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Featured I LHC starts 2017 data-taking

  1. May 23, 2017 #1

    mfb

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    "Stable beams" has been declared 30 minutes ago.
    Similar to 2016, the initial collision rate is low (0.2% the design rate). The machine operators have to check that everything works and nothing presents a danger to the machine before more protons can be filled in. It will probably take a few weeks to reach the same collision rates as achieved last year.

    Meanwhile, the experiments start collecting some initial data. 0.2% sounds like nothing, but for some analyses this is ideal. The LHC experiments are not only limited by the number of collisions, they are also limited by the amount of data they can read out and process. This is about 1 kHz for ATLAS and CMS (about 13 kHz for LHCb, 200 Hz for ALICE). At the design values, this means 99.9975% of all collisions are discarded: Only the most collisions with the highest particle energies can be kept. The other collisions are still interesting, however. Currently the high-energetic collisions are rare, which means there is more space to record other processes.
     
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  3. May 23, 2017 #2

    Drakkith

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    Why does it take several weeks to reach the design collision rates? Just safety checks and such?
     
  4. May 23, 2017 #3

    Vanadium 50

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    Yes. The stored energy in the beam is enormous (or it is when they circulate thousands of bunches) so they creep along slowly.
     
  5. May 23, 2017 #4

    mfb

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    The full beam is powerful enough to heat several tons of graphite by a few hundred Kelvin. You want to be really sure it doesn't hit anything it is not supposed to hit.

    Safety is the main point, but not the only one. There are always stray electrons in the beam pipe, and they can heat the magnets. Starting at lower intensities reduces this issue and prepares the machine to go to higher intensities. See this and this post in the 2016 thread for details. We might see a few days of dedicated "scrubbing" runs, but last year it worked without them.
     
  6. May 23, 2017 #5
    (!) How quickly?
     
  7. May 23, 2017 #6

    Vanadium 50

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    Instantaneously.
     
  8. May 23, 2017 #7
    Great googly moogly! The graphite is the beam dump? And all that energy is contained in a tiny amount of hydrogen nuclei?
     
  9. May 23, 2017 #8

    mfb

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    The graphite is the beam dump, yes (well, both beam dumps - one per direction, every following number is per beam dump).
    A block 70 cm x 70 cm x 7 meters, with a mass of 7.5 tons. Water-cooled and surrounded by more than 750 tons of steel, iron and concrete.

    The bunches gets 600 meters of flight distance to spread out, and kicker magnets at the start make sure different bunches impact the block at different places. You see the time-structure of the beams here (axes=position at beam dump):

    swept-beam.jpg

    All that energy (320 MJ, the energy of 80 kg of TNT) in 0.5 ng of hydrogen ions (that much hydrogen wouldn't even fill the volume of a grain of sand at room temperature+pressure). If the beam is dumped, it hits the absorber within one revolution (90 microseconds).
     
  10. May 23, 2017 #9

    Vanadium 50

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    And one of the things they are doing is examining the pattern in mfb's plot very, very carefully to ensure that they understand exactly where the beam is going before they add more beam to the machine.
     
  11. May 23, 2017 #10
    Thank you. Up till now, I never had an intuitive feel for what 10 Tev actually meant in macroscopic terms. When you talk about high energy physics, you're not exaggerating!
     
  12. May 23, 2017 #11

    mfb

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    Well, 6.5 TeV is a tiny energy - in macroscopic terms, per proton it is huge. We get a large macroscopic energy if we consider that the LHC has up to 2800 bunches per beam with 110 billion protons per bunch.
     
  13. May 23, 2017 #12
    Of course you're right, what I meant was that you're throwing a totally insignificant amount of hydrogen ions (in macroscopic terms) at a graphite block, hard enough to raise its temperature hundreds of degrees. Extremely impressive, and a good real-world indication of how much energy it takes to "see" (make?) something like a top quark.
     
  14. May 25, 2017 #13
    I read about beam dump before. One surprising thing is that despite all this whacking with TeV-scale protons (more than enough to knock many neutrons off or outright disintegrate carbon nuclei), beam dump block does not become dangerously radioactive afterwards. (It _is_ radioactive, but not to the point where you can't stand near it).

    One question I did not find answer to, is the entire beam dump assembly in vacuum?
     
  15. May 25, 2017 #14

    mfb

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    It has hundreds of tons of shielding around it. Without that shielding, I would avoid standing next to it. Graphite doesn't get activated much, but still a bit.

    The vacuum pipe goes into the shielding. I guess it ends somewhere and the protons shoot through the endcap.

    dump.jpg
     
  16. May 25, 2017 #15

    mheslep

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    What endcap material is used/suitable, that has the structural strength to support the vacuum yet not significantly absorb beam energy?
     
  17. May 25, 2017 #16

    mheslep

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    I'm unable to quickly find any mention of how thermal expansion is handled for the 27 km vacuum tube. I imagine tight temperature control in the tunnel is used, though a loss of thermal control allowing ~3degK change leads to a meter of length change in steel.
     
  18. May 25, 2017 #17

    Vanadium 50

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    It looks like there might be a bellows in your picture - above the leftmost green post.
     
  19. May 25, 2017 #18

    mheslep

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    You mean MFB's LHC picture?
     
  20. May 25, 2017 #19

    Vanadium 50

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    Yes, in post #14,
     
  21. May 25, 2017 #20

    mfb

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    Thermal expansion is a major issue. Not so much for the outermost tube, where you can control the temperature, but for the beam pipe with the magnets. You have to install them at room temperature, and then cool them to 2 K. There are many bellows to handle the shrinking magnets.
    The beam dump has bellows every 12 meters (no magnets in that region), the image in post 4 shows one of them. Vacuum design report, page 13.
    Here is the design report. The graphite is kept in an inert gas environment. A vacuum was considered but not used: The graphite is designed for a temperature of up to 1250 °C, and an air leak shortly after a beam has been dumped could lead to a fire.
    Unfortunately they don't mention the window material.

    Here the high energies are an advantage. Most protons will pass through as minimally ionizing particles. A small fraction will interact with a nucleus, and produce several minimally ionizing particles - that is still fine as long as the seal is short compared to the hadronic interaction length The peak heating rate occurs deeper into the absorbers.

    There is also an interesting comment on activation:


    We had a nice stable run over night, 0.4% of the design luminosity with 12 bunches in the machine. We might get collisions with 50-100 bunches in the night to Saturday or Sunday.
     
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