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Featured I LHC starts 2017 data-taking

  1. May 25, 2017 #21

    mheslep

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    Thank you. Fascinating machine.
     
  2. May 25, 2017 #22

    mheslep

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    Thanks, yes, the bellows is visible. The LHC then has many parts moving with respect to each other in expansion: inner tube with crogenics, outer tube, green supports. Tube travel (in the mm range per 12 m section but it's there with delta T) still has to occur with respect to either the tube wrt supports, or the supports wrt to the floor. The vacuum report you supplied references the "supports" for the tube, both fixed and "mobile", without elaboration as to what mobile means. In the post 14 photo I can't pick up any indication of a travel mechanism (e.g bearings) between tube and green support. Does the base of a "mobile" support travel (seems unlikely)? Or does a support simply flex, wrt the nearest fixed support?
     
  3. May 25, 2017 #23

    mfb

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    Figure 12.10 looks like the beam pipe could move on some supports (along the beam pipe direction).
     
  4. May 25, 2017 #24

    Vanadium 50

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    The idea is to minimize the motion of the shells, which are roughly at room temperature, and let the cold internals adjust via an expansion bellows. There are constraints which make this an idealization rather than a strict rule, but that's the idea.
     
  5. May 27, 2017 #25

    mfb

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    We had two long runs with stable beams in the last 24 hours, 75 bunches, up to 3% the design luminosity.
    More than 0.01/fb worth of data collected for ATLAS and CMS, about a trillion collisions per experiment.

    The main focus is still on commissioning, but in parallel they increase the number of bunches.


    Edit Monday morning: We got another run over the night, 336 bunches, 15% the design luminosity. More than 0.05/fb collected.

    Up to 1236 bunches per beam have been tested, but only at low energy.

    The plan now is to do scrubbing. The beam pipe condition is that bad.
     
    Last edited: May 29, 2017
  6. May 29, 2017 #26

    mfb

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    The main commissioning part is done.
    2100 bunches in beam 2, not too far away from the design value of 2800. Scrubbing will be done with nearly the full beams. Can take a few days, but it is not always easy to predict how fast it works. Afterwards the plan is to increase the intensity with stable beams, which means we'll start collecting many collisions.
     
  7. May 29, 2017 #27
    "An optical transition radiation (OTR) beam monitor located in front of the dump [33] will detect off-normal dilutions."

    Hmmm. There exist _photos_ and maybe even _videos_ of these dumps?
     
  8. May 29, 2017 #28

    mfb

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    The devices produce fgures like the one in post 8. I don't think that is very photo-like, although it shows the distribution as function of the 2D position.
     
  9. May 30, 2017 #29
    What about taking actual pics or videos of the dump, as beam leaves the vacuum tube and travels through "air" (nitrogen, I guess) into the TDE? Will it be visible in air? How much Cherenkov radiation? Or you think it will look "dangerous" and thus be a bad PR?
     
  10. May 30, 2017 #30

    mfb

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    The beam dump elements are close together, I don't think their is an air gap to take pictures. The beam is dumped within 0.1 milliseconds, I doubt you would see actual beam effects. The glowing hot beam dump element surfaces: maybe (if there would be an air gap).


    The short scrubbing runs yesterday helped a lot already.
    Stable beams with 315 bunches right now, initial luminosity was close to 20% the design value, about 0.1/fb collected in total. It is planned to go to 600 bunches on Thursday. Going beyond that might require more scrubbing.
     
    Last edited: May 30, 2017
  11. Jun 3, 2017 #31
    How did they determine the specific number of stages of rings to build (three I think) and the specific diameter plus length of linac eg why not more smaller rings or fewer big ones. I know its optimal but is there a simple way to explain the physics or just simulation came up with this configuration?

    Also what angle do the counter rotating beams collide at, doesn't seem to be head on the way the geometry looks at the beam cross over points.
     
  12. Jun 3, 2017 #32

    Vanadium 50

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    Most of the accelerators used as boosters today were front-line research machines in the past, now repurposed. IKt's not optimal. But it's a lot cheaper than ripping out the old accelerator and putting in a new one that is 10% bigger or smaller.
     
  13. Jun 3, 2017 #33
    To put this in perspective, at what time after the big bang would these sort of energies be seen, theoretically?
     
  14. Jun 3, 2017 #34

    mfb

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    It depends on the running conditions, typically 300 µrad, or 0.017 degrees. The angle is necessary to avoid collisions with the previous / following bunch (relative to the bunch they should collide with) - see the first image here, marked "long range". 300µrad for half the bunch spacing leads to a separation of 1.1 mm at 3.75 m distance to the collision point.
    Somewhere in the first pico- to nanoseconds, depending on the process studied.


    After some problems with power supplies and other hardware, we had another run with stable beams this morning, 300 bunches, 17% the design luminosity.
    We might get collisions with 600 bunches during the night.
     
  15. Jun 3, 2017 #35

    Vanadium 50

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    The crossing angles are around 300 microradians. One important aspect of designing an accelerator complex is that you don't want huge increases in energy at a single stage. keeping it a factor of 20 or less is good practice.
     
  16. Jun 3, 2017 #36
    Thanks explanations and links, most interesting.

    If money wasn't a factor what would be the most optimal config to get the beam up to energy, how is this determined. I guess I could ask the same about rocket stages - is it the same physics principles based in thermodynamics?

    How does the 20% figure come about?

    LHC fanboy here.
     
  17. Jun 3, 2017 #37

    mfb

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    20%? Do you mean the factor 20? The magnets have to adjust their magnetic field according to the beam energy very accurately (10-5 precision) to keep the particles on track, at very low fields (relative to the maximum) that can be challenging. You also have to take into account if your particle speed still changes notably during the acceleration process.

    If money wasn't a factor you could build a 15 km long linear accelerator directly leading to the LHC. Then you can fill it in two steps (one per ring), in seconds instead of 20 minutes, and with more bunches. Or, if we get rid of any realism, make the linear accelerator ~300 km long and directly put the protons in at their maximal energy. Then you also save the 20 minutes of ramping up and 20 minutes of ramping down.
    The beam dump would need some serious upgrades to handle a higher turnaround.
     
  18. Jun 3, 2017 #38
    Construction, design, beam steering, beam intensity and collision geometry...etc would be optimal with a LINAC in a world of no constraints?

    Rings are the compromise solution to real world constraints?

    Is there any possibility of building a research facility that would then become a alternative structure post research, eg build a big LINAC straight thru the Alps north and south which could then become a commercial transport tunnel when the research is competed.
     
  19. Jun 3, 2017 #39

    fresh_42

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    They have degrees up to 50°C or something in the stones of the new Gotthard base tunnel. I just try to imagine how you would cool the entire tunnel to 0.3K or so, on 57 km! And this is just one mountain. My guess is it would be easier to construct a linear accelerator in Death Valley than under the Alps.
     
  20. Jun 3, 2017 #40

    Vanadium 50

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    A circular machine has two advantages over a linac. The first is cost - it lets you use the small part that actually accelerates again and again on the same proton. Superconducting magnets are expensive, but accelerating structures are even more expensive. The second is beam quality - by requiring each proton to return to the same spot (within microns) every orbit you get a very high quality beam. This is done by setting up a complex negative feedback scheme: if a particle drifts to the left, it feels a force to the right, and vice versa. Linacs don't do this - a beam particle that drifts to the left keeps going to the left, and if your accelerator is long enough to be useful, it's likely that this drifting particle hits a wall.

    Proposals for future linacs include something called "damping rings" so that before the final acceleration, you can get the beam going in a very, very straight line.

    The factor of ~20 comes about for several reasons. One is, as mfb said, problems with persistent fields. If your magnets are good to 10 ppm at flattop, and the ring has an injection energy 10% of flattop, at injection it's only good to 100 ppm. Make that 5% and now it's 200 ppm. The lower the energy, the harder it is to inject. And even without this problem, it would still be harder to inject because the beam is physically larger (we say it has more emittance). Finally, there is some accelerator physics that makes you want to keep this ratio small. There is something called "transition", where you essentially go from pulling on the beam to pushing on it. At the exact moment of transition, you can't steer the beam, so you lose it after a fraction of a second. The bigger the energy range, the more likely you have to go through transition. The LHC is above transition, but if you injected at a low enough energy, you'd have to go through transition. That number is probably of order 50-100 GeV.
     
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