How Things Work: Particle Accelerators

Physics is pretty complicated stuff. So complicated, in fact, that physicists often have to resort to violent tactics in order to probe the subatomic universe: they smash atoms. Doing physics with an atom smasher is a little bit like dropping a monitor off the eighth floor stairwell of Wean Hall and trying to figure out how it works from the little pieces of glass, wire and circuit-board left over. But the atoms go a lot faster, there are a lot more pieces at the end, and above all, it costs a lot more tax money.

Atom smashers, more accurately known as particle accelerators, boost charged particles like electrons and positrons or heavy ions and their anti-particles to almost the speed of light and then heave them into one another. If they collide, an explosion of so-called ?daughter? particles is produced, many of which decay into further daughters, and so on. This explosion occurs inside a strong magnetic field, which causes all of the charged particles produced to bend in circular arcs,the curvature of which can be used to deduce which particle is what. This typically happens on the order of a billion times each second.

What happens with all those data? Well, they typically get written to giant data storage tapes and collect dust until someone finds a reason to look at them ? which usually isn?t that long, as any run of data-taking is always driven by the desire to learn something new about a specific set of physical phenomena. For instance, the fact that protons and neutrons are actually made out of quarks was discovered in 1968 at the Stanford Linear Accelerator by smashing protons into electrons and deducing that the protons aren?t just point particles, but have to have an additional substructure of three quarks, specifically two ?up? quarks and one ?down? quark. Later, as theory and experiment developed, other quarks such as ?charm,? ?strange? and ?bottom? (sometimes called ?beauty?) were discovered. Finally in 1995 physicists at Fermilab who were colliding protons into anti-matter protons found evidence of the heaviest quark, the ?top? quark. This discovery put the last piece into a quark-puzzle that makes up part of the ?Standard Model? of particle physics, which is an attempt to unify the electromagnetic, weak nuclear, and strong nuclear forces into one grand whole.

So how do you accelerate an electron to near the speed of light? You use the same thing that turns on your vacuum cleaner: an oscillating electric field. In the case of a modern accelerator like the Tevatron outside of Chicago, the electric field lives in a series of ?super-conducting radio frequency cavities,? or SRF cavities. SRF cavities are often ellipsoidal (this shape helps reduce the amount of damage due to resonances that can zap the walls of the cavity and render it unusable), and made of some super-conducting metal such as niobium which is cooled by liquid helium to become a super-conductor. The electric fields are actually generated outside of the cavities, and then directed into them via metal wave-guides. The phase and frequency of the oscillating electric field are tuned such that when an electron passes through the cavity, it experiences maximum acceleration the whole time it is in the cavity. While the electron is making its transition into the next cavity, the field flips back to its initial configuration in order to start the process all over again.

Not only do you have to accelerate your particles before you get to smash them, you also have to keep them in efficient little bunches traveling at the right speed and in the right direction. To do this we space our SRF cavities out in a line in the case of a linear accelerator, or a circle in the case of a circular accelerator along with a series of dipole, quadrupole, and sextupole magnets that bend, focus, and change the chromaticity of our beam, respectively. This is exceptionally difficult to do precisely and accurately due to the fact that when you try to bunch any collection of all positively or all negatively charged particles together against their will, they will repel one-another as a consequence of their own intrinsic electric fields.