other and opposite charges attract. So the negative end of one magnet will attract the positive end of another magnet. These attraction and repulsion forces come from the magnetic field surrounding the magnets. The interesting thing is that when electrically charged particles move through space, they also generate a magnetic field like that of a permanent magnet. Electricity and magnetism are intimately linked and the opposite is also true; a magnet moving through space generates electricity.
So if a charged particle, such as an electron, is sent through a narrow tube surrounded by electromagnets, it can be directed on a certain curved path by changing the strength and direction of the magnetic fields of the electromagnets. The magnetic field surrounding the charged particle will interact with that of the electromagnets and the particle will be pushed or pulled. This is how the direction of a particle is controlled in an accelerator so that it follows the proper trajectory.
However, how does the charged particle get going down the tube and pick up speed? Well, in a similar fashion, the plates of the tube are electrically charged in an intelligent way so that the charged particle will be attracted down the tube and repelled from behind so that it does not reverse direction and go the wrong way. The forces generated by the electrical fields of the charged plates accelerate the particle as it progresses down the tube. The electrical plates are controlled such that batch after batch of particles can be sent along in a rippling fashion, allowing scientists to conduct multiple experiments at once.
Most particle accelerators are not the ridiculously expensive and humongous variety. You likely have at least one accelerator in your home. Cathode ray tube (CRT) televisions and computer monitors - quickly becoming things of the past - contain simple particle accelerators. An electron gun shoots electrons onto a fluorescent screen to produce light and ultimately a pretty set of moving pictures for your viewing pleasure. The beam of electrons is deflected to the proper position on the screen using coils in a similar way to the big accelerators used by the state-of-the-art research labs, just on a much smaller scale and with a lot less energy.
There are many different varieties of particle accelerators, both large and small, but they roughly fall into two main categories: linear and circular. Just as expected, linear particle accelerators send particles in a straight line towards a target, and circular accelerators send particles in a circle. The circular accelerators have the advantage of being able to send particles round and round in a loop until the desired velocity is achieved, whereas linear accelerators need to be very long and use a lot more energy to achieve the same results. However, circular accelerators do have a cost for sending the particles in a curved path, and it is called synchrotron radiation. High-energy particles moving at high speeds through a magnetic field, like in a circular accelerator, emit radiation. For controlled experiments, this is definitely not a wanted feature. It is difficult enough for physicists to narrow in on the desired outcomes of these high-energy experiments. Luckily, the amount of radiation is higher for tighter curves and shrinking the curvature of the tube minimizes the level of radiation. That is why the circular accelerators have huge diameters, some as large as several kilometers. At the research level, accelerators of both forms have produced breakthrough results. A popular option is to utilize a linear accelerator that feeds into a circular accelerator to get the best of both worlds.
The last topic that requires discussion concerning how particle accelerators work is what the target is. In the end, the particles are usually accelerated to high speeds in order to hit something. What they are supposed to hit varies depending on the application or experiment. In some cases, the particles will hit a screen or a detector, like in the discussion of a television CRT. In other cases, high-speed particles will actually be directed towards each other for a massive collision. At the time of impact, a lot of energy is released and other interesting and unique events occur, providing insights into the nature of quantum particles and the universe. Particle accelerators used in this way are called colliders.
With this basic understanding of how particle accelerators work, the next news broadcast covering the latest experiments at the Large Hadron Collider in Geneva, Switzerland will not be so unintelligible.