Hi,
It seems that a very important parameter of the LHC is the "Luminosity" hence the "Integrated luminosity over one fill". I can see why: somehow, this must be linked to the number of good collisions that the detectors will record in one shift! Now here is my question:
It seems that the measured luminosity goes down quickly; very quickly!
In fact, during the first 8 hours of the fill #1372, the luminosity decreased by a factor of two, while the beam intensity, only went down by 5 percent. Why ?
Thanks for your answer.
Loss of Luminosity
Re: Loss of Luminosity
That's because the beam intensity is only one of parameters which affects luminosity. Another very important one is called emittance and it specifies how well the beam is focused in the center of the beam path (and in proton bunches). Over time the beam becomes "blurred" and it decreases chances for collisions even if number of protons stays the same.
Re: Loss of Luminosity
>>>> "...Over time the beam becomes "blurred"..."
your explanation makes a lot of sense, but it is not complete. I would like to understand better. Tell me if I am wrong:
TRANSVERSE: During the "stable beam" phase, the deflection dipole magnets, and the focusing quadrupole magnets are held at a constant current. also there are some electronic active steering systems. Why would this result in a slowly increasing transverse section of the beam? If you have a focusing system that works, well then your system stays focussed!
LONGITUDINAL: The beam is driven by a few pairs of microwave cavities. At every turn, the protons are pushed or pulled by the electric field. This action is "synchronous", resulting in bunches. I do not see why those bunches would spread over the time!
Of course the laws of physics say that there is an "uncertainty" limit for the total volume (section + length) of a bunch, but if both systems (RF and tuning) work (and they work for 15 hours!), then I do not see why the beam parameters would change with time.
The only effect that is easily understandable is when the protons disappear from the beam (collisions), but as I mentioned in the first message, this drop is really small. So I repeat my question again: What is the main factor responsable for the loss of luminosity?
Thanks for reading, and happy LHC.
your explanation makes a lot of sense, but it is not complete. I would like to understand better. Tell me if I am wrong:
TRANSVERSE: During the "stable beam" phase, the deflection dipole magnets, and the focusing quadrupole magnets are held at a constant current. also there are some electronic active steering systems. Why would this result in a slowly increasing transverse section of the beam? If you have a focusing system that works, well then your system stays focussed!
LONGITUDINAL: The beam is driven by a few pairs of microwave cavities. At every turn, the protons are pushed or pulled by the electric field. This action is "synchronous", resulting in bunches. I do not see why those bunches would spread over the time!
Of course the laws of physics say that there is an "uncertainty" limit for the total volume (section + length) of a bunch, but if both systems (RF and tuning) work (and they work for 15 hours!), then I do not see why the beam parameters would change with time.
The only effect that is easily understandable is when the protons disappear from the beam (collisions), but as I mentioned in the first message, this drop is really small. So I repeat my question again: What is the main factor responsable for the loss of luminosity?
Thanks for reading, and happy LHC.
Re: Loss of Luminosity
Focusing magnets and RF only keep the mean value of the beam in place. When the beam is injected into the ring, its protons fly more or less parallel to each other, but over time they become displaced and focusing magnets and cavities don't manage to put them back into the place, they just make them to run on closed orbits around the beam mean. The longer these orbits are, the lower the luminosity.