LHC basic setup principle

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Now the text tells that "By contrast, protons with slightly different energies arriving earlier or later will be accelerated or decelerated so that they stay close to the desired energy. In this way, the particle beam is sorted into packs of protons called "bunches".
Here it refers to the main accelerating cavities of the LHC, their working principle sounds exactly like that of a klystron.

So here is my question , would it be fair to say that the LHC itself works like a giant bent/circular klystron tube?
Here is the way I understand it,
first they make the protons, then they are accelerated by linacs and "injected" into the synchrotron ring/s from where they are fed into the largest main ring?
This large main ring (the one 27km in circumference) is a circular tube with evenly spaced RF cavities working at 400Mhz and bending quadropole magnets so after enough rounds trips by the protons in the main ring they get bunched up by the cavities and accelerated to reach their maximum energy?


What I gather from CERN's homepage is that initially they only had the much smaller proton synchrotron ring and then they built the larger "super proton synchrotron" which I guess was just another synchrotron but larger and more powerful and then they built the now famous 27km main ring which I gather is just another synchrotron but now on "steroids".


Finally is it true that both the larger and smaller LHC rings are all synchrotrons and have the same basic function just that their size and power and parts count differ? differences in details like for example I read the smaller sps ring uses ordinary "room" temp magnets while we know the main ring uses superconducting ones.
 

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Not everything that accelerates particles in bunches is a klystron. The particles are grouped into bunches early in the linear accelerator already. You can't accelerate a continuous beam to useful energies for the LHC. Apart from the very first steps everything uses bunches.

The main ring has a short section of RF cavities at one place - they are not the limit for anything. Most of the length is taken up by dipole magnets bending the beam into a circle. How fast they can ramp up to the full strength determines the time the LHC needs for acceleration.
Quadrupole magnets are used for focusing.
Finally is it true that both the larger and smaller LHC rings are all synchrotrons and have the same basic function just that their size and power and parts count differ?
Two of them even have it in their name. Yes.

Normal conducting magnets can be ramped much faster. The PS has a cycle time of less than a second, the SPS can accelerate particles a few times in its ~10 second cycles. Compare this to the ~20 minutes the LHC needs to ramp up its superconducting magnets.
Running the LHC with normal conducting magnets would consume way too much energy to be practical.
Running the SPS with superconducting magnets would make filling the LHC a pain (you need many SPS cycles to fill the LHC rings).
 
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@mfb I remember reading that they indeed bunch up the particles way before they enter the main ring, I guess they also synchronize the arrival of the bunches as they enter the main ring as otherwise not synchronizing it many bunches would be de-accelerated by flying through the cavities and the wrong time ?

but with respect to the klystron ring as I called it, leaving time and energy efficiency out of the equation, if you would simply introduce a steady stream of protons in to the ring and then let them fly around , wouldn't eventually the cavities cause bunching up of the protons and also the acceleration of the bunches?


So from your text I take that instead of the 16 cavities being evenly spaced around the ring they are all located in a row in a single place?
The quadropole focusing magnets must be evenly spaced around the ring after a number of dipole magnets right? as I would imagine after some curvature the beam needs some focusing done on it?



As for your last paragraph , hmm this is interesting, I always thought that the reason why the main ring takes about 20 mins for the protons to reach their max speed is because they can't achieve it from a single pass through the RF cavities and so need multiple passes to gain the full available energy, but your saying that the bending and focusing magnet strength is at play, or are both of these factors at play?

IIRC superconducting magnets could lose their superconductivity and return to normal state if either the field gets too strong or increases too fast?
But I was under the impression that the ring magnets are on and "ramped up" like all the time , why would they need to ramp them up while accelerating at the same time?


thanks.
 
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Yes of course everything needs to be synchronized.
This is an interesting challenge in the lead/proton mode (lead ions in one ring, protons in the other) as they have a different speed. At injection it's impossible to keep them synchronous to each other, so the LHC cannot collide them at the injection energy. After bringing them to the full energy the speed is close enough to synchronize their revolutions again.
So from your text I take that instead of the 16 cavities being evenly spaced around the ring they are all located in a row in a single place?
Yes. The protons pass the cavities millions of times during the acceleration process, gaining a tiny bit of energy each time, it doesn't matter much where they are. Having them in the same place is easier to work with. They could easily install more of them, but it wouldn't help, the limits are from the magnets. This is different at electron-positron colliders where synchrotron radiation is significant and the particles lose a significant fraction of their energy each revolution.

Quadrupole magnets need to be everywhere along the ring, otherwise you would lose the particles.
But I was under the impression that the ring magnets are on and "ramped up" like all the time , why would they need to ramp them up while accelerating at the same time?
Check how the bending radius depend on the energy and the magnetic field strength.
 
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@mfb I take that your mentioned electron positron collider produced synchrotron radiation is as you say significant because electrons having the same strength of electric charge but being much less heavy can be accelerated to much higher speeds (relativistic? ) so at such speeds bending them by say the same angle as LHC protons would produce much more EM radiation?


As for the "bending radius" the way I understand it is that for a fixed radius and fixed particle weight (like that of proton) any increase in it's KE aka speed would require an increase in the deflection B field strength.
So at some point I guess the bending magnets cannot keep up with the speed and the protons would simply run into the walls and be lost eventually?
 
  • #6
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It's all ultra-relativistic for the high energy accelerators, but synchrotron radiation scales with the gamma factor which is much higher for electrons and positrons.
As for the "bending radius" the way I understand it is that for a fixed radius and fixed particle weight (like that of proton) any increase in it's KE aka speed would require an increase in the deflection B field strength.
Right, that's why you need to ramp up the magnets together with increasing the energy.
So at some point I guess the bending magnets cannot keep up with the speed and the protons would simply run into the walls and be lost eventually?
And that's limiting the energy for proton accelerators.
 
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@mfb Oh right now I got it, the reason the deflection B field must be ramped up proportionally to the proton speed is because if it was steady from the beginning the not yet fast enough protons would get too much deflected inwards and instead of crashing into the outer parts of the tube they would crash into the inner one.


How precise does the magnet field increase need to be around the whole circle because a mismatched field strength between say one part of the torus with another might result in the proton bunch crashing into the wall?



ps. I guess what we need is a straight tube , nature doesn't like bends... but the problem is that the cavities cannot transfer enough force on the small protons with just a single pass even if those cavities would be like hundreds stacked in a row right?
If i'm not mistaken the largest part of the whole LHC electric bill goes to the bending magnets and the cryogenics to keep them superconducting?
 
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How precise does the magnet field increase need to be around the whole circle because a mismatched field strength between say one part of the torus with another might result in the proton bunch crashing into the wall?
The average curvature radius is ~3 km. The beam pipe is about a centimeter wide. That's means the magnetic field needs to match the energy better than one part in 105.
but the problem is that the cavities cannot transfer enough force on the small protons with just a single pass even if those cavities would be like hundreds stacked in a row right?
Cavities for the ILC are reaching ~35 MeV/m. Divide 7 TeV by 35 MeV/m and you get a linear accelerator 200 km long. And then you need two of them. It also means protons are single-use now: At 1011 protons every ~30 ns the beam power would be 3.7 TW per beam. You can recover some of that energy, but its still completely unrealistic in every aspect.
If i'm not mistaken the largest part of the whole LHC electric bill goes to the bending magnets and the cryogenics to keep them superconducting?
Yes. It needs less power than LEP despite having much stronger magnets because superconducting magnets are so energy-efficient.
 

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