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Size of particle accelerators 
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#1
Feb614, 10:04 AM

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I have a very lamans understanding of particle accelerators, one of the things I gather is that their large size allows for the particles to be accelerated to higher speeds. This can't be done easily at smaller sizes because a smaller circumference will mean a more extreme angle for the particles to bend round (apologise for probably appalling phrasing on this topic). Is this the case? If so is making progressively larger accelerators the only option or could better technologies achieve the same result?
This question is prompted by hearing a statement that at some point in the future particle physics of this sort would come up against a hard limit; that limit being the circumference of the Earth itself (assuming one build an accelerator along the equator). The statement wasn't said as though this would actually happen but more illustrating the point. I've done a bit of googling but it's hard to find a resource that isn't too popsci that is accessable. 


#2
Feb614, 10:48 AM

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You have it approximately correct. The loss of energy in the beam due to bending can be quite substantial the higher the energy (speed) of the beam. Synchrotron losses are always a major problem for such particle accelerators.
Note that circular accelerators aren't the only game in town. The history of high energy physics experiments has always gone circularlinearcircularlinearetc. The last major linear accelerator was the one at SLAC that has been shut down. So since LHC was the last major accelerator facility constructed, the push right now is for that to be followed by another linear accelerator, thus the push for the ILC. Linear accelerators do not have the energyloss issues from bending, but of course, it has issues with a finite length and how high of an energy it can achieve for such a finite length. Zz. 


#3
Feb614, 06:40 PM

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Ah cool, glad to see I wasn't off base. Did a bit of reading about synchrotron radiation, is there any technology being worked on that could minimise this effect so that a larger circular accelerator could be made more efficient? Regarding linear accelerators I guess the higher efficiency is bought by lower speeds due to the finite length?



#4
Feb614, 10:15 PM

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Size of particle accelerators
I thought that there is no way to bypass the synchrotron radiation (at least for the circular particle accelerators)?



#5
Feb714, 07:27 AM

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Not sure what you are asking about linear accelerators. As with the circular accelerator, you get to higher energies by making it longer using current technologies. There are new, exotic schemes of doing particle acceleration (laser/electron/dielectric wakefields, for example), but those won't be practical for several more years since they are still in the R&D stage right now. Zz. 


#6
Feb714, 06:54 PM

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You have to distinguish between electron and proton colliders here (antiparticles are always included).
Synchrotron radiation is limiting the energy in synchrotrons for electrons  for a fixed radius, energy loss goes with the 4th power of the energy, at some point you lose as much in the curves as you can add in the straight sections. Due to this 4th power, going towards even higher energies needs linear accelerators. The chinese think about a circular collider a bit above the LEP energy (optimized for higgs production), but well below the ILC values. For protons, synchrotron radiation is something you have to take into account, but it does not limit the energy. For protons, the field strength of the dipole magnets is the limit  for a fixed radius, more energy needs stronger magnetic fields. You can upgrade your magnets a bit, but not by orders of magnitude, so you need larger accelerators. Still way better than linear accelerators. Linear accelerators have an energy that is roughly proportional to their length  a longer accelerator just allows to put more cavities inside to accelerate particles. As all particles are just used once, they need significantly more power, have just one spot for an experiment (the ILC is planned with movable detectors to have two experiments) and have some other issues (damping, for example  the ILC will have two small synchrotrons to do that). Plasma wakefield acceleration could be gamechanging. A factor of 1000 in terms of gained energy per length... SLAC, DESY and CERN are all building/doing experiments for this, we'll have to see what they can find out. Another interesting idea is the CLIC concept  use one strong, lowenergetic beam to generate electromagnetic oscillations, guide them to another weak, highenergetic beam to accelerate this. Even more futuristic (in my opinion) is the idea of a muon synchrotron. Muons are heavier than electrons, so they can be accelerated to significantly higher energies (roughly like proton accelerators). The disadvantage? Muons just live for microseconds. No way to use anything similar to the LHC magnets with their 20 minutes to ramp them up. 


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