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Some LHC questions

M

Mark R

Diamond Member
Photos look sweet, but I've got a few questions that are probably very simple to people who know.

What happens if they need to shut down and dump energy rapidly? Beam kinetic energy is supposed to be something like 700 MJ, or about as much as a freight train. I'd imagine that if the beam hit the side of the pipe (? due to magnet failure), the damage won't just buff out. So, how to they dump it? Magnetic brakes? Just let it plough into a massive lump of concrete or water bath?

How do you maintain it? Surely once you switch it on and run it up to power, the entire thing (or at least the areas near the magnets and experiments) will become radioactive. Something to do with the X-rays produced during operation causing disintegration of the nuclei in anything they hit? Is it just a matter of leaving it idle for a few weeks so that the radioactivity decays?

Really basic one this. How do you send protons in opposite directions around the same ring - the steering magnets only work in one direction? Maybe I'm missing it from the pictures, but as far as I can see, you'd need 2 beam pipes, each with its own set of magnets, so that the anticlockwise pipe has magnets in the opposite orientation to the clockwise pipe.
 
They've worked it all out with a shit load of tests, experiments, and simulations. They won't need to stop it, there won't be emergencies. This isn't the first particle collider, if you want some information, look at how current ones run. There is a lot of radiation, but that won't affect a piece of metal, you should know that. There are areas closed off to personal because of radiation though. As for magnets, they aren't one direction, it's based on two way current.
 
at SLAC (Stanford Linear Accelerator), they have rectangular blocks of iron or steel (something ferrous) at
the end of some of the smaller beam lines. some of them look like sponges - as if they were extremely
corroded. ( of course it's possible the physicist who told me that was kidding )

the main beam points directly at the Hoover Institution.

700 MJ ? that would charge a lot of disposable cameras !
 
700 MJ? That's actually a good bit higher than what we're shooting for (or is even feasible today). At full luminosity, collisions should be around 14 TeV, which is only around 2.2*10^-6 J. Still, give a proton 7 TeV of energy and it's going to be moving really fast!

The magnets are superconducting, so they actually take a very long time to turn off. Even when you stop supplying them with power, they continue running because there is effectively 0 resistance.

If the beam hit anything inside of the pipe, it would probably be destroyed. If the magnets suddenly lose power, we'd basically just stop pumping in new protons.

There are actually two beams of protons, but they're separated by very little distance. They collide at 8 points along the ring.

X-Rays don't disintegrate matter... but they can damage instruments, and that is always a concern. At ATLAS (big experiment), we're predicting that the giant lead scintillating crystals at CMS (the other big experiment) will become useless very quickly due to the high radiation environment combined with the nature of these crystals. We're also concerned about our forward calorimters (liquid argon) becoming saturated. When ionizing radiation passes through a LAr calorimeter, the Ar ionizes and a current is created (electrons in one direction, Ar ions in the other). We're concerned that there will be a large amount of recombination, where an electron and Ar ion recombine into neutral Ar, which is a real concern in a high radiation environment. Naturally we could lose a large amount of our energy in the forward region if this is a problem. Hopefully it's not a big deal (we have a lot more calorimeters anyway)

I'm not sure exactly what goes into stopping the entire experiment. I do know that you're not allowed to go underground for 2 weeks after shutoff, and then you have to wear a film badge. In past experiments it was normally possible to walk around with a film badge even during operation.

Try digging around on cern.ch, they have a lot of public outreach for the LHC I think. Or try google
 
Originally posted by: Eeezee
700 MJ? That's actually a good bit higher than what we're shooting for (or is even feasible today). At full luminosity, collisions should be around 14 TeV, which is only around 2.2*10^-6 J. Still, give a proton 7 TeV of energy and it's going to be moving really fast!

The magnets are superconducting, so they actually take a very long time to turn off. Even when you stop supplying them with power, they continue running because there is effectively 0 resistance.

If the beam hit anything inside of the pipe, it would probably be destroyed. If the magnets suddenly lose power, we'd basically just stop pumping in new protons.

There are actually two beams of protons, but they're separated by very little distance. They collide at 8 points along the ring.

X-Rays don't disintegrate matter... but they can damage instruments, and that is always a concern. At ATLAS (big experiment), we're predicting that the giant lead scintillating crystals at CMS (the other big experiment) will become useless very quickly due to the high radiation environment combined with the nature of these crystals. We're also concerned about our forward calorimters (liquid argon) becoming saturated. When ionizing radiation passes through a LAr calorimeter, the Ar ionizes and a current is created (electrons in one direction, Ar ions in the other). We're concerned that there will be a large amount of recombination, where an electron and Ar ion recombine into neutral Ar, which is a real concern in a high radiation environment. Naturally we could lose a large amount of our energy in the forward region if this is a problem. Hopefully it's not a big deal (we have a lot more calorimeters anyway)

I'm not sure exactly what goes into stopping the entire experiment. I do know that you're not allowed to go underground for 2 weeks after shutoff, and then you have to wear a film badge. In past experiments it was normally possible to walk around with a film badge even during operation.

Try digging around on cern.ch, they have a lot of public outreach for the LHC I think. Or try google
Click to expand...

I think 700 MJ is the energy of all the particles in the beam at any given time. So 10^8 Joules and 10^-6 Joules per particle gives 10^14 particles in the beam at any given time, which I guess seems right.

You can quench a superconducting magnet 🙂 You can also just start pushing current the other direction. It only takes a few minutes to safely ramp the current down in one of them, and I'm sure you could do it much more quickly if you had to.

I did a little bit of work at TRIUMF (world's largest cyclotron AFAIK) and yeah, you can't go inside for a few weeks after shut down. Luckily for me it was cool enough to go in and see, so that was neat.
 
Thx guys.

Lol. 700 MJ is total beam energy - not per particle (or even per bunch energy). 700 MJ per particle would be impressive, but I wouldn't want to be anywhere near when such a particle hit anything.

Turns out they have some nice technical info. The beam is dumped by switching on a deflection magnet (with very good ramp time) which will direct the beam down a side branch off the ring. There it is rapidly scanned by deflection magnets (like a CRT) onto a huge, low-density graphite target. The scanning is needed so that the whole target area is used, otherwise the target would be incinerated. 1000 tons of concrete are used to provide radiation shielding to the targets.

I was wrong about the X-rays. You don't get too many off the ring (just the experiments), and they're not of sufficient energy to disintegrate the surrounding matter anyway. However, the problem is the beam interacting with contaminants to the vacuum in the beampipe (e.g. leakage of air or helium, or outgassing from the pipe itself), or by protons scattered by focusing magnets which subsequently hit the pipe wall. These proton-contaminant collisions are hugely energetic and produce all kinds of scattered crap - high-energy scattered protons, neutrons, other hadrons, etc. Fair enough, keep people out while the beams are circulating. However, the real problem is the neutrons which will transmute the nuclei in the equipment, air and tunnel walls into a soup of radioactive elements. Hence the need for exclusion even after shutdown. Looks like there are very detailed simulations of this effect, and that radioactivity will have decayed acceptably for brief entry to the tunnel after 24-48 hours.

I wasn't able to find any understandable pics of the dipole magnets - but I assume there must be one orientation for the clockwise pipe and opposite orientation for teh anti-clockwise pipe. I did take fiewwolfsm's advice to see how other accelerators had dealt with this problem - so I checked the Fermi tevatron and the LEP. And the answer was that they didn't deal with this problem, they simply avoided it by using oppositely charged particles in each of the 2 directions.

 
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