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Featured I LHC starts 2017 data-taking

  1. Jun 3, 2017 #41

    mfb

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    I guess you mean quadrupole (+potential higher order) magnets? Long linear accelerators do this as well.
    They just keep the beam together, they don't reduce the emittance (like damping rings do for electrons), but the LHC doesn't reduce that either.
    500 km through the Alps to replace about 35 km of LHC plus preaccelerators, built at convenient spots near Lake Geneva? Even if the tunnels would be wide enough to be used for transport afterwards (they are not), and even if there would be demand for a 500 km tunnel, that project would be way too expensive for particle physics or transportation. And that is just the tunnel - you need 500 km of accelerating structures. There is absolutely no way to fund that.
     
  2. Jun 4, 2017 #42

    Vanadium 50

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    That, plus things like stochastic cooling. Yes, you can add correctors to linear accelerators, but the ratio of corrector lengths/accleration lengths is much higher in a circular accelerator. Perhaps the two most directly comparable accelerators are LEP and SLC at the Z pole. Despite the fact that the electrons underwent significant synchrotron radiation, LEP still ended up with a smaller beam energy spread than SLC.

    So I think my statement that the requirement that the beam makes it around the ring at the same point that it started gives you better beam quality is an advantage that a circular design has over a linear design is borne out.
     
  3. Jun 4, 2017 #43

    mfb

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    For electrons, synchrotron radiation is a great cooling method. For protons it is not - protons in the LHC have a cooling time of days but they don't stay in the machine that long. The FCC would be the first proton machine where synchrotron cooling gets relevant.



    They tried to get collisions with 600 bunches over the night, but didn't achieve it due to powering issues. The plan is to get 600 bunches next night.
     
  4. Jun 4, 2017 #44
    What does a simple conservation of energy equation look like at LHC at point of collision.

    Proton energy = ionisation energy + rest mass + brehmstalung losses + relativistic energy + coulomb energy + nuclear binding energy + ....?
     
  5. Jun 4, 2017 #45

    mfb

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    Apart from the rest mass, all these things don't apply to protons colliding in a vacuum. The rest mass contributes 0.94 GeV to the total proton energy of 6500 GeV. You can call the remaining 6499.06 GeV "kinetic energy" if you like.


    The machine operators are preparing the machine for collisions with 600 bunches now.
     
  6. Jun 4, 2017 #46
    Huh?

    To have the proton smash surely you need to overcome both coulomb & binding energy at least?
    They are not zero.
     
  7. Jun 4, 2017 #47

    mfb

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    Binding energy of what? There is nothing bound.

    The coulomb potential between the protons is of the order of 0.001 GeV, completely negligible. Nuclear binding energies, if anything would be bound, would be of the same order of magnitude.
     
  8. Jun 4, 2017 #48
    Binding energy to break the nucleus apart in collision.
     
  9. Jun 4, 2017 #49

    mfb

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    There is just one nucleon in the nucleus, there is nothing to break apart.
    The protons are not broken into pieces in any meaningful way. Completely new particles are created in the collision.



    Stable beams with 600 bunches, 30% of the design luminosity for ATLAS/CMS, 125% of the (lower) design luminosity for LHCb.
     
  10. Jun 4, 2017 #50
    This would apply only to ion beams (Pb).
    But anyway, please realize that at some 3-7 TeV energies per nucleon, any binding energy of nucleus is utterly insignificant. Even the entire rest energy of the nucleus is much lower than the "kinetic" energy of that magnitude (it's about 0.03% of it).
     
  11. Jun 4, 2017 #51

    strangerep

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    Just want to say: I love your running commentary. :oldbiggrin:
     
  12. Jun 5, 2017 #52

    mfb

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    Thanks.
    I add it when something happened since the last post - or make a new post if it is a major milestone.


    The lumninosity plots are now online. LHCb data seems to be missing.

    ATLAS had some issues with its magnet in the muon system, it was switched off last night. As long as the luminosity is low, that is not a large loss, and analyses that don't need muons can probably use the data. The magnet is running again now.

    We'll probably get some more collisions with 600 bunches next night..
    Edit: There they are. 30% design luminosity again.

    Edit2: Now we get more scrubbing. Afterwards hopefully 900 and 1200 bunches.
     
    Last edited: Jun 6, 2017
  13. Jun 7, 2017 #53
    Was just wondering since I noticed the beam went up:

    07_June_2017-pp_luminosity_integrated_date_2017.png
     
  14. Jun 7, 2017 #54

    mfb

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    That should be the run from the night to Monday.
     
  15. Jun 7, 2017 #55
    What units is luminosity measured in?

    Graph shows fb, physical splanation please.

    Fill number?
     
  16. Jun 7, 2017 #56

    mfb

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    Inverse femtobarn (fb-1). I wrote an Insights article about it.
    1/fb corresponds to roughly 1014 collisions.

    Fill number is just counting how often protons have been put in the machine. After beams are dumped, the number is increased by 1 for the next protons to go in. "Fill 5750" is more convenient than "the protons we had in the machine at June 5 from 8:23 to 13:34".
     
  17. Jun 12, 2017 #57

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    After a few days of scrubbing, we are back to data-taking.

    Scrubbing went well. We had a record number of 3.37*1014 protons per beam, with 2820 bunches per beam (slightly exceeding the design value of 2808).
    The heating of the magnets went down by ~40% in the most problematic region, enough to continue operation with higher beam intensities.

    Currently with a short (1 hour) run with 10 bunches each, then they'll complete the 600 bunch step (~3 hours), and then go on with 900 and 1200 bunches. Each step gets 20 hours of stable beams to verify nothing goes wrong. These two steps combined should deliver about 0.5/fb worth of data. Progress in the first weeks is always a bit slow, but it starts to get an interesting dataset.

    Edit: We got 900 bunches. 68% design luminosity, about 50 inelastic ("destructive") proton-proton collisions per bunch crossing (design: ~25). Unfortunately the beam was dumped after just 20 minutes of data-taking for safety reasons. Now they are working on a cooling issue, that will take several hours.

    Edit2: More 900 bunches (980 actually), nearly 0.15/fb of data collected on Wednesday. We'll probably get 1200 late Thursday to Friday.
     
    Last edited: Jun 14, 2017
  18. Jun 15, 2017 #58

    mfb

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    We had some runs with 900-980 bunches in the last two days, about 65% the design luminosity. Each step gets 20 hours before the number of bunches is increased. 900 is done now, the next step is 1200 bunches, probably this evening.
    Edit in the evening: Stable beams with 1225 bunches, 75% the design luminosity. A bit lower than expected.

    ATLAS and CMS both reached 0.5/fb of data. Not much compared to last year's 40/fb, but we are still in the very early phase of data-taking.



    The machine operators found another way to increase the number of collisions a bit. The bunches have to hit each other with a minimal crossing angle to avoid additional collisions outside the design point. That means the bunches don't overlap completely (see this image). With the HL-LHC in 2025+ it is planned to "rotate" the bunches, but that needs additional hardware not available now.
    In long runs (many hours), the number of protons per bunch goes down over time - some are collided, some are lost elsewhere in the machine. That means the long-range interactions get less problematic, and the crossing angle can be reduced. This increases the number of collisions by a few percent. It does not change the maximal luminosity, but it reduces the drop of the luminosity over time.



    The LHC could get a very unusual record this year: The luminosity record for any type of collider.
    Electrons and positrons are much lighter than protons. That means they emit more synchrotron radiation when they travel around in a circular collider. Faster electrons radiate more and get slower in the process. That is a very effective "cooling" mechanism, as a result you can put the electrons very close together, increasing the luminosity. KEKB set the world record of 2.11*1034/(cm2*s) in 2009 - with electrons/positrons.
    The LHC could reach this value in 2017 - with protons, where it is much harder. As far as I know, it would be the first proton-collider ever to set the absolute luminosity record.
    KEKB is currently upgraded, and the new version (SuperKEKB) is supposed to reach 100*1034/(cm2*s), way above everything the LHC can achieve, but it will probably need until late 2018 to beat its old record, and several more years to reach its design value. There is a small time window where LHC could get the record for a while.
     
    Last edited: Jun 15, 2017
  19. Jun 17, 2017 #59

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    The LHC is progressing fast this week. The 20 hours at 1200 bunches were completed today, and the machine switched to 1550 bunches. Collisions in ATLAS and CMS reached ~100% the design luminosity this evening. If everything goes well, we get 1800 bunches on Monday, clearly exceeding the design luminosity.

    The luminosity record last year was 140% the design value, with a naive scaling we need 2150 bunches to reach this, and 2820 bunches will give 180% the design luminosity. Similar to last year, I expect that the luminosity goes up more, as they'll implement more and more improvements. The absolute luminosity record is certainly realistic.

    Both experiments collected 1.25/fb of data now, and the trend is going upwards rapidly.

    Edit: They shortened the 1500 bunch step and went directly to 1740 after just ~10 hours. Initial luminosity ~110% the design value.
     
    Last edited: Jun 18, 2017
  20. Jun 20, 2017 #60

    mfb

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    2029 bunches!
    125% of the design luminosity. Approaching the 2016 record.

    ATLAS and CMS now have 2.2/fb, twice the data they had three days ago. It is also half the total 2015 dataset.

    The machine operators expect that they can go to 2300 bunches without issues. Afterwards the heat load from the electrons in the beam pipe could get too high. Then we need more scrubbing or simply more data-taking at 2300 bunches - that acts as scrubbing as well.
     
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