LHC Size Limitation

[mooeypoo]

I'm moving this question here, it seems more suitable than the original 'news' thread I took the source from (thanks again Severian).

 

I've read this: http://www.hep.ucl.ac.uk/iop2010/talks/14.pdf

 

In it (slide 6) the claim is made that one of the factors limiting new and bigger and better hadron colliders is the size. And it seems that, indeed, the LHC is huuuuuuuge -- which also made it an incredible effort to build both in terms of actual work and in terms of money.

 

But there's something here that I'm not sure I understand. The collider is circular, which means that in terms of a moving particle, there's no 'change' in the path no matter where it is located. It's being accelerated along the way until it collides with another particle.

 

Why, then, can't we -- instead of building a *bigger* circle -- just send one particle on 100 loops around the circle, and then release another particle to be collided with after the old particle is accelerated? Why is it the size that matters so much? If forces are applied on the particle along the path, then instead of size, we can just ahve more rotations, save money, effort, and have more LHCs, and therefore more option for research, results, and wonderful science.

 

 

I assume, however, that if it was that simple, it would have been done. Then... what am I missing here? Why is the size so important? Why can't we just do more rotations instead of making the colliders bigger?

 

~moo

[Cap'n Refsmmat]

I'd presume it's because the force required to keep the particle going in a circle goes up with [imath]v^2[/imath]. Instead of increasing the force, using stronger magnetic fields, and losing more energy to synchrotron radiation, it's easier to increase the radius.

 

Particles in the LHC do run multiple loops before collision. But there's a limit to what you can achieve by increasing the fields, due to synchrotron radiation.

 

There's probably other factors I don't know about as well.

[mooeypoo]

I'd presume it's because the force required to keep the particle going in a circle goes up with [imath]v^2[/imath]. Instead of increasing the force, using stronger magnetic fields, and losing more energy to synchrotron radiation, it's easier to increase the radius.

 

Particles in the LHC do run multiple loops before collision. But there's a limit to what you can achieve by increasing the fields, due to synchrotron radiation.

 

There's probably other factors I don't know about as well.

There's something there about the minimum radius required to be effective (I believe it's in the next page, pg7). I guess there's a requirement to bend the particle through the loop and that requires a specific force.

 

But isn't the particle staying in the "middle" of the track anyways? That is, we're already building the accelerator to bend the particle through the circle successfully - why not repeat the loops further..

 

Is it a matter of energy requirements to increase the forces, or are we actually losing something when we bend the particle with those forces, and the goal is to use weaker forces anyways?

 

Let's say we want to build an even better accelerator than the LHC -- do we go for size (have it circle the globe, eventually! or do we go for increased energy? What are the actual limitations in making a better accelerator in that aspect, and why are all the new ones getting bigger and bigger?

[Mr Skeptic]

Then... what am I missing here? Why is the size so important?

 

Heh.

 

Why can't we just do more rotations instead of making the colliders bigger?

 

Well first of all, you need your energy input to be more than the synchrotron radiation. Synchrotron radiation is proportional to the acceleration, which depends on the radius, so with a larger radius you reduce this loss.

 

There's probably other reasons too. The accelerator accelerates particles in bunches. There needs to be activation of the accelerating components in a sequence moving at very close to the speed of light. I'm not sure what size is necessary for this.

 

There's a limit to the sort of fields a material can hold before being destroyed. I recall hearing about a system to induce an electric field in a plasma via microwaves because it could make a stronger field than the best solid materials. The stronger the fields, the smaller you can make your accelerator.

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Is it a matter of energy requirements to increase the forces, or are we actually losing something when we bend the particle with those forces, and the goal is to use weaker forces anyways?

 

Yes. When you accelerate a charged particle, it emits EM radiation.

[Severian]

It is indeed synchrotron radiation which is the problem. All accelerated charges radiate, so you need to make sure that the acceleration is spent on making the particle go faster rather than going around in a circle.

 

In fact, your idea of not wasting money by circulating the beam multiple times is already used. The beam makes about 11000 circuits per second and can be in the beam pipe for hours, so it is spending a lot of time going round and round being accelerated faster.

 

But as it gets faster and faster you need to use more and more of the acceleration to keep the beam going in a circle rather than just flying off. And eventually you find you are losing more energy via synchrotron radiation than you are putting in.

 

This is the reason that the LHC is a hadron collider. Heavy particles don't need to travel so fast to have as much energy, so don't need to go around the beam as often as lighter particle would (of the same energy). Therefore they lose less synchrotron radiation, and you can get them up to higher energies before you lose the battle. LEP, which accelerated electrons in the same ring, was about at the limit of what you could do for accelerating electrons.

 

Although you can get protons to higher energies, they are not as nice as electrons because they are composite particles. The LHC is really colliding the gluons which are inside the protons, and although we know the energy of the protons very well, it is hard to know the energy of the colliding gluons (since the energy is split between many gluons inside the proton). Also, hadron collisions tend to be very messy and difficult to dig out the interesting physics.

 

For precision measurements we would really like to have another electron-positron collider, like LEP, but at higher energies. But this will have to be linear, to avoid the synchrotron radiation, and work is underway designing the International Linear Collider (ILC). It is a tough job, since you can only accelerate them once without relying on being able to send them around the pipe again and again.

 

I was an author of the TESLA Technical Design Report, which was a first attempt to design such a machine. It wasn't funded at the time, mainly for political reasons, but the technology that we developed is largely going over into the ILC design.

[PaulS1950]

Would it be possible to use a spiral shaped accelerator to feed a linear accelerator to gain the required acceleration and resulting energy levels?

[Mr Skeptic]

Would it be possible to use a spiral shaped accelerator to feed a linear accelerator to gain the required acceleration and resulting energy levels?

 

Yes; it's called a cyclotron. The particles travel in a spiral, although the cyclotron itself is not a spiral.