字幕表 動画を再生する 英語字幕をプリント Particle accelerators are machines that take electric and magnetic fields, and use them to accelerate beams of particles to almost the speed of light. Controlling and focusing beams like that is a bit of a challenge, but it we can do it, we can use them not just for scientific research, but also say, in hospitals to generate x-rays and proton beams for cancer treatment, and loads of other industries as well. This is a very visual demonstration of how we can control and focus particles. It's called a Paul trap, after the inventor Wolfgang Paul. Imagine you're riding on a beam of particles in an accelerator. This saddle shape is analogous to the effect of the magnetic field as it comes up to a focusing magnet. If it's too far over to one side, it'll get pushed back to the centre. To far to this side, it'll get pushed back to the centre. But, it can't focus in both directions at once, so if I were to just let this go... it's going to fall off. It's unstable. It turns out you can't focus a beam of particles in both directions at the same time with either a static electric or magnetic field. But we can focus beams in an accelerator, and the way we do it is by alternating the gradient of the field in time. In this demonstration, we do it by spinning the device. The thing to note here is we have to get the forces right and at the right time. For example, if I rotate this any slower, the particle will fall off. And it's gonna fly off quite explosively if I drop it in when it's going any faster. By rotating the saddle we're alternating between focussing and defocusing effect, which is very similar to the effect of quadrupole magnets in an accelerator. That's a pretty crude model of a Paul trap, but we can use a bit more of a sophisticated setup to look at the beam motion in accelerators in a bit more detail. This is a quadrupole linear Paul trap and it has four rods on which we put an AC oscillating potential on each opposing pair of the rods in order to provide the focusing. Into there, between the four rods I'm going to place some pollen particles. I'm just going to charge up some particles using a Teflon wand, which I'll charge with static electricity. What we're looking at is a bunch of pollen particles which are trapped by the oscillating AC fields. If those fields were static, the particles would just fly off in one direction or another, but because we're oscillating them at the right voltage and with the right freqency, we actually manage to set up a stable region where the particles can remain more or less indefinitely in the centre. Even when they have an amplitude that's not zero they're still stable in the centre. So they're moving inwards and outwards but alternating between vertical and horizontal, so when they move outwards vertically they move inwards horizontally. So the way that they're oscillating back and forth, vertically and horizontally, is very much like the movement of the beam in the accelerator But it's as if we're sitting on top of the beam as we move through it as we move through it rather than seeing the beam move along. So that's why it looks stationary. Turning down either the voltage or the frequency is like slowing down the speed of the rotating saddle trap that we had before. This is not just a really beautiful demonstration, but in my research, I've actually built a more precise version of this, trapping actual ions so we can study some of the properties of beams in real accelerators, and using that we can study new types of accelerators before they've even been built.