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  • 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.

Particle accelerators are machines that take electric and magnetic fields,

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B2 中上級

粒子加速器で粒子をトラップする方法 (How To Trap Particles in a Particle Accelerator)

  • 43 4
    Ian Jyun Li に公開 2021 年 01 月 14 日
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