Queries on the operation of the LHC with pilot beams before Run 3

[Wrichik Basu]

We all are aware of the fact that the LHC is now seeing pilot beams in preparation for Run 3 next year. Recently, I came to know about a Twitter account, @lhcstatus2, which automatically posts updates about the current operation of the LHC. I am interested in learning about some of the different things that it has tweeted till date. For each case, I am writing what I understand; please let me know if I am wrong/partially correct. Also, feel free to add some knowledge too.

1. Injection Physics Beam​

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What I understand: Physics beam is the one that will result in stable beams. The beams will be ramped in energy, and then will be brought into collisions at the IPs for generating physics data. In the SPS, this beam is generated using Q20 optics.​

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2. Injection Probe Beam​

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What I understand: Not very sure about this. Beams to probe the injection system? No idea. Also, can you throw some light on the filling scheme, and what does it signify?​

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3. Injection Setup Beam​

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What I understand: Beams to check the setup of different beam instrumentation/diagnostics elements around the ring (like collimators, BPMs and BLMs)? And again, what about the filling scheme?​

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4. Beam mode: Ramp down​

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What I understand: Ramping the beam is increasing the energy of the beam. However, I am not sure how they ramp down the beam. Which elements around the ring help in decreasing the energy of the beam? And why is it necessary to decrease the beam energy?​

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5. Beam mode: Adjust​

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What I understand: Adjust what?​

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6. What is cogging?​

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MKI is definitely an injection kicker magnet. But what is cogging?​

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7. Beam mode: Cycling​

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https://twitter.com/lhcstatus2/status/1454399709234311175?s=20​

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What I understand: No idea.​

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8. Beam mode: Flat top​

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https://twitter.com/lhcstatus2/status/1454389847989362694?s=20​

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What I understand: After the beam is injected, and its energy is ramped up, probably the beam is in the flat top mode, meaning that it has reached its highest energy. Am I right?​

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9. Are splashes good for the health of collimators?​

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https://twitter.com/CMSExperiment/status/1450793930501464068?s=20​

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Splashes and collisions are not the same. As per this not-so-technical article from CERN for the layman, splashes are created when the collimator jaws before the ATLAS and CMS IPs are pushed into the beams, so that a spray of particles travel down the experiment, and are captured by the calorimeters. Definitely these beams are not the 3.5 TeV beams, otherwise the collimators would suffer severe damage. But even then, there will be some damage to the collimators because, as far as I know, they are generally designed for intercepting off-momentum particles, particles outside RF buckets, or for halo cleaning; they are not meant to take the complete beam (the beam dump is designed for that purpose). Before Run 3 next year, will the collimator jaws be replaced again?​

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Also, is such a spray of particles created even when the beam halo is intercepted during normal LHC operation? If yes, how are those secondary showers prevented from traveling down the beam line?​

 

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Did you search for e.g. "LHC beam modes"? The first hit has a list with explanations.

Also, can you throw some light on the filling scheme, and what does it signify?

It's a somewhat cryptic description of where in the ring the bunches are. The fill pattern always needs one large gap which is used to dump the beam (a time where no bunches fly past the beam dump kicker magnets while they ramp up). It needs smaller gaps coming from SPS injections (both SPS and LHC kicker magnets need to ramp up/down in these gaps). It needs even smaller gaps from the PS to SPS transition for the same reason. You get up to 72 bunches with 25 ns spacing, then a 300 ns gap (PS to SPS), that repeats a few times until you get a ~1000 ns gap (SPS to LHC), repeat that larger pattern, and finally add a ~3000 ns gap (LHC to beam dump). Here is some more information about fill patterns
Haven't looked into the pattern names for a while, but if I remember correctly:
"single" means one bunch per PS group or even one bunch per LHC injection?
"4b" is 4 bunches in total (per ring)
2_2_2 are 2 colliding bunches in ATLAS/CMS, LHCb, ALICE (or ATLAS/CMS, ALICE, LHCb? Not sure).
Don't know what noLR is.

9. Are splashes good for the health of collimators?

They are done with a very low beam intensity.

Collimators do produce secondary particles during regular operation and filtering them out (with additional collimator elements) is a science on its own. Luckily they have a much lower energy so they can't follow the beam for long.