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36C3 - The Large Hadron Collider Infrastructure Talk

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    36c3 preroll music
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    Herald: Our next talk's topic is the Large
    Hadron Collider infrastructure talk. You
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    probably know the Large Hadron Collider
    over at CERN. We heard quite a bit of it
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    in the recent talks. This time we will
    have a deep dive into the infrastructure.
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    You can assume our next speakers are doing
    a great job. Basically, it's pretty
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    obvious because we are not stucked into a
    great into a giant supermassive black
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    hole. So please welcome with a very warm
    applause, Severin and Stefan.
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    Applause
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    Severin: Yeah. Hello, everyone. Thanks for
    coming. So many people here, quite nice.
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    In the last couple of years we had several
    talks about, yeah, basically the physics
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    perspective of LHC, how physicists analyze
    data at LHC, how physicists store all the
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    data, et cetera. And we would like to give
    like more an engineering perspective of
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    the whole LHC. So three years ago we had a
    talk by Axel about how physicists analyze
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    massive big data and then last year we had
    a talk conquering large numbers at LHC by
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    Carsten and Stefanie. And we would, as
    I've mentioned already, we would like to
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    give like more an engineering perspective.
    We are Stefan and Severin. We're both
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    electrical engineers working at CERN.
    Stefan is working in the experimental
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    physics microelectronics section and he
    will give a second talk tomorrow about
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    designing high reliability digital
    electronics together with Szymon tomorrow
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    morning at 11:30. And I'm, as I mentioned
    already, also working at CERN. I'm working
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    in the electronics for machine protection
    section. I will describe briefly later. A
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    short disclaimer; the LHC is a pretty big
    machine and we try to explain it as good
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    as possible. 45 minutes is not really
    enough to talk about everything because I
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    think you can basically take one of the
    topics we are talking here about now and
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    talk for 45 minutes only about one
    specific topic, but we try to give an
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    overview as good as possible. So imagine
    you want to build an accelerator in your
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    backyard. OK, maybe not in your backyard
    because LHC is quite big, so 27 kilometers
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    in diameter is quite big, but basically we
    figured out three main challenges you have
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    to take. First of all, we have to
    accelerate particles because otherwise
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    it's not a particle accelerator. Second,
    we have to keep the particles on a
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    circular trajectory. And then third, we
    have to make sure that the particles which
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    are inside our beam tube or beam pipe
    don't collide with anything which is
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    there, for example, to beam pipe itself,
    air molecules, etc. And the solution we
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    adopted for LHC there is, that we
    accelerate the particles with a high power
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    radio frequency cavities. Then we have a
    beam control system which is quite
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    sophisticated using superconducting
    magnets and then we have the beam pipe
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    itself, which is evacuated, so it's under
    vacuum conditions to avoid any collisions
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    we have inside with gas molecules, etc. A
    brief overview about the location itself.
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    So probably many of you know already that
    CERN is next to Geneva. So it's in the
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    western-southern part of Switzerland. When
    we zoom in a little bit more, then we have
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    here an artificial like picture of LHC
    itself in the red circle there. To put it
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    a little bit in a perspective, we have a
    relatively big airport there. You can see
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    there, it's a 2200 metre long runway. We
    have Geneva Lake next to it. And that's
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    only one small part of Geneva Lake, but
    nevertheless, and what also quite nice, we
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    see Mont Blanc from LHC, er, from CERN.
    When we zoom in a little bit more, then we
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    basically have the big, circular collider
    there. That's LHC itself. And we have pre-
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    accelerators, I will talk in a few minutes
    about. Basically we have two main
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    campuses: we have Meyrin Site, which is in
    Switzerland, and we have to Prevessin
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    site, which is in France. Then at LHC
    itself, we have eight service points. We
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    also call it just points, to briefly go
    through all of them; we have point one
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    where we have the experiment called ATLAS,
    one of the big and major experiments at
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    LHC. Then at the exactly opposite side of
    ATLAS, we have CMS at point five. Then we
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    have a little bit smaller experiment,
    which is ALICE. It was basically
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    constructed for lead ion runs. We will
    talk about this later. And then we have
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    another relatively small experiment, it's
    called LHCb. And that's the only non-
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    symmetrical experiment at LHC. These are,
    I think, the four experiments you already
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    maybe heard of. Then there are four or
    three other smaller experiments. We have
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    LHCf at point one, it's a forward
    scattering experiment at point one. So
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    basically, they're taking data like
    scattered particles from ATLAS itself.
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    Then we have TOTEM. It's also a forward
    scattering experiment and point five, then
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    we have, sorry, we have MOEDAL, which is
    the experiment at point eight. They're
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    looking for magnetic mono-poles. Then we
    have TOTEM, sorry for that, at point five.
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    And then we have a relatively new
    experiment which is called PHASER. It's
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    actually under construction and it will be
    used, starting from 2021 and it's forward
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    scattering experiment, which, where they
    try to detect neutrinos. Then we have
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    point four, there we have the RF cavities
    to accelerate the particle beam itself. We
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    have the beam dump area. So when there is
    like a fault in a machine or we just want
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    to dump the beam, then we used the mean
    dump system at point six. And then we have
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    two more general service areas. It's point
    three and point seven. LHC would not be
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    possible without the pre-accelerator
    complex. So we have a relatively big one
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    and it's also sometimes relatively old. On
    the left hand side of the slide you can
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    see LINAC2, it's an old linear accelerator
    which was used until last year. It's not
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    now phased out. And now we have LINAC4,
    which is also a linear accelerator and it
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    has a little bit higher acceleration. Then
    we have the proton synchrotron booster.
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    It's the first circular collider. So you
    can see two pictures there. What is
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    relatively special about PSB is that we
    have there two, sorry, four beam pipes
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    instead of just one beam pipe. Then we
    have the proton synchrotron accelerator,
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    which is the next stage for acceleration.
    It then has only one one beam pipe. And
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    then we are going from PS we are going to
    SPS, which is the super proton synchrotron,
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    which is has circumferences of seven
    kilometers. There we basically accelerate
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    the particles the last time and then they
    are injected it in the LHC itself. We
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    mentioned a few accelerators already,
    basically all everything we just
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    highlighted here. But CERN is a little bit
    more. So CERN is famous for LHC, I would
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    say. But there is much more than only the
    LHC. So only about 15 percent of the
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    protons, which are accelerated in the pre-
    accelerator complex, are really going to
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    the LHC itself. So there is much more:
    there is material science, there is anti
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    matter research and all different other
    kinds of research going on. Of course,
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    everything has to be controlled. It's
    called a CCC, the CERN control center.
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    It's located at, the Prevessin site;
    looks like that. Basically, we have, four
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    Cs looking to each other and there the
    operators are sitting 24/7 and operate the
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    whole machine. So basically, the whole
    pre-accelerator complex, all the energy
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    cryogenics and LHC itself. Before you ask,
    everything is running on Scientific Linux.
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    So we have basically our own Linux
    distributed distribution, which is used
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    there and it of course, it's open source.
    Talking about the LHC beam itself, we have
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    two beams: one is running clockwise and
    the other one is to running anti-
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    clockwise because we don't have a fixed
    target experiment where we basically let
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    the accelerated particles colliding with
    like a fixed target, like metal or
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    something like that. We have controlled
    collisions at four points, we mentioned
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    before. Most of the year we have proton
    runs. So we have protons and protons
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    colliding each towards each other. And
    then we have at the end of the year,
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    nearly starting from November to December,
    we have lead ion run. The protons itself
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    is not really like a fixed, straight line
    of particles. We have something called
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    bunches. You can imagine a little bit like
    spaghetti. It's basically the same length
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    of a spaghetti, but it is much thinner
    than a spaghetti. And each bunch, when you
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    have a proton run, then each bunch
    consists of approximately 100 billion
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    protons. And when you have lead ion runs,
    then we have approximately 10 million lead
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    ions in LHC. And last year we operated
    with 2565(sic) bunches in the LHC itself.
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    The LHC tunnel. We already talked about
    the tunnel itself. It is 27 kilometers
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    long and you can see maybe a little bit on
    this graph, that we have some, we have
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    eight straight sections and we have eight
    arcs in the tunnel. Basically the straight
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    sections are always there, where we have
    like service cavities or we have areas and
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    also the experiments. Because it's not so
    good visible in this picture, I put the
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    picture here. Basically, that's a straight
    section of LHC. You can basically just see
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    the beam pipe itself, with aluminum foil
    around it, and there are also no magnets.
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    And when we look in the arc section of
    LHC, then you see here the arc itself and
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    I think it's quite famous picture of LHC
    itself because we have blue dipole magnets
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    there. The tunnel itself is an old tunnel,
    used previously by LEP, the large electron
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    proton collider. It has a diameter of 3.8
    metres and the circumference is
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    approximately 27 kilometers. Inside the
    tunnel we have, first of all, cryogenics,
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    so we have big tubes, stainless steel
    tubes to carry all the cryogenic. So
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    liquid helium and gaseous helium. Then we
    have the magnet itself to bend the
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    particles and then we have electrical
    installations to carry like signals from
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    the magnets to have safety systems,
    electricity, etc, etc. Geography is a
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    little bit complicated in the area because
    we have in the western part of LFC we have
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    the Jura mountain range and this Jura
    mountain range has a relatively hard
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    material. It's not made out of, not made,
    but nature. I mean it's a limestone, so
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    it's relatively complicated to dig into
    this material, in comparison to all the
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    other areas at LHC. So when you would
    basically put a straight section of LHC,
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    then you have to dig much more into the
    relatively hard limestone. So that's why
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    it was decided that the LEP or LHC tunnel
    is tilted a little bit. So we have a tilt
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    angle of one 1.4 percent there. The depth
    is approximately between 50 meters at
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    point one or point eight, up to 170 metres
    deep at point four. We already talked a
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    little bit about magnets, but we would
    like to go a little bit more in the
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    details now. So why do we need magnets?
    Um, maybe you learned at school that when
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    you have a magnetic field and you have
    charged particles and you can bend
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    particles around an arc in a magnetic
    field. Depending on the charge of the
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    particles, you bend them around on the
    right side or left side, that's this
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    famous right hand and left side rule, you
    maybe learned during school. And at LHC we
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    cannot use a normal magnets like typical
    magnets. We have to use electromagnets
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    because normal magnets would not be strong
    enough to build in an electromagnetic
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    field which is feasible to bend the
    particles around the whole tunnel. At LHC
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    we use, in the dipole magnets, a magnetic
    field of 8.3 Tesla. And to do this we need
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    a current of 11850 amps. We have basically
    two different types of magnets. We have
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    bending magnets. So the dipole magnets I
    mentioned quite often already. And then we
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    have injection and extraction magnets.
    They are also dipole magnets there, but
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    they are a little bit differently
    constructed, because the injection and
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    extraction magnets have to be quite fast,
    because they have to basically be powered
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    up at full, the full magnetic field in
    several microseconds. Then we have higher
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    order magnets which are quadrupole pool,
    magnet, sextupole magnets and octupole
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    magnets, et cetera, et cetera. And they
    are used for focusing and defocusing the
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    beam itself. In total, we have 1200 dipole
    magnets at LHC. We have around 850
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    quadrupole magnets and we have 4800 higher
    order magnets. But they are normally quite
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    short or so, shorter than the the other
    magnets. The dipole magnets consist of two
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    apertures. They are used to bend to beam
    around, so I already said. In the middle
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    of the magnet itself we have a cold bore.
    So there is basically there are the
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    particles flying around. Then there is a
    metallic structure. You can see this in
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    the picture. It is just a shiny metallic
    sphere you see there. And then we have
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    next to the cold bore, we have the tool,
    the two apertures to bend the particle and
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    build the magnetic field itself. The
    dipole magnets have a length of 50 meters
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    and the manufacturing precision is plus
    minus one fine, one point five percent,
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    er, one point five millimeters. Then we
    have quadrupole magnets. They are used for
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    focusing and defocusing the beam. The
    problem is that we have bunches, were are
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    basically equally charged particles inside.
    And the Coloumb force tells us, that when
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    we have equally charged particles, then
    they are basically want to fade out from
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    each other and in the end they would just
    hit the beam pipe itself and we could
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    maybe destroy the beam or cannot do any
    collisions. So what we do is we use a
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    quadruple magnets as, yeah, similar to
    lenses, because we can focus and defocus
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    the beam. The quadrupole magnets, the name
    already suggested it, that we have
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    basically four apertures. So we have on
    the left and the right side two and then
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    we have on top and bottom we have also a
    few of them. To go a little bit into
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    detail about the focusing and defocusing
    scheme. In the beginning we have a
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    particle beam which is not focused, but we
    want to focus it. Then we go to the first
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    quadrupole magnet. So we focus the beam.
    And this is only done in one axis. That's
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    a little bit a problem. So, in the second
    axis we don't have any focus and we have a
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    defocusing effect there. And then we have
    to use a second quadrupole magnet for the
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    other axis, in this case the Y-axis to
    focus the beam even further. And you can
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    even see this here in the Z-axis, that's
    basically the cut off the beam itself. You
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    can also see that in the beginning we have
    on the left side, we have an unfocused
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    beam and then we focus it in one axis, so
    we have like a little bit more ellipse and
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    then we focus in the other direction, then
    we have a different ellipse. So we have to
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    use several quadruple magnets in a row to
    really focus the beam in the way we want
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    to have it. In the LHC magnets, we have
    quite high currents. We we need these
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    currents, because otherwise we cannot bend
    to the very high energetic particle beam
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    and to use normal conducting cable, it
    would not be possible to basically build a
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    magnet out of it. So what we do is, we use
    materials which are called superconducting
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    materials, because they're for very good
    effect. They go to basically zero
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    resistance at a specific temperature
    point. And after this point or when we
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    basically go lower, then the current can
    flow without any losses inside of it. But
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    to reach the state, we have to cool the
    magnets quite heavily, which is not so
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    easy, but it can be done. And on the right
    side you basically see a very historic
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    plot. That was 1911 in Denmark, a
    researcher called Heike Onnes detected for
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    the first time superconducting effect in
    mercury. And it was detected at 4.19
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    Kelvin. To show you a little bit the
    comparison between a normal conducting
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    cable and a superconducting cable, as we
    put the picture here. So that is basically
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    the same amount of cable you need to use
    to carry, the thirteen thousand amps and
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    to do the same or to transport the same
    amount of energy we also can use a very
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    small superconducting cable and I think
    it's quite obvious why we use here
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    superconducting cables. At the LHC we use
    Niobium tin(sic) as material. And this
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    material basically goes into a
    superconducting state at 10 Kelvin. But to
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    have a safe operation to LHC, we have to
    cool it down at 1.9 Kelvin.
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    Superconducting magnets have some
    benefits, but also some downsides, so
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    sometimes they change their state because
    there are small rigid vibrations and the
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    magnet or the temperature's not precise
    enough or the current is too high, then
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    they change that state and it's called
    "Quench". And, we basically can detect a
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    Quench when we measure the voltage across
    the magnet, because the resistance changes
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    at this point. So when there is a Quench
    then the resistance changes quite rapidly,
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    in milliseconds and we can detect this
    voltage rise with sophisticated
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    electronics. On the right side, you see a
    board I'm working on. So basically here,
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    we have a measuring system to measure the
    voltage across the magnet. And then we
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    have a detection logic implemented in FPGA
    to basically send triggers out and open an
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    Interlock loop. Interlock loop is a system
    at LHC. You can imagine that little bit
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    like a cable going around the whole tunnel
    and there are thousands of switches around
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    this Interlock loop. And as soon as one of
    the detection systems basically opens to
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    the interlock loop, then basically the
    whole machine will be switched off. And
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    what means switched off is basically, that
    we will power down the power converter,
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    but then the energy is still in the
    superconducting magnet and it has to be
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    taken out of the superconducting magnet.
    And therefore, we use dump resistors to
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    extract the energy. And here you can see a
    picture of such a dump resistor. It's
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    quite big. It's in a stainless steel tube,
    oil cooled. It's approximately three or
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    four meters long. And basically, when
    there was a Quench, and the energy was
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    extracted via these resistors, the whole
    resistor is heated up by several hundred
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    degrees and it needs several hours to cool
    it down again. Power converters; the power
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    converters are used to power the magnet
    itself. So they can produce a current of
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    approximately 13000 amps and a voltage of
    plus minus 190 volts. And you can see a
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    picture how here, how big it is. One
    downside with the power converters is that
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    they have to be, not downside but one
    difficulty is, that they have to be very
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    precise, because every instability in the
    current would have or has a direct effect
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    on the beam stability itself. So basically
    the beam would be not diverted in the
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    right amount of length. So that's why they
    have to be very precise and have to have a
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    very precise stability. So here I just
    pointed out, like in 24 hours, the power
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    converter is only allowed to have a
    deviation of 5 ppm. And in comparison, for
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    13000 amps we have a deviation of 65 milli
    amps. So the power converters have to be
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    very precise. And to do that, we had to
    develop our own ADC, because at the time
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    when LHC was built, there was no ADC on
    the market which was able to have this
  • 19:51 - 19:56
    precision and also the whole ADC is put
    into a super-precise temperature
  • 19:56 - 20:03
    controlled areas and it is calibrated
    quite regularly. Okay, cryogenics. We
  • 20:03 - 20:06
    already talked about that we have
    superconducting magnets and they have to
  • 20:06 - 20:11
    be cooled down quite low. So the
    superconducting magnets we have at LHC has
  • 20:11 - 20:17
    have to be cooled down to 1.9 Kelvin. And
    we are doing this when we like start the
  • 20:17 - 20:22
    LHC then we cool down on the first hand
    with liquid nitrogen. So approximately six
  • 20:22 - 20:26
    thousand tonnes of liquid nitrogen are put
    through the magnets to cool them down
  • 20:26 - 20:33
    to 18 Kelvin and afterwards we cool the
    magnets down with liquid helium. And
  • 20:33 - 20:39
    liquid helium is at 1.9 or 1.8 Kelvin. And
    to put it a little bit in a comparison,
  • 20:39 - 20:42
    outer space, so when we measure like the
    temperature of space, we have
  • 20:42 - 20:48
    approximately 2.7 Kelvin in outer space.
    So LHC is much colder than outer space.
  • 20:48 - 20:52
    The whole cooldown needs approximately one
    month and each dipole magnet, which is 15
  • 20:52 - 20:57
    meters long, shrinks several centimeters
    during that. Which also has to be taken
  • 20:57 - 21:03
    into account, because otherwise pipes
    would break. The cryogenic system is that
  • 21:03 - 21:09
    we have at each of the eight points at LHC
    we have compressors to cool down the
  • 21:09 - 21:14
    liquid helium or the helium itself. And
    then we compress the helium and pump it
  • 21:14 - 21:18
    down. We have one gaseous helium stream,
    which is at 15 Kelvin and we have liquid
  • 21:18 - 21:24
    helium stream at approximately 4.5 Kelvin.
    And then we pump it underground and then
  • 21:24 - 21:29
    we have something called Cold Compression
    System. And the Cold Compression System
  • 21:29 - 21:36
    even further reduces the pressure of the
    helium that we have in the end a helium,
  • 21:36 - 21:41
    which is at 1.8 Kelvin. So it can really
    cool down the magnet itself. And helium
  • 21:41 - 21:46
    has a very interesting effect because at
    2.1 Kelvin, it becomes something called
  • 21:46 - 21:52
    superfluid. So it basically can run around
    like holes, for example, or walls. It can
  • 21:52 - 21:58
    basically flow against gravity, which is
    quite interesting. And it has also very
  • 21:58 - 22:04
    high thermal conductivity and that's also
    why we use superfluid helium here. And
  • 22:04 - 22:08
    that's why we cool down the whole magnets
    that low. And one other interesting effect
  • 22:08 - 22:14
    is also that the LHC tilt angle, which is
    1.4 percent, has to be taken into account
  • 22:14 - 22:18
    because we have very low pressure inside
    all the tubes or all the system, at 16
  • 22:18 - 22:25
    millibars. But we have sometimes to pump
    the helium against gravity or going down.
  • 22:25 - 22:28
    So we also have to take into account the
    LHC tilt angle to not have wrong pressure
  • 22:28 - 22:35
    levels at the whole LHC itself. Okay.
    Stefan: All right! So, you probably
  • 22:35 - 22:39
    already got the idea, that what we've done
    in the last 20 minutes, was only solve the
  • 22:39 - 22:42
    first of the three challenges we had,
    which was actually bending the beam around
  • 22:42 - 22:49
    the circular trajectory. So I'm trying to
    go to the other challenges we have lined
  • 22:49 - 22:54
    up in the beginning. And the first one of
    that is the actual acceleration of the
  • 22:54 - 23:00
    particle beam. And large synchrotrons,
    e.g. like the LHC, they use radio
  • 23:00 - 23:05
    frequency or RF systems to do this
    acceleration. And I'm just going to do a
  • 23:05 - 23:11
    quick recap of the LHC beam and RF and how
    they interact. So Severin mentioned
  • 23:11 - 23:16
    already that the particles in LHC actually
    come in bunches. So in like packets that
  • 23:16 - 23:20
    contain about hundred billion protons and
    those bunches are spaced when they are
  • 23:20 - 23:26
    running around the LHC approximately 25
    nanoseconds apart. And starting from that
  • 23:26 - 23:31
    the tasks of the RF system are basically
    twofold. It first has to ensure that these
  • 23:31 - 23:36
    bunches are kept tightly together in a
    process that we call longitudinal
  • 23:36 - 23:40
    focusing. And the second task is to care
    for the actual acceleration of the
  • 23:40 - 23:45
    particle bunches. So from their injection
    energy, when they come from one of the
  • 23:45 - 23:49
    pre-accelerators up to their final energy,
    that they are supposed to collide at
  • 23:49 - 23:56
    during the physics run. So in general, you
    can imagine RF as being a quickly
  • 23:56 - 24:03
    alternating electric and magnetic field
    components. And in the LHC, this RF energy
  • 24:03 - 24:08
    is basically injected into what is called
    a cavity, which is a resonant structure.
  • 24:08 - 24:12
    And there the particle beams travels
    through, while the field quickly
  • 24:12 - 24:17
    alternates and the RF signal, or the
    energy, basically interacts with the
  • 24:17 - 24:22
    particle beam. So perhaps you know that
    the protons are positively charged and
  • 24:22 - 24:26
    then a negative polarity of the field
    would attract these protons, while the
  • 24:26 - 24:32
    positive field location would basically
    move them away. And this has ... well,
  • 24:32 - 24:36
    after just injecting and with the
    frequency of this RF field being the same
  • 24:36 - 24:41
    as the speed that the particles actually
    go round the LHC, solves the first of the
  • 24:41 - 24:45
    two problems, which was the the focusing
    because actually the particles that are
  • 24:45 - 24:49
    too slow arrive only when the field is
    already changed to the opposite polarity
  • 24:49 - 24:53
    and actually get accelerated a bit, while
    the particles that are too fast, they are
  • 24:53 - 24:58
    actually being decelerated a bit. And this
    is a process that we call the longitudinal
  • 24:58 - 25:03
    focusing, which makes sure that the
    bunches stay neatly packed together. And
  • 25:03 - 25:06
    of course this would be relatively
    inefficient if we would only change the
  • 25:06 - 25:12
    polarity of this field once for each of
    the proton bunches that pass by. Which is
  • 25:12 - 25:16
    why we do it ten times. So the polarity
    basically changes ten times or the
  • 25:16 - 25:21
    frequency is ten times higher than the
    bunch crossing frequency. And by doing
  • 25:21 - 25:26
    that, we make sure that the change of this
    field is much faster and therefore the
  • 25:26 - 25:33
    particle bunches are packed much closer
    together and the focusing is better. So
  • 25:33 - 25:35
    here you can see these cavities that were
    shown in the previous picture as a
  • 25:35 - 25:40
    schematic, how they're actually placed in
    the tunnel. So eight of these huge
  • 25:40 - 25:44
    cavities are used per beam and they are
    the actual thing that is used to couple
  • 25:44 - 25:50
    the RF energy into the beam and transfer
    it to the particles. They are also
  • 25:50 - 25:54
    operating superconductively, so at
    cryogenic temperatures, to reduce the
  • 25:54 - 26:00
    thermal stress and the losses that would
    otherwise occur in their materials. And
  • 26:00 - 26:01
    these are actually – even though they are
    so big, similar to the magnets that had to
  • 26:01 - 26:06
    be very precisely manufactured – these
    also have very small manufacturing
  • 26:06 - 26:12
    tolerances and have to be precisely tuned
    to the RF frequency that is used to
  • 26:12 - 26:17
    inject. So and the second part of this,
    that actually produces this high power RF
  • 26:17 - 26:22
    signal. For that is used what we call
    Klystrons. So Klystrons are basically RF
  • 26:22 - 26:29
    amplifiers. They are built from high power
    RF vacuum tubes and they amplify this 400
  • 26:29 - 26:34
    MHz signal that is used to transfer energy
    to the particles. And each of those
  • 26:34 - 26:40
    Klystrons produces about 300 kW of power
    and you can probably imagine how much that
  • 26:40 - 26:44
    power for an individual unit that is, if
    you know that your microwave oven has like
  • 26:44 - 26:49
    2 or 3 kW. And of course, as we have eight
    cavities per beam and one Klystron always
  • 26:49 - 26:55
    feeds one cavity, we in total have 16 of
    those Klystrons and they are in principle
  • 26:55 - 27:04
    able to deliver a total energy of 4.8 MW
    into the LHC beam to accelerate it. But if
  • 27:04 - 27:07
    we take a small step back for now, we have
    only solved the first of the two problems,
  • 27:07 - 27:13
    which was to keep the bunches neatly
    focused. Because currently the particles
  • 27:13 - 27:17
    have been injected and the frequency is at
    some specific frequency and actually they
  • 27:17 - 27:22
    are only running basically in sync, the
    two. So what we do after all the particle
  • 27:22 - 27:26
    bunches from the pre-accelerators have
    been injected into LHC, is that we ever so
  • 27:26 - 27:31
    slightly increase the frequency, which of
    course also means that the particles need
  • 27:31 - 27:35
    to accelerate together with the RF signal.
    And this is the mechanism that we use to
  • 27:35 - 27:40
    accelerate them actually. And the change,
    that is required to do this, is very tiny,
  • 27:40 - 27:44
    actually. So it is less than a thousandth
    of a percent sometimes, that is used to
  • 27:44 - 27:48
    change the frequency to actually make them
    go so much faster. So from their
  • 27:48 - 27:54
    relatively low injection energy up to the
    top energy plateau that they need to have
  • 27:54 - 28:00
    to produce the actual physics collisions.
    And an interesting question to ask here is
  • 28:00 - 28:04
    where does this signal actually comes from
    if it needs to be so precisely tuned to
  • 28:04 - 28:10
    some specific frequency? Who generates it
    or who controls it? And that opens up the
  • 28:10 - 28:16
    whole complex of the timing of the LHC, of
    the machine. So actually this first signal
  • 28:16 - 28:21
    that I mentioned, this RF signal, it
    originates in a Faraday cage. So an
  • 28:21 - 28:26
    especially shielded area somewhere on the
    Prévessin site of CERN. And from there it
  • 28:26 - 28:33
    is distributed to the low-level RF
    subsystem with the Klystrons and the
  • 28:33 - 28:39
    cavities. But inside this room, there are
    also a number of other signals generated.
  • 28:39 - 28:43
    The first one of that being this Bunch
    Crossing Clock, which is the actual clock
  • 28:43 - 28:48
    that signals one pulse, basically every
    time, it changes polarity one time a
  • 28:48 - 28:55
    proton bunch moves across a specific
    location inside the LHC. And another one
  • 28:55 - 28:59
    is the so-called orbit clock, which always
    indicates the start of the first or when
  • 28:59 - 29:05
    one proton bunch has basically re-arrived
    at the same position and has completed one
  • 29:05 - 29:11
    orbit. And you may ask the question why
    this is an important piece of information.
  • 29:11 - 29:17
    But if you think back to this image that
    Severin has already shown, about the
  • 29:17 - 29:22
    accelerator complex, the big challenge
    that all this brings is also the whole
  • 29:22 - 29:25
    synchronization of all these machines.
    Because you have to imagine that while
  • 29:25 - 29:30
    these proton bunches run around the LHC
    and new ones are supposed to be injected
  • 29:30 - 29:34
    from the outside, from another pre-
    accelerator, this has to be very precisely
  • 29:34 - 29:38
    synchronized. So all these pre-accelerator
    systems actually share a common
  • 29:38 - 29:43
    synchronized timing system that allows
    them to precisely inject a new packet of
  • 29:43 - 29:49
    bunches at the right position, at the
    right location into the LHC. And this a
  • 29:49 - 29:53
    bit how such a timing distribution system
    looks like. It is only a very small
  • 29:53 - 29:56
    excerpt of what it looks like, but it
    gives you an idea that somewhere
  • 29:56 - 30:01
    underground in the LHC there is rooms full
    of equipment that is just used to
  • 30:01 - 30:06
    distribute timing signals between
    different parts of the accelerator. And of
  • 30:06 - 30:12
    course, as CERN is forward-thinking and
    realized that future colliders will need
  • 30:12 - 30:15
    quite a bit more of all this
    synchronization and that the requirements
  • 30:15 - 30:20
    for how precisely everything needs to be
    synchronized is ever growing, they
  • 30:20 - 30:23
    actually developed their own timing
    distribution standard which is also
  • 30:23 - 30:28
    openly available and available for
    everybody to use. So if you're interested,
  • 30:28 - 30:34
    look that up. But of course, not only the
    accelerator itself is interested in this
  • 30:34 - 30:40
    information about what particles are where
    and how quickly they interact or how
  • 30:40 - 30:45
    quickly they go around. But also all the
    experiments need this information, because
  • 30:45 - 30:50
    in the end they want to know "Okay, has a
    collision occurred at some specific time
  • 30:50 - 30:55
    in my experiment?" and actually providing
    this timing information about when bunches
  • 30:55 - 31:00
    have crossed their experiment locations is
    also vital for them to really time tag all
  • 31:00 - 31:06
    their collision data and basically track
    which bunches were responsible for what
  • 31:06 - 31:12
    kind of event or what event throughout
    their whole signal storage and processing
  • 31:12 - 31:17
    chain, let's say. Good. So that is
    basically challenge 2 out of the way. So
  • 31:17 - 31:20
    that was the acceleration of the actual
    particles and all the associated issues
  • 31:20 - 31:26
    with timing. And the third issue we
    mentioned was that the particles need to,
  • 31:26 - 31:31
    let's say, be kept from colliding with
    anything but themselves or the other beam.
  • 31:31 - 31:36
    And that is what we, why we need vacuum
    systems for. So, again, it is not as
  • 31:36 - 31:41
    simple as just putting a vacuum somewhere.
    Of course not. Because in fact, there is
  • 31:41 - 31:46
    not only one vacuum system at LHC, but
    there are three. So, the first two of
  • 31:46 - 31:51
    those are perhaps a bit less interesting
    to most of us. They are mainly insulation
  • 31:51 - 31:57
    vacuum systems that are used for the
    cryogenic magnets. So they isolate,
  • 31:57 - 32:03
    basically thermally isolate the magnets at
    those very cool temperatures from the
  • 32:03 - 32:09
    surrounding air to avoid them getting more
    heat load than they need to. And there is
  • 32:09 - 32:12
    an insulation vacuum also for the helium
    distribution lines that are actually
  • 32:12 - 32:16
    distributing, delivering the helium to
    these magnets. And then the third one,
  • 32:16 - 32:20
    which is perhaps the most interesting one,
    is the beam vacuum. So the one where
  • 32:20 - 32:25
    actually the beam circulates inside the
    LHC. And this is a cross section of what
  • 32:25 - 32:30
    this beam vacuum typically looks like. So
    it is approximately this size, so a very
  • 32:30 - 32:37
    ... handful, let's say. And the question
    you may ask "OK, if I want to keep all the
  • 32:37 - 32:42
    like the particles in my particle beam
    from colliding with anything they are not
  • 32:42 - 32:47
    supposed to, for example, rest molecules
    of remaining air there, how many molecules
  • 32:47 - 32:51
    can there still be?" So somebody has to
    make up that number. And typically you
  • 32:51 - 32:56
    express this as a quantity called the
    beam lifetime, which basically says if you
  • 32:56 - 33:01
    were only to keep those particles
    circulating in the accelerator, how long
  • 33:01 - 33:05
    would it take until they have all
    dispersed and lost their energy due to
  • 33:05 - 33:10
    colliding with rest gas molecules? And it
    was decided that this should be at a value
  • 33:10 - 33:15
    of 100 hours, is what the beams should
    basically be able to circulate without
  • 33:15 - 33:19
    collisions, without being lost. And this
    gave the requirement for pressures down to
  • 33:19 - 33:25
    about 100 femtobar, which is a very small,
    very, very tiny fraction of the
  • 33:25 - 33:30
    atmospheric pressure we have here, which
    is about 1 bar. And to actually get to
  • 33:30 - 33:34
    this level of vacuum, it requires multiple
    stages and multiple components to actually
  • 33:34 - 33:42
    get there. So the initial vacuum inside
    these beam tubes, which are basically
  • 33:42 - 33:49
    going throughout the whole LHC tunnel, has
    the volume of approximately the Notre-Dame
  • 33:49 - 33:54
    cathedral. So the first step of getting
    all the air out of these beam tubes is
  • 33:54 - 34:00
    using turbomolecular pumps. And then there
    needs to be more mechanisms to reduce the
  • 34:00 - 34:04
    pressure even further, because these pumps
    are not able to reduce the pressure to the
  • 34:04 - 34:09
    levels required. And they actually use a
    relatively clever trick to do that, which
  • 34:09 - 34:16
    is the use of cryopumping. So the, ... I
    cannot show that? Okay. So the outer wall
  • 34:16 - 34:20
    of this beam pipe cross section that you
    see here is actually also where the very
  • 34:20 - 34:27
    cold helium inside the magnets is outside
    of. And what that does is, it leads to an
  • 34:27 - 34:32
    effect called cryopumping. So actually any
    rest gas molecule that hits this wall
  • 34:32 - 34:36
    actually condenses there. And as the
    molecules condense there, they are of
  • 34:36 - 34:41
    course removed from the atmosphere inside
    this beam pipe, which removes them from
  • 34:41 - 34:45
    the atmosphere and increases the quality
    of the vacuum. And with the use of this
  • 34:45 - 34:48
    and then the warm sections, the use of
    getter coatings, which are basically able
  • 34:48 - 34:54
    to trap gas molecules, you are able to
    reach the crazy vacuum levels that are
  • 34:54 - 34:59
    required to make this happen. But they
    realized also during the design that one
  • 34:59 - 35:05
    big problem – for the first time in an
    accelerator – another effect will create a
  • 35:05 - 35:09
    significant problem for the vacuum, which
    is the generation of synchrotron
  • 35:09 - 35:15
    radiation. So synchrotron radiation is a
    byproduct of when you do bend a particle
  • 35:15 - 35:20
    beam, it results in a phenomenon called
    synchrotron radiation. And when this
  • 35:20 - 35:24
    synchrotron radiation, as it goes straight
    on and is not bent, hits the walls of this
  • 35:24 - 35:30
    vacuum system, or in this case of the beam
    pipe, it actually liberates molecules from
  • 35:30 - 35:34
    there and reintroduces them into the
    vacuum, which of course then makes the
  • 35:34 - 35:40
    vacuum worse again. An additional problem
    that gives the synchrotron radiation is,
  • 35:40 - 35:45
    that it also gives a significant heat
    load, and if you need to dissipate all
  • 35:45 - 35:50
    this heat that is generated through the
    very cold helium, this is not a very
  • 35:50 - 35:54
    efficient process. Because making this
    helium so cool, is actually a very energy
  • 35:54 - 35:59
    intensive process. And just removing a
    single watt of thermal power through the
  • 35:59 - 36:03
    superfluid helium costs about 1 kW of
    energy. So that is not the most efficient
  • 36:03 - 36:08
    part. And this is why the cross-section
    you have just seen includes another large
  • 36:08 - 36:11
    component, which also technically belongs
    to the vacuum system, which is called the
  • 36:11 - 36:16
    beam screen. And this beam screen is
    basically another tube running inside the
  • 36:16 - 36:21
    beam pipe, of which we have, of course,
    two, which run inside the magnet cold
  • 36:21 - 36:25
    bores. And it shields the synchrotron
    radiation heat load from the outer walls,
  • 36:25 - 36:31
    which are at 1.8 Kelvin, while this pipe
    itself is actively cooled to only about 20
  • 36:31 - 36:36
    Kelvin of temperature, which is much more
    efficient to dissipate this heat. So it is
  • 36:36 - 36:39
    basically a steel tube about one
    millimeter thick. It has these pumping
  • 36:39 - 36:47
    holes, where hydrogen gas molecules can go
    out of, and on the inside it has a copper
  • 36:47 - 36:52
    coating, which is used to reduce its
    electrical resistance, which is required
  • 36:52 - 36:56
    because the beam, while it circulates,
    also induces current that would otherwise
  • 36:56 - 37:00
    flow inside this tube, which is really, if
    you think about it, only a simple tube and
  • 37:00 - 37:04
    it would increase the heat load again. So
    a lot of engineering already has to go
  • 37:04 - 37:12
    into a very simple piece of ... a thing
    like that. So after having spoken so much
  • 37:12 - 37:16
    about all the things required to just make
    a beam circulate and accelerate and so on,
  • 37:16 - 37:21
    now it's probably also time to talk a
    little bit about the beam itself and how
  • 37:21 - 37:27
    to control it and how to instrument, how
    to measure things about this beam. Even
  • 37:27 - 37:32
    without going yet about collisions and
    doing actual physics experiments. So the
  • 37:32 - 37:37
    first important bit that is able to
    basically control or influence the beam
  • 37:37 - 37:42
    here is what's called the beam cleaning or
    collimation system. So typically such a
  • 37:42 - 37:47
    particle beam is not very clean. It always
    travels associated with what is called
  • 37:47 - 37:52
    halo of particles around this core area
    that is less than a millimeter wide where
  • 37:52 - 37:57
    most of the intensity is focused. And
    these particles outside we want to remove,
  • 37:57 - 38:00
    because they otherwise would be lost
    inside the magnets and for example, would
  • 38:00 - 38:06
    lead to quenches of the superconducting
    magnets. And for collimation, we basically
  • 38:06 - 38:11
    use small slits that are adjustable and
    are located at two main locations of the
  • 38:11 - 38:15
    LHC. So they have collimation systems
    there, with vertical and horizontal slits
  • 38:15 - 38:22
    that can be adjusted in width, in order to
    scrape off all the particles that they do
  • 38:22 - 38:25
    want to get rid of and extract out of the
    beam, while only the core part can
  • 38:25 - 38:30
    circulate and produce clean collisions
    without any background, that otherwise
  • 38:30 - 38:36
    would need to be accounted for. And then
    there is a whole other open topic of beam
  • 38:36 - 38:40
    instrumentation. So when you run a
    particle accelerator, you want to measure
  • 38:40 - 38:45
    various quantities and performance figures
    of such a beam. And that is crucial for a
  • 38:45 - 38:49
    correct operation and for the highest
    performance, getting the highest
  • 38:49 - 38:52
    performance from an accelerator. And there
    are a lot of different types of those, and
  • 38:52 - 38:59
    I want to go quickly about ... over why we
    have them and what we do with them. So the
  • 38:59 - 39:04
    first and most basic measurement you want
    to do, is the beam current measurement. So
  • 39:04 - 39:09
    the beam current is a basic accelerator
    beam intensity measurement. So it gives
  • 39:09 - 39:13
    you an idea of how strong the beam that is
    running inside your accelerator is. And it
  • 39:13 - 39:18
    is measured using these DCCTs or DC
    current transformers. And their basic
  • 39:18 - 39:22
    principle of operation is, that while the
    particles move through this torus, which
  • 39:22 - 39:26
    is actually a coil or a transformer,
    induces a voltage there that you can
  • 39:26 - 39:31
    measure and then use to quantify the
    intensity of this beam. And the big
  • 39:31 - 39:35
    challenge here is that the dynamic range,
    this instrument needs to capture, is
  • 39:35 - 39:40
    really large, because it has to operate
    from the lowest intensity pilot injection
  • 39:40 - 39:46
    beams up to the full energy, full number
    of bunches running inside the LHC. So it
  • 39:46 - 39:52
    has to cover six orders of magnitude of
    measurement dynamic range. Then the second
  • 39:52 - 39:57
    thing when talking about collisions is the
    luminosity measurement. So luminosity is a
  • 39:57 - 40:02
    quantity basically said to measure the
    rate of interaction of the particle beams.
  • 40:02 - 40:07
    So to give you an idea of how often
    interactions happen inside the experiments
  • 40:07 - 40:11
    or where you want them to happen. And this
    measurement is used to first of all,
  • 40:11 - 40:14
    adjust this interaction rate to a target
    value, which is optimal for the
  • 40:14 - 40:19
    experiments to function and to equalize
    the interaction rates in different
  • 40:19 - 40:24
    experiments. So different experiments also
    are specified to have the same interaction
  • 40:24 - 40:30
    rate so they can get the same let's say
    statistical quality of their data. So it's
  • 40:30 - 40:34
    used to equalize those. And then as a
    third thing, this system is also used to
  • 40:34 - 40:38
    measure the crossing angle of the beam. So
    as you may know, at some point, when the
  • 40:38 - 40:43
    beams are collided, they collide at an
    angle, that is very small. And this angle
  • 40:43 - 40:48
    is actually measured also very precisely
    in order to adjust it correctly. And it is
  • 40:48 - 40:50
    measured to less than a thousandth of a
    degree, which is again a very impressive
  • 40:50 - 40:55
    feat, given that the detection principle
    of this measurement is only measurement of
  • 40:55 - 40:59
    some neutral particles that are a result
    of the particle interaction of the beam
  • 40:59 - 41:05
    ... of the collision. Okay, so that is
    number two. Then number three that we have
  • 41:05 - 41:10
    is the beam position monitor. Because
    along the LHC, you also always want to
  • 41:10 - 41:15
    know, where the beam is at any given time.
    So you want to measure the position of the
  • 41:15 - 41:19
    beam inside the beam pipe in order to
    optimally adjust it to the position you
  • 41:19 - 41:24
    want to have it. And for that we use these
    beam position monitors of which we have
  • 41:24 - 41:28
    more than a thousand installed along the
    LHC. So they are typically capacitive
  • 41:28 - 41:32
    probes or electromagnetic strip lines. As
    you can see on top and bottom
  • 41:32 - 41:37
    respectively. And they basically are
    distributed along the LHC and provide
  • 41:37 - 41:40
    position of the particle beam along the
    accelerator, which can then be used to
  • 41:40 - 41:48
    tune, for example, the magnets. All right.
    Then we have beam profile. So after the
  • 41:48 - 41:51
    position, that gives you an idea where the
    beam is, you also want to know its
  • 41:51 - 41:57
    intensity distribution. Basically when you
    do a cut through the beam pipe somewhere,
  • 41:57 - 42:02
    you want to know how the intensity profile
    looks like. And for that we have basically
  • 42:02 - 42:07
    two measurement systems. One measures the
    profile in X and Y directions. So if you
  • 42:07 - 42:10
    really would do a cut and it gives you
    something like this and it's, for example,
  • 42:10 - 42:15
    done with wire scanners, which is literal,
    very thin wire that is moved through the
  • 42:15 - 42:20
    beam. And then the current that the beam
    moving through this wire, generates is
  • 42:20 - 42:25
    used to generate such a profile map, when
    scanning with this wire. The other one is
  • 42:25 - 42:30
    the longitudinal profile, which gives you
    an idea about the quality of your RF
  • 42:30 - 42:34
    system and there you want to know how the
    intensity profile of your beam looks like.
  • 42:34 - 42:38
    If you were looking only at one spot of
    the accelerator and the beam would pass
  • 42:38 - 42:43
    by, and you would basically see over time
    how the intensity looks like. And then as
  • 42:43 - 42:48
    the last bit of beam instrumentation,
    there is the beam loss monitors. So they
  • 42:48 - 42:53
    are these yellow tubes, that are located
    on the outside of mostly all the magnets,
  • 42:53 - 42:58
    of the dipole and quadrupole magnets and
    so on. Again, there's more than a thousand
  • 42:58 - 43:03
    of those. And the idea here is that you
    need a lot of detectors that are basically
  • 43:03 - 43:08
    small ionization chambers, which detect
    any showers of secondary particles that
  • 43:08 - 43:13
    are generated when one of the high energy
    protons are lost somewhere in the magnet
  • 43:13 - 43:17
    materials. So these are really used for
    protection of the system, because if a
  • 43:17 - 43:21
    specific threshold of energy loss is
    detected, then the accelerator needs to be
  • 43:21 - 43:26
    quickly shut down. Which is why they have
    to react in a matter of nanoseconds in
  • 43:26 - 43:31
    order to keep the accelerator safe.
    Because any interaction of the particle
  • 43:31 - 43:36
    beam with for example, the magnets could
    just destroy huge amounts of money and of
  • 43:36 - 43:42
    time that would be needed to rebuild. And
    as a last and final thing, we have spoken
  • 43:42 - 43:46
    one or two times already about shutting
    down the LHC. Which sounds also trivial at
  • 43:46 - 43:52
    first, but really is not. So, the last
    thing here is, what we call the Beam Dump.
  • 43:52 - 43:57
    So the energy content that is contained in
    those particular beams, it can be used,
  • 43:57 - 44:03
    could be used if it were shot on a copper
    target, it could just melt 1000 kilograms
  • 44:03 - 44:08
    or one ton of copper instantly. So during
    beam extraction, the process of getting
  • 44:08 - 44:12
    the particle beam outside out of the LHC,
    this energy needs to be dissipated
  • 44:12 - 44:17
    somehow. And for that, this special Beam
    Dump area is constructed. So there are
  • 44:17 - 44:21
    fast kicker magnets, that are used to ...,
    that are able to ramp up in a really,
  • 44:21 - 44:25
    really short amount of time of
    microseconds. And then the beam is
  • 44:25 - 44:30
    carefully and in a controlled manner
    directed into a set of concrete blocks,
  • 44:30 - 44:35
    that is basically big enough to dissipate
    all this energy, when required. And in the
  • 44:35 - 44:39
    process of doing so, it also heats up to
    about 800 degrees Celsius, and then of
  • 44:39 - 44:46
    course, also needs the associated time to
    cool down again. Good. So as you may or
  • 44:46 - 44:50
    may not know, currently the LHC is not in
    operation. So LHC currently is undergoing
  • 44:50 - 44:56
    its second long shutdown phase, or LS2.
    But what we do when the LHC is in
  • 44:56 - 45:00
    operation, is that we have these status
    dashboards, that you can see here, that
  • 45:00 - 45:05
    are distributed all around CERN, and can
    be used by anyone, any passer-by, to
  • 45:05 - 45:10
    basically monitor what the current
    operation mode or the current situation of
  • 45:10 - 45:16
    the accelerator is. And can be used also
    to quickly see if like an operator needs
  • 45:16 - 45:20
    to go somewhere or is needed, or how the
    shift planning for the next shift works
  • 45:20 - 45:24
    out and so on. And on the right side you
    would see what this currently looks like.
  • 45:24 - 45:31
    So basically black screen saying next beam
    expected in spring 2021. And the good
  • 45:31 - 45:34
    thing about these status pages is that you
    can actually see them from your home,
  • 45:34 - 45:40
    because they're also openly available, as
    most of the stuff we do at CERN. So if you
  • 45:40 - 45:45
    are interested, then perhaps in a year
    from now or a bit longer than a year, it
  • 45:45 - 45:48
    would be quite interesting to follow all
    the commissioning process of when they are
  • 45:48 - 45:54
    trying to start the LHC back up, and
    follow that process from your home.
  • 45:54 - 45:58
    Otherwise, if you now feel the urge to
    maybe visit CERN, pay some of the things
  • 45:58 - 46:02
    we talked about a visit, or are just
    generally interested, CERN offers a
  • 46:02 - 46:07
    variety of tours free of charge. So if
    you're interested in that, visit that web
  • 46:07 - 46:11
    site and we would be happy to welcome you
    there. And with that, thank you very much
  • 46:11 - 46:15
    for your attention.
  • 46:15 - 46:18
    Applause
  • 46:18 - 46:24
    Severin: Punktlandung.
    Herald: Thank you, Stefan and Severin. If
  • 46:24 - 46:30
    you have questions, there are six
    microphones in the room. Please make a
  • 46:30 - 46:35
    queue, and we start with the Signal Angel.
    Signal Angel, please, first question.
  • 46:35 - 46:40
    Signal Angel: There is said to be a master
    red button for shutting down the whole
  • 46:40 - 46:46
    system in case of heavy problems. How
    often did you push it yet?
  • 46:46 - 46:49
    Stefan: Master red button?
    Severin: Master button ...
  • 46:49 - 46:55
    Signal Angel: Like a shut down button.
    Severin: I cannot really understand you. I
  • 46:55 - 46:59
    think the question was about how often
    basically we used the Beam Dump system to
  • 46:59 - 47:01
    basically get rid of the beam, is that
    correct?
  • 47:01 - 47:04
    Signal Angel: I guess so.
    Stefan: He said there is a master button.
  • 47:04 - 47:06
    Signal Angel: I guess so.
    Stefan: I think there's a master button in
  • 47:06 - 47:08
    the...
    Severin: There is not only one master
  • 47:08 - 47:12
    button, there are several master buttons.
    These are switches, called beam interlock
  • 47:12 - 47:18
    switch. Basically, at every operator's
    screen, there is basically one beam
  • 47:18 - 47:24
    interlock switch. I don't know. I think
    sometimes they get rid of the beam just
  • 47:24 - 47:30
    because, I mean. When we have LHC at full
    operation – Stefan talked about the
  • 47:30 - 47:34
    luminosity – so what is happening, that in
    the beginning we have a very high amount
  • 47:34 - 47:40
    of luminosity, So many particles collide
    on each other. But over time, like after
  • 47:40 - 47:45
    12 or 15 hours or whatever, basically the
    luminosity ..., so the amount of particles
  • 47:45 - 47:49
    which collide with each other, is going
    down and down. So the luminosity
  • 47:49 - 47:54
    decreases. And then at some point in time,
    basically the operators decide, that they
  • 47:54 - 47:59
    will now get rid of the actual beam, which
    is inside LHC and basically will recover
  • 47:59 - 48:02
    the whole machine and then restart the
    machine again. And this is done sometimes,
  • 48:02 - 48:08
    I don't know, every 12 hours, sometimes
    after 24 hours. Something like that, yes.
  • 48:08 - 48:11
    Herald: Cool. And microphone number four,
    I think.
  • 48:11 - 48:18
    Q: Yes. So where's the energy coming from?
    So do you have your own power plant, or
  • 48:18 - 48:20
    so?
    Severin: So, no, not really, not really.
  • 48:20 - 48:24
    Basically, we get all the power from the
    French grid. So we have relatively big
  • 48:24 - 48:33
    power trails coming from the French grid.
    So we get 450 kV of power there. So
  • 48:33 - 48:35
    basically the voltage is quite high and
    then we have our own transformers on site.
  • 48:35 - 48:40
    And I think only, ... a little bit smaller
    fraction of the energy is coming from the
  • 48:40 - 48:44
    Swiss grid. So basically we use most of
    the energy which is coming from the French
  • 48:44 - 48:47
    grid.
    Q: Okay. Thank you.
  • 48:47 - 48:49
    Herald: Thank you for your question. And
    microphone number one, please.
  • 48:49 - 48:57
    Q: Hi. Thank you for your presentation. If
    I'm not wrong, you say the beam can warm a
  • 48:57 - 49:05
    block of concrete to 800 Celsius. Would it
    be possible to use it as a weapon?
  • 49:05 - 49:10
    Stefan: laughs Very likely not. And CERN
    very much condemns these actions in any
  • 49:10 - 49:14
    form, I guess. So CERN operates in a
    purely peaceful mission and would never
  • 49:14 - 49:18
    think about using their particle beams as
    a weapon. And even if they could, it is
  • 49:18 - 49:22
    probably not the most practical thing to
    do, I guess. laughs
  • 49:22 - 49:27
    Herald: But if your telephone is again
    hanging up, you can destroy it, right?
  • 49:27 - 49:29
    Stefan: laughs
    Herald: And microphone number six, I
  • 49:29 - 49:33
    think.
    Q: Yes. So you said, you can stop in
  • 49:33 - 49:40
    nanoseconds, but just the light would go
    just 30 centimeters, you know, a
  • 49:40 - 49:45
    nanosecond. How will you be able to stop
    in this small time?
  • 49:45 - 49:49
    Stefan: Ah, no, no. So what I was talking
    about is that these magnets that are used
  • 49:49 - 49:55
    to extract the beam out of the LHC, they
    have reaction times, or ramp up times that
  • 49:55 - 50:01
    are in the order of 1, 2, 3 microseconds.
    So not nanoseconds, but microseconds. And
  • 50:01 - 50:06
    really only then basically the particles
    still circulate, worst case one full turn,
  • 50:06 - 50:10
    and only then moving outside of the
    accelerator.
  • 50:10 - 50:17
    Herald: And microphone number one again.
    Q: So do you have any photos of the front
  • 50:17 - 50:23
    of the dump block? It has to look like
    it's got hit a lot.
  • 50:23 - 50:26
    laughter
    Severin: No, not really. I think it's one
  • 50:26 - 50:32
    of the only pictures we could find about,
    the Beam Dump system. And these areas, I
  • 50:32 - 50:37
    think it's not really opened any more. So
    since operation of LHC, which was in
  • 50:37 - 50:43
    basically LHC started in 2008, and since
    then, the Beam Dump system was not opened
  • 50:43 - 50:49
    again because it's completely sealed in
    stainless steel. And that's why it wasn't
  • 50:49 - 50:52
    opened anymore.
    Heral: Cool. Question from the interwebs.
  • 50:52 - 50:59
    Signal: Regarding power supply. How do you
    switch or fine-control the currents? Are
  • 50:59 - 51:03
    you using classic silicone transistors,
    off-the-shelf IGBTs?
  • 51:03 - 51:07
    Sverin: Um, yes. laughs
    laughter
  • 51:07 - 51:13
    Severin: Yes. Uh, the system was developed
    at CERN. And I think that's quite common
  • 51:13 - 51:16
    at CERN that we basically developed all
    the technology at CERN or try to develop
  • 51:16 - 51:20
    nearly everything at CERN. But then
    production, for example, is put into
  • 51:20 - 51:26
    industry. And yes, these are relatively
    classical power converters. The
  • 51:26 - 51:30
    interesting or like challenging part about
    the current power converters is really
  • 51:30 - 51:33
    that the current has to be measured quite
    precisely and also controlled quite
  • 51:33 - 51:39
    precisely so there we use also DCC TS.
    Which we have also mentioned before. But
  • 51:39 - 51:42
    basically all this controlled mechanism
    there. That's one of the big challenges
  • 51:42 - 51:48
    there.
    Herald: Cool. Microphone number one again.
  • 51:48 - 51:56
    Q: You talked about the orbit clock that
    detects when the bunch is completed one
  • 51:56 - 52:00
    round. How is it possible to detect which
    is the first bunch?
  • 52:00 - 52:04
    Stefan: Yeah. So it is it is actually not
    detected, but this clock is actually
  • 52:04 - 52:07
    something that is constructed. So we
    basically what we do is, we count these
  • 52:07 - 52:13
    cycles of the of the RF cycle. Maybe I can
    open this slide. So somewhere there is a
  • 52:13 - 52:19
    counter that basically knows how many 40
    MHz clock cycles a full rotation takes.
  • 52:19 - 52:22
    And then at some point decides this is
    number one. And that's also where they
  • 52:22 - 52:26
    start counting when they inject bunches
    into the LHC. So there's no marker, let's
  • 52:26 - 52:30
    say. But there is a certain structure to
    the beam. So you could potentially do
  • 52:30 - 52:33
    that. So, for example, for these longer
    periods where the kicker magnets need to
  • 52:33 - 52:37
    ramp up, they have something they call the
    abort gap. So a number of bunches that are
  • 52:37 - 52:41
    never filled but are always kept empty. So
    the magnets have enough time to deflect
  • 52:41 - 52:44
    the beam when the next bunch comes around.
    So you could probably measure that, but
  • 52:44 - 52:47
    it's much easier to do it the other way
    around.
  • 52:47 - 52:55
    Herald: Microphone number four, please.
    Q: You said you had quite tight needs for
  • 52:55 - 53:01
    the timing clock. Is it tight enough? That
    the speed of light was the limit with the
  • 53:01 - 53:04
    distances between locations or that was
    not a concern?
  • 53:04 - 53:09
    Stefan: No, it is a concern. So because
    just distributing a cable for 27
  • 53:09 - 53:15
    kilometers produces like just considerable
    run times of electrical signals. All the
  • 53:15 - 53:18
    delays of all the cables need to be
    measured precisely for their delay and
  • 53:18 - 53:23
    then calibrated out so all the experiments
    get their clocks at the right time,
  • 53:23 - 53:27
    shifted, compensated for the delay time
    that it just takes to get the signal
  • 53:27 - 53:33
    there.
    Herald: And again, the interwebs.
  • 53:33 - 53:40
    Signal Angel: Is it too dangerous to stand
    near the concrete cooling blocks, like
  • 53:40 - 53:47
    radioactive wise or, I don't know.
    Severin: Yes.
  • 53:47 - 53:49
    laughter
    Stefan: Not recommended.
  • 53:49 - 53:55
    Severin: Not recommended. We have a very
    good interlock system. Also, the doors,
  • 53:55 - 53:58
    all the doors have switches. So basically
    when one door is basically like opened
  • 53:58 - 54:03
    then basically the whole machine will be
    shut down. So we have very critical and
  • 54:03 - 54:11
    safety related access system at LHC. Maybe
    you watch Angels and Demons. This
  • 54:11 - 54:17
    Hollywood movie that we have, the eye
    scanners are shown. It's a little bit. I
  • 54:17 - 54:22
    mean, it's Hollywood. But, we have eye
    scanners. So iris scanners. So every time
  • 54:22 - 54:26
    like we want to go to the tunnel, for
    example, then we have to let also our iris
  • 54:26 - 54:30
    be scanned because otherwise we will not
    be able to go to the tunnel. So there's a
  • 54:30 - 54:34
    very sophisticated access system to really
    go to the tunnel. So when there is
  • 54:34 - 54:37
    operation, the whole tunnel access is
    completely blocked.
  • 54:37 - 54:41
    Herald: Good, microphone number one,
    please.
  • 54:41 - 54:48
    Q: What is the exact reason to have each
    of the experiments, every side. I mean, so
  • 54:48 - 54:52
    far apart on the LHC. I mean, on
    opposite sides.
  • 54:52 - 54:58
    Severin: Um, basically, you are talking
    about Atlas and CMS. The reason for that
  • 54:58 - 55:02
    is because when, these two experiments
    were constructed, there was a little bit
  • 55:02 - 55:10
    of fear that particles basically interact
    at the two experiments. So that they
  • 55:10 - 55:14
    really are like the most far away. We like
    to have a very big distance from each
  • 55:14 - 55:18
    other. So there is no interaction between
    them. That's why we basically put them at
  • 55:18 - 55:21
    point one and point five. That's the
    reason why.
  • 55:21 - 55:26
    Herald: If I can see it correctly.
    Microphone number five.
  • 55:26 - 55:32
    Q: Yes, hello. I've seen that you're also
    using the CAN bus. What are you using the
  • 55:32 - 55:37
    CAN bus for in CERN?
    Stefan: I know of at least one use, but it
  • 55:37 - 55:43
    is inside an experiment. So there are, as
    far as I know, investigations under way to
  • 55:43 - 55:48
    use the CAN bus to do the actual control
    of the detectors of one experiment. I
  • 55:48 - 55:51
    don't know if there is a use inside the
    accelerator itself. So apart from the
  • 55:51 - 55:56
    experiments. But perhaps if you come by
    afterwards we can find one.
  • 55:56 - 56:00
    Q: Thank you.
    Herald: Microphone number one.
  • 56:00 - 56:06
    Q: Do you have any official data about how
    many tons of duct taper used in
  • 56:06 - 56:08
    daily operations?
    laughter
  • 56:08 - 56:14
    Severin: No. No.
    Herald: What about zip ties?
  • 56:14 - 56:21
    Severin: Many. Yeah. Millions. Billions.
    Herald: Okay. As far as I can see... Ah,
  • 56:21 - 56:25
    the intercepts again with a question.
    Signal Angel: Do you know your monthly
  • 56:25 - 56:27
    power bill?
    laughter
  • 56:27 - 56:31
    Severin: No, not no. No, sorry.
    Stefan: No. But it is, I think, in fact
  • 56:31 - 56:38
    the contribution of France, which is the
    main contributor in terms of energy. That
  • 56:38 - 56:41
    it is part of their contribution to
    contribute the electricity bill basically
  • 56:41 - 56:44
    instead of paying money to CERN. That's as
    far as I know.
  • 56:44 - 56:48
    Severin: Yes. And also we shut down the
    LHC and the accelerator complex through
  • 56:48 - 56:53
    like the wintertime. And one of the
    reasons for that is because electricity is
  • 56:53 - 56:57
    more expensive during wintertime in France
    than in summer.
  • 56:57 - 57:04
    Herald: In this case, I can't see any
    other questions. I have a maybe stupid
  • 57:04 - 57:10
    question. You said earlier you have to
    focus and defocus the beam. But as we
  • 57:10 - 57:13
    know, you accelerated already the
    particles. Why do we have to focus the
  • 57:13 - 57:17
    beam?
    Severin: Because every time when we have a
  • 57:17 - 57:22
    dipole magnet, then basically we bend the
    particle around an arc. But then we also
  • 57:22 - 57:25
    defocus a little bit. And also, the
    coulomb force is still a problem because
  • 57:25 - 57:30
    we have equally charged particles in the
    bunch or in the whole beam itself. So they
  • 57:30 - 57:34
    will by themselves basically go out of
    each other. And if you would not focus it
  • 57:34 - 57:36
    again, then basically we would lose the
    whole beam in the end.
  • 57:36 - 57:45
    Herald: Oh, thank you. I don't see any
    questions. Internet? In this case thank
  • 57:45 - 57:50
    you very, very much, Stefan and Severin.
    Please. With a warm applause. The Large
  • 57:50 - 57:51
    Hadron infrastructure talk.
  • 57:51 - 57:52
    Applause
  • 57:52 - 57:53
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Title:
36C3 - The Large Hadron Collider Infrastructure Talk
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Video Language:
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Duration:
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