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Featured I LHC starts 2017 data-taking

  1. Jul 12, 2017 #81

    dlgoff

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    @mfb and @Vanadium 50

    Thanks for your replies. I don't mean to hijack this thread, but these things are what I live for.
     
  2. Jul 12, 2017 #82

    dlgoff

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    Speaking of composition (materials), don't these very low pressures evaporate some components? or degrade them?
     
  3. Jul 12, 2017 #83

    mfb

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    Steel and copper (outside the experiments) and beryllium (at the experiments) don't evaporate notably, especially at cryogenic temperatures (some parts of the beam pipe are at room temperature, however). The LHCb VELO detector uses an AlMg3 foil, no idea about that but it has a small surface anyway. I don't see how vacuum would degrade these materials.
     
  4. Jul 16, 2017 #84

    mfb

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    Recovery from technical stop is still ongoing. The RF system (radio frequency cavities to accelerate the beam) cannot handle the 2556 bunches we had before, the problem is under investigation. With 2317 bunches it works, for now the LHC is running with this lower number of bunches. Still enough to collect a lot of collisions. ATLAS and CMS reached 7/fb, LHCb collected 0.24/fb.


    I made a thread about results from EPS.

    Edit on Wednesday: Finally back at 2556 bunches.
     
    Last edited: Jul 19, 2017
  5. Jul 31, 2017 #85

    mfb

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    Both ATLAS and CMS reached 10/fb, about 1/4 of the 2016 dataset. 16 more weeks for data-taking are planned. At pessimistic 2/(fb*week), we get the same number of collisions as last year, at optimistic 3.5/(fb*week) we get 65% more.
    If everything in the LHC would work perfectly 100% of the time, more than 5/(fb*week) would be possible, but that is unrealistic with such a complex machine.


    We had a short break for machine development and van-der-Meer scans:
    Cross section measurements are an important part of the physics program, and they require an accurate luminosity estimate. What the machine can deliver from normal operation has an uncertainty of a few percent. That is good for the machine operators, but for physics you want to get the uncertainty to be smaller - 2% is nice, 1% is better. The luminosity depends on a couple of machine parameters:$$\mathcal{L} = \frac{N_1 N_2 f N_b S}{4 \pi \sigma_x \sigma_y}$$
    ##f## is the revolution frequency - fixed and known to many decimal places.
    ##N_b## is the number of bunches per beam - known exactly.
    ##N_1## and ##N_2## are the numbers of protons in the bunches, they can be measured via the electromagnetic fields they induce when moving around the ring.
    ##S \leq 1## is a factor that takes the crossing angle into account, it can be calculated precisely. See also post 58.
    ##\sigma_x## and ##\sigma_y## are the widths of the bunches in x/y direction. There is no good direct way to measure that accurately.

    To estimate the width of the bunches, the machine operators shift the relative positions of the beams around at the collision points while the experiments monitor the collision rate as function of the shift. A fit to the observed rates leads to the widths. This procedure was named after Simon van der Meer.
     
  6. Aug 11, 2017 #86

    mfb

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    A few updates: The LHC experiments got collisions at a high rate, and the machine operators found some methods to improve the rate further.

    ATLAS and CMS reached 15.5/fb. 11 days since they had 10/fb, this means 0.5/(fb*day) or 3.5/fb per week.
    Wednesday 6:46 to Thursday 6:46 this week we had a record of 0.83/fb in 24 hours. As comparison: In these 24 hours, the LHC experiments had 4 times the number of Higgs boson and 8 times the number of top quarks the Tevatron experiments had - in their 20 years of operational history.

    LHCb surpassed 0.5/fb, nearly 1/3 of the 2016 dataset.

    The stepwise reduction of the crossing angle, discussed earlier was studied in more detail. Previously it was reduced in steps of 10 millirad (150 -> 140 -> 130 -> ...). That increases the collected data by about 3.5%. The process now works so smoothly that it became possible to reduce it in steps of 1 millirad, always following the optimal angle. This increases the number of collisions by additional 1.5%. That doesn't sound much, but all these small improvements add up.

    The number of protons per bunch went up a bit. We reached a record of 3.1*1014 protons per beam at high energy, or 320 MJ per beam. Correspondingly, the initial luminosity reached a new record, 174% the design value.
    The machine operators tried to get even more, but that lead to problems, so they added a day of scrubbing.

    Another thing discussed is the focusing of the beams at the collision points. Based on the analysis of the machine development block, it can be improved a bit more. That could increase the luminosity by ~20%. 1.74*1.2=2.09. There is still hope for the absolute luminosity record!
     
  7. Sep 1, 2017 #87

    mfb

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    ATLAS and CMS reached 20/fb. We have gained 4.5/fb since the previous post 21 days ago, or 1.5/fb per week, even below the pessimistic estimate from above. You can see this clearly in the luminosity plots as well.

    A problem appeared in a region called 16L2, which lead to the dump of many fills, often before collisions started. Although the cause is not well understood, the process is always the same: Some beam particles are lost in this region, and a few milliseconds (tens of revolutions) later many more particles are lost roughly at the opposite side of the ring - more than acceptable, this triggers a beam dump. This can happen from either beam 1 or beam 2, although they fly in separate beam pipes in 16L2.
    The problem appeared early in the year already, but until August, the dump rate could be managed by adjusting the control magnets in this region a bit. With increasing beam currents, it got more problematic and the machine operators wanted to get rid of the problem. The losses look gas-induced. The gas can stick to parts called "beam screen", and get released during the run, the collision of the beam with gas particles leads to the observed losses. The usual approach is to heat this beam screen, then all the gas evaporates, and gets pumped out or sticks to even colder parts of the beam pipe where it stays.
    That was done on August 10 - and then everything got worse. Now more than half of the fills were dumped due to 16L2, even at lower numbers of bunches. The smaller fraction of time in stable beams plus the reduced number of bunches lead to the slower accumulation of collision data in the last three weeks. The leading hypothesis is gas in other components of 16L2 that redistributed when heating the beam screen and other components, leading to even more gas there.

    What to do?
    • The problem could be solved by heating up the whole sector and pumping it out properly. That would probably take 2-3 months, doing it now would mean most of the time planned for data-taking this year is gone. Unless data-taking becomes completely impossible this won't be done before the winter shutdown.
    • The machine operators see if there is a stable running condition that works for now. The last few runs with 1550 bunches were promising, at this rate the LHC would be limited to ~2/fb per week, but that is still a reasonable rate that would double the 2016 dataset by the end of the year.
    • Gaps between bunches can reduce losses, e. g. "8 bunches in a row, then 4 slots free, then 8 bunches in a row, then 4 slots free, ...". This might be tested. It would also mean the number of bunches has to be reduced compared to the initial plan, but if it reduces the number of dumps sufficiently it can be worth it.
    • There are some special runs planned/proposed for 2018, some at lower energies and some with a very low collision rate, for something like 1 week in total. They might be shifted to 2017 as they won't be affected by the 16L2 issue as much as the regular operation at high energy and collision rate.
    • The machine operators discuss what else can be done.

    LHC report: Something in the nothing
     
  8. Sep 13, 2017 at 8:46 AM #88

    mfb

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    ATLAS and CMS reached about 24/fb.
    The mitigation approaches, especially the "8 bunches, then 4 slots free, repeat" pattern worked, in the last days ~2/3 of the time could be spent with data-taking. The luminosity is lower, but still at the design value. There are still some dumps due to 16L2 but they don't break everything any more.

    A scheduled machine development block started, followed by a few days of technical stop. About 9 weeks for data-taking left in 2017. Unless there are some new ideas how to solve the 16L2 issue, I guess they will just keep the current configuration, it should lead to about 2-2.5/fb per week, so we will still get more than the 40/fb of last year.

    LHC Report: operation with holes
     
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