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Featured I LHC starts 2017 data-taking

  1. 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.
     
  2. 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?
     
  3. May 29, 2017 #28

    mfb

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    The devices produce figures 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.
     
  4. 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?
     
  5. May 30, 2017 #30

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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
  6. 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.
     
  7. 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.
     
  8. Jun 3, 2017 #33
    To put this in perspective, at what time after the big bang would these sort of energies be seen, theoretically?
     
  9. 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.
     
  10. Jun 3, 2017 #35

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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.
     
  11. 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.
     
  12. 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.
     
  13. 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.
     
  14. Jun 3, 2017 #39

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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.
     
  15. Jun 3, 2017 #40

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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.
     
  16. 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.
     
  17. Jun 4, 2017 #42

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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.
     
  18. 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.
     
  19. 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 + ....?
     
  20. 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.
     
  21. 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.
     
  22. 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.
     
  23. Jun 4, 2017 #48
    Binding energy to break the nucleus apart in collision.
     
  24. 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.
     
  25. 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).
     
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