Tuesday, June 19, 2012

particle accelerators work

How particle accelerators work: Particle accelerators (like the LHC at CERN) are big news right now, but what are they and how do they work? If you watched the little video about JJ Thomson on Monday (see below), you'll have seen the basic idea already. What we saw was: 1) Inside a glass tube, we can heat a negative electrode (the cathode) and "boil" electrons off its surface. 2) The electrons spe...ed through the tube, attracted to a positive electrode (anode). 3) If we take a magnet, we can make the beam bend up or down.

OK, so what if we make our tube into a loop shape and place a ring of magnets around the edge of it to make the electrons go round in a circle? What if we keep switching the current on and off intermittently to accelerate the electrons? And when they reach an incredible speed, what if we open a "door" and let a few escape into another chamber, so they smash into atoms or other fast-moving particles? That, very crudely, is the basic idea of a ring-type particle accelerator (other accelerators, like SLAC in California, just speed the particles in a straight line). These things might look amazingly complex, but really they're that simple. And when the particles collide, then the fun really begins!

So what are accelerators really like? Here's a photo from Maximilien Brice at CERN (© CERN 2011) showing part of the particle beam from the LHC at CERN. I've added some simple notes so it's clearer. We have a ring carrying two beams of particles (protons and anti-protons) in opposite directions (the red and blue lines). The ring is surrounded by cooled, superconducting magnets that keep the particles in place. Those magnets are surrounded by thermal insulation (to keep them cool) and the whole thing is inside a vacuum container. The entire ring is 27km (17 miles) in circumference and uses over 1200 dipole magnets like this...

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