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B Particle accelerators and timing for particles

  1. Jun 11, 2017 #1
    So, I found this document that talks about rf cavities at lhc https://home.cern/about/engineering/radiofrequency-cavities
    Can you please explain me what i underlined Means?
    If i understood when the particles achieve the velocity the scientist want them to achieve they get into the cavity when the E field is 0 (perfect timing to get the wave at 0) so they don't get accelerated is this Right?

    So is it right saying that you can select the speed you want to achieve for the particle by selecting the frequency of the Radio Wave?

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  2. jcsd
  3. Jun 11, 2017 #2


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    Yes, although that's not the complete story. A proton at "full energy" will, indeed, experience no acceleration temporarily while in an RF cavity, but the LHC is a circle and has many bending magnets to bend the beam around the LHC tunnel. When the path of any moving charged particle is bent (in this case it undergoes transverse acceleration due to the magnetic field from the bending magnet), it radiates. This is called synchrotron radiation, and it removes energy from the particle. As a result, protons slow down after each bend and must be accelerated back up to full speed.
  4. Jun 12, 2017 #3


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    You cannot change the frequency much, it is given by the geometry of the resonator, you have to keep the cavities synchronized with the up to 2820 bunches that follow each other with 25 ns separation and with the overall revolution frequency.

    For all practical purposes, protons in the LHC move at the speed of light - the speed difference is tiny. Protons at higher energies are a little bit faster - but they are also harder to bend around in the magnets, which means they will travel closer to the outer parts of the ring: They have a longer way to go per revolution, and this effect dominates: Protons at higher energies move backwards towards the end of a bunch. To stop that, you want to slightly accelerate the protons at the front and slightly slow down protons at the end. You need the cavities even if you don't want to change the overall energy of the particles (you also have to compensate synchrotron losses as marcusl mentioned, but they are just ~3 keV/revolution at full energy, while the cavities can deliver many MeV per revolution). This negative feedback stabilizes the particle bunches.

    To increase the particle energy, you have to ramp up the magnets in the ring to keep the particles on track. That will influence the path and therefore the timing of the particles. If you do it right, they are accelerated just by the right amount to follow the increasing magnetic field. Otherwise they get lost in the ring, and damage the accelerator. Luckily you still have the negative feedback mentioned before.
  5. Jun 12, 2017 #4
    How do you know how much to ramp up the magnets? Is there a way to detect the beam position inside the ring so you have closed-loop control?
  6. Jun 12, 2017 #5


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    Sure, the beam position is monitored all the time. There are multiple approaches, the easiest one is to have a wire close to the beam - the passing bunches induce a current there. You can also let the particles fly through some sort of cavity and measure the induced electromagnetic fields there.

    In terms of destructive measurements: If the beam current is very low, it can be sent to absorbers and the distribution can be measured there by changing the absorber positions. The beam doesn't circulate in that case, it is injected and moves until it hits the absorbers.
  7. Jun 12, 2017 #6
    Very interesting, thanks.
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