# Help regarding intuition on particle accelerators

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1. Jun 14, 2015

### Obliv

I realize there have been multiple threads on this and believe me I tried my hardest to find these answers from them and other resources.

Questions: 1) Is the van de graaff generator a particle collider? I am under the impression that it is meant to follow up on the Cockroft-Walton accelerator and provide insight on the nucleus. I know that the cockroft-walton accelerator collided protons with lithium atoms. What baffles me is that the van de graaff generator is just two spheres that produce a potential difference with air as the insulator. How does this provide collision between particles?

edit: from http://bt.pa.msu.edu/pub/papers/steeremsc/steeremsc.pdf page 4-6 he talks about this accelerator and doesn't mention collisions so i assume now that it was not used for that. What else could it be used for? Is electrical breakdown desired, to produce sparks?

2) how do oscillating field accelerators work? I looked this up and cannot find the reason for how they circumvent the issue of electrical breakdown that the electrostatic accelerators consistently met with. I don't understand how they work in the first place so an in detail answer would be greatly appreciated.

This is for a final project for my ap physics class if anyone is wondering

Last edited: Jun 14, 2015
2. Jun 14, 2015

### ZapperZ

Staff Emeritus
This really belongs in the Classical Physics forum, rather than high energy physics forum. 99% of particle accelerators in this world have nothing to do with high energy physics or particle collider. Furthermore, if this is part of a school work, it must be done in the HW/Coursework forum and must abide by the rules of that forum.

You are confusing the name "particle accelerators" with "particle collider". Particle accelerators are generic. It does what the name implies, accelerator particles. By how much, to what energy, etc.. etc. depends on what the accelerated particles are used for.

RF accelerators require that the oscillating electric field lines along the axis of the accelerator beamline. It also results in the particles being accelerated only for a certain portion of the RF period, since the direction of the electric field changes direction.

Zz.

3. Jun 14, 2015

### e.bar.goum

1) You can use a Van de Graaf generator as a particle accelerator, yes (that is, you collide a moving beam of particles and a stationary target. "Collider" normally implies two beams. I don't think it's impossible to use a Van de Graaf as a collider, but I don't think it has been done). Van de Graafs are still used in nuclear physics today.

The way it works is reasonably simple. As I'm sure you know, you can use a Van de Graaf to produce a large positive potential on the terminal. If you place the terminal in a pressure vessel, you can use something like Sulfur hexaflouride (SF6) as the insulator, and get very high positive potentials (25 MV was the record. Van de Graafs in operation today top out at about 15 MV). So, you've got a positive terminal. Then, you can introduce charged ions. Now, depending on the geometry of the accelerator, these may be positive or negative. If you place your source of ions in the terminal, then they need to be positive, and they will be repelled away from the terminal, giving you acceleration. Add a few magnets, and some focussing elements, and you have a rather nice accelerator.

Now, you can play a rather clever trick. Rather than placing the terminal at either end of the acceleration tube, you can place the terminal in the middle. Then, if you inject negative ions before the terminal, you get one stage of acceleration, and if you strip off some electrons (turning the ion into a positive ion) the ions will experience a second stage of acceleration! This more than doubles your bang for your buck. This is called a tandem accelerator, and many of the electrostatic accelerators used for nuclear physics research look like this. Here's a rather simple diagram showing the geometry (made by me).
https://dl.dropboxusercontent.com/u/34677838/14UD.png [Broken]

As for (2) What have you read? What didn't you understand? This is kind of a big question.

Last edited by a moderator: May 7, 2017
4. Jun 14, 2015

### Obliv

thank you so much. I understand much more about the van de graaff generator.

From what I gather, there is a limit to how high the electric field strength can go for any gas (including SF6) before breaking down the insulator and turning it into a conductor, which allows the sparking from a van de graaff generator. My question is, how does an oscillating field accelerator overcome this? We are accelerating beams of particles to thousands of GeV somehow in the CERN accelerator complex. How is this achieved? I understand that there are multiple accelerators involved, not just the LHC. Is this still too broad?

Last edited: Jun 14, 2015
5. Jun 14, 2015

### e.bar.goum

Right. This is where the beauty of circular accelerators lie. Particles don't just experience one instance of acceleration. You don't need super high fields if you can just add a little bit of energy each time the particles pass. At the LHC, there are 8 cavities per beam, and each of them delivers 2 MV (~5 MV/m). That's not a huge amount as far as these things go, but they are superconducting. So, to get up to 7 TeV, you whip the beam around the ring thousands and thousands of times, increasing the energy each time. And you're correct, the LHC isn't the only accelerator involved. Beams are delivered at 450 GeV to the LHC, after they've been accelerated with a series of smaller accelerators (last decades "biggest accelerator ever" is the next decades injector)

6. Jun 14, 2015

### Obliv

so in theory could we just stack a bunch of cockroft-walton accelerators linearly, and accelerate them multiple times to achieve greater energies than a single one? I thought that you had to apply greater energy each time to increase the speed of the beam though? That has no basis, it is just an assumption. So this is not necessarily a new type of acceleration, it is just multiple electric fields working together at different strengths (not exceeding the limit before electrical breakdown) that we call oscillating? I can't tell you how helpful you've been :) thank you so much

7. Jun 14, 2015

### e.bar.goum

Well, no. The oscillating part is important. That's how the whole thing works. The cavities at the LHC operate at 400 MHz, and that's for good reason. In a synchrotron (the kind of accelerator at the LHC) without RF acceleration, you would have an extremely crappy beam, to put it lightly.
Think of the RF field as a sine function in V(t). For a particle to get the right acceleration, it needs to land in the resonator at exactly the right time. Too soon, it gets a smaller acceleration, too late, it gets a bigger acceleration. Thus, slow particles get sped up, fast particles get slowed down (relatively). In this way, RF cavities are essential to minimising beam losses, ensuring the correct energy of the beam, and maintaining the longitudinal shape of the beam (the "bunch" shape). You can see that as beams get faster, the timing of the RF voltage will change.

Further, RF cavities give you higher acceleration gradients (1-100 V/M)

Oh, and also, of course RF cavities to experience breakdown at very high fields. There's something called the Kilpatrick limit. If you want higher fields, you need higher frequencies. $f = 1.64 E_k^2 e^{-8.5/E_k}$.
Image credit: EuCARD

http://eucard-old.web.cern.ch/eucard-old/activities/communication/public/Images/RF.jpg

8. Jun 14, 2015

### Staff: Mentor

9. Jun 14, 2015

### e.bar.goum

And, indeed, the next generation High Energy Physics accelerators will likely be multi-stage linear colliders - CLIC and ILC.

ETA: And there are hundreds of small RF linear accelerators. To bring it back to the first part of the OP: There are even coupled Van de Graaf and RF accelerators, with the RF accelerator acting as a booster.

10. Jun 15, 2015

### nsaspook

I posted some pictures of a small industrial 1MeV RF accelerator that's injected by a 90keV DC accelerator.

Each cavity section has an electrostatic quadrapole between to refocus the beam for the next cavity. The first two are the 'buncher' to the chop the incoming DC beam into the proper length bunches for the later ones.

Last edited: Jun 15, 2015
11. Jun 15, 2015

### e.bar.goum

Nice. Here's a fairly horrid top-down photo of one used for nuclear physics research. 6 MV/q, and fed by a 15MV tandem.

I like your example, nsaspook. Proof that accelerators come in all shapes and sizes!

12. Jun 15, 2015

### nsaspook

She's a little brute, the top end calibration beam is a Xe3+ ion @ 3MeV. The company I worked at before had 1.5 a MeV tandetron accelerator system that worked well but it had a Mg charge exchanger that was a pain to keep running.

Last edited: Jun 15, 2015