RF frequncy of LHC: 400.789 MHz

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shilpi
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RF frequncy of LHC: 400.789 MHz

Post by shilpi » Wed Dec 04, 2013 1:59 pm

Hi,

I have read that the driver frequency of the LHC RF is 400.789 MHz
and not an integral number (400 MHz exactly)

I just wanted to understand why it is like this...

Thanks,
Shilpi

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DCWhitworth
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Re: RF frequncy of LHC: 400.789 MHz

Post by DCWhitworth » Wed Dec 04, 2013 10:43 pm

Well the RFC frequency determines the bucket size, which in turn determines the bunch length that can be effectively captured. You want as long a bucket as possible but there are limitations because a lower frequency means a larger RF cavity which leads to greater challenges with applying superconducting technology.
DC

The LHC - One ring to rule them all !

shilpi
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Re: RF frequncy of LHC: 400.789 MHz

Post by shilpi » Thu Dec 05, 2013 12:38 pm

DCWhitworth wrote:Well the RFC frequency determines the bucket size, which in turn determines the bunch length that can be effectively captured. You want as long a bucket as possible but there are limitations because a lower frequency means a larger RF cavity which leads to greater challenges with applying superconducting technology.
Hi DCWhiworth,

Thanks for the reply. What I want to ask is
the frequency is not exactly 400 MHz. But it is 400.789 MHz.
I had heard somewhere that there can be a problem if it is exactly 400 MHz,
so that is why it is made a non-integral number.

I wanted to understand as to what problems can arise if it is an integral number.

Thanks,
Shilpi

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DCWhitworth
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Re: RF frequncy of LHC: 400.789 MHz

Post by DCWhitworth » Thu Dec 05, 2013 6:06 pm

shilpi wrote:
DCWhitworth wrote:Well the RFC frequency determines the bucket size, which in turn determines the bunch length that can be effectively captured. You want as long a bucket as possible but there are limitations because a lower frequency means a larger RF cavity which leads to greater challenges with applying superconducting technology.
Hi DCWhiworth,

Thanks for the reply. What I want to ask is
the frequency is not exactly 400 MHz. But it is 400.789 MHz.
I had heard somewhere that there can be a problem if it is exactly 400 MHz,
so that is why it is made a non-integral number.

I wanted to understand as to what problems can arise if it is an integral number.

Thanks,
Shilpi
Well the frequency was selected for the reasons I gave above so presumably it was quite carefully calculated and that number was arrived at. Why isn't it exactly 400 ? Why should it be ?

The reasons I gave are from "The Large Hadron Collider: a Marvel of Technology" edited by some fellow called Lyndon Evans ;) The section on the RF system quote 400 MHz several times but never 400.789

But then again the frequency of the RF isn't static.
DC

The LHC - One ring to rule them all !

pcatom
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Re: RF frequncy of LHC: 400.789 MHz

Post by pcatom » Wed Dec 11, 2013 6:23 pm

The beam must always see the same RF each time it passes. What this means is that the RF frequency, the Circumference of the machine and the speed of the particles are related.

The ratio of the RF wavelength to the circumference is called the harmonic number - in LHC this is 35,640 exactly. Once this is fixed, the exact RF frequency will be determined by the exact circumference of the machine (which is only known once the machine is actually built) and the speed of the particles (which means that the RF frequency actually changes as the beam is accelerated).

Why 35640? Well there has to be a relationship between the harmonic numbers of the different machines in the accelerator chain in order to make bunch-to-bucket transfers from one machine to another. This cog-wheeling means that only certain combinations of harmonics are possible.

By the way, the frequency for Pb ions is substantially different from protons. This was quite a complication for p-Pb operation where the two beams have a different revolution frequency during injection and acceleration. During collisions the 2 beams must have the same revolution frequency (and hence the same RF frequency) in spite of having different speeds. This is achieved by moving the Pb slightly inwards and the protons slightly outwards such that the circumference of the machine for each beam was different.

Hope this helps ...

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DCWhitworth
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Re: RF frequncy of LHC: 400.789 MHz

Post by DCWhitworth » Wed Dec 11, 2013 10:15 pm

Thank you pcatom, great insight as always and glad to see you're still keeping an eye on us ;)
DC

The LHC - One ring to rule them all !

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