Why did we need a 8.3 T magnetic field in the LHC beam pipe?

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Discussion Overview

The discussion revolves around the required magnetic field strength of 8.3 T in the LHC beam pipe, exploring the calculations and considerations that lead to this specific value. Participants examine the implications of the LHC's design and its operational parameters, including the geometry of the tunnel and the arrangement of magnets.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant calculates the magnetic field needed based on the LHC's circumference and beam energy, arriving at 5.4 T and questioning their methodology.
  • Another participant notes that the LHC tunnel is not a perfect circle and mentions the need for various types of magnets, which may affect the magnetic field distribution.
  • A participant expresses curiosity about the origin of the specific value of 8.3 T, suggesting that it seems arbitrary and seeks clarification on the calculations leading to it.
  • Another reply suggests counting the number of dipole magnets and considering their effective length to understand how the 8.3 T figure is derived, indicating that the field strength may not be uniform along the entire length of the magnets.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the calculations leading to the 8.3 T magnetic field, with multiple viewpoints and methods presented. The discussion remains unresolved regarding the exact reasoning behind the specific magnetic field strength.

Contextual Notes

Participants highlight the complexity of the LHC's design, including the presence of different types of magnets and the geometry of the tunnel, which may influence the calculations and assumptions made regarding the magnetic field strength.

curious.cat
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I tried to work backwards and verify the magnetic field required by the LHC beam pipes. A circumference of 27km implies a radius R=4300 m. The beam energy E is 7 TeV per proton beam. Considering approximation E approximately equal to pc to get momentum (ultra-relativistic case). Plugging into p=qBr, I am getting a magnetic field of 5.4T as opposed to 8.3T. Clearly, I am doing something wrong. Could someone please point out where I am faltering? Thank you very much.
 
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The LHC tunnel is not a perfect circle. It was built for LEP, where long straight sections are needed for electron/positron acceleration (to counter synchrotron radiation losses). And even in the curves, the dipole field is not everywhere as you need quadrupoles and a few more elements in the ring.
 
Thank you so much for your reply. :) Actually, I was wondering more along the lines of why 8.3, it seems like a weird number. Is there some calculation after which we arrive at this figure?
 
Count the number of dipole magnets in the ring (1232), look up their effective length (the field is not present at full strength along the whole length), take the product as effective circumference, and you should get some number close to 8.3 T. Even with the full length of 15 meters per magnet you get a reasonable agreement.
 
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Thank you so much!
 

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