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36c3 preroll music
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Herald: Our next talk's topic is the Large[br]Hadron Collider infrastructure talk. You
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probably know the Large Hadron Collider[br]over at CERN. We heard quite a bit of it
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in the recent talks. This time we will[br]have a deep dive into the infrastructure.
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You can assume our next speakers are doing[br]a great job. Basically, it's pretty
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obvious because we are not stucked into a[br]great into a giant supermassive black
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hole. So please welcome with a very warm[br]applause, Severin and Stefan.
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Applause
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Severin: Yeah. Hello, everyone. Thanks for[br]coming. So many people here, quite nice.
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In the last couple of years we had several[br]talks about, yeah, basically the physics
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perspective of LHC, how physicists analyze[br]data at LHC, how physicists store all the
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data, et cetera. And we would like to give[br]like more an engineering perspective of
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the whole LHC. So three years ago we had a[br]talk by Axel about how physicists analyze
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massive big data and then last year we had[br]a talk conquering large numbers at LHC by
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Carsten and Stefanie. And we would, as[br]I've mentioned already, we would like to
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give like more an engineering perspective.[br]We are Stefan and Severin. We're both
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electrical engineers working at CERN.[br]Stefan is working in the experimental
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physics microelectronics section and he[br]will give a second talk tomorrow about
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designing high reliability digital[br]electronics together with Szymon tomorrow
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morning at 11:30. And I'm, as I mentioned[br]already, also working at CERN. I'm working
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in the electronics for machine protection[br]section. I will describe briefly later. A
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short disclaimer; the LHC is a pretty big[br]machine and we try to explain it as good
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as possible. 45 minutes is not really[br]enough to talk about everything because I
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think you can basically take one of the[br]topics we are talking here about now and
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talk for 45 minutes only about one[br]specific topic, but we try to give an
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overview as good as possible. So imagine[br]you want to build an accelerator in your
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backyard. OK, maybe not in your backyard[br]because LHC is quite big, so 27 kilometers
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in diameter is quite big, but basically we[br]figured out three main challenges you have
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to take. First of all, we have to[br]accelerate particles because otherwise
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it's not a particle accelerator. Second,[br]we have to keep the particles on a
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circular trajectory. And then third, we[br]have to make sure that the particles which
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are inside our beam tube or beam pipe[br]don't collide with anything which is
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there, for example, to beam pipe itself,[br]air molecules, etc. And the solution we
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adopted for LHC there is, that we[br]accelerate the particles with a high power
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radio frequency cavities. Then we have a[br]beam control system which is quite
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sophisticated using superconducting[br]magnets and then we have the beam pipe
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itself, which is evacuated, so it's under[br]vacuum conditions to avoid any collisions
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we have inside with gas molecules, etc. A[br]brief overview about the location itself.
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So probably many of you know already that[br]CERN is next to Geneva. So it's in the
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western-southern part of Switzerland. When[br]we zoom in a little bit more, then we have
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here an artificial like picture of LHC[br]itself in the red circle there. To put it
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a little bit in a perspective, we have a[br]relatively big airport there. You can see
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there, it's a 2200 metre long runway. We[br]have Geneva Lake next to it. And that's
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only one small part of Geneva Lake, but[br]nevertheless, and what also quite nice, we
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see Mont Blanc from LHC, er, from CERN.[br]When we zoom in a little bit more, then we
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basically have the big, circular collider[br]there. That's LHC itself. And we have pre-
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accelerators, I will talk in a few minutes[br]about. Basically we have two main
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campuses: we have Meyrin Site, which is in[br]Switzerland, and we have to Prevessin
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site, which is in France. Then at LHC[br]itself, we have eight service points. We
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also call it just points, to briefly go[br]through all of them; we have point one
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where we have the experiment called ATLAS,[br]one of the big and major experiments at
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LHC. Then at the exactly opposite side of[br]ATLAS, we have CMS at point five. Then we
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have a little bit smaller experiment,[br]which is ALICE. It was basically
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constructed for lead ion runs. We will[br]talk about this later. And then we have
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another relatively small experiment, it's[br]called LHCb. And that's the only non-
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symmetrical experiment at LHC. These are,[br]I think, the four experiments you already
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maybe heard of. Then there are four or[br]three other smaller experiments. We have
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LHCf at point one, it's a forward[br]scattering experiment at point one. So
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basically, they're taking data like[br]scattered particles from ATLAS itself.
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Then we have TOTEM. It's also a forward[br]scattering experiment and point five, then
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we have, sorry, we have MOEDAL, which is[br]the experiment at point eight. They're
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looking for magnetic mono-poles. Then we[br]have TOTEM, sorry for that, at point five.
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And then we have a relatively new[br]experiment which is called PHASER. It's
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actually under construction and it will be[br]used, starting from 2021 and it's forward
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scattering experiment, which, where they[br]try to detect neutrinos. Then we have
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point four, there we have the RF cavities[br]to accelerate the particle beam itself. We
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have the beam dump area. So when there is[br]like a fault in a machine or we just want
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to dump the beam, then we used the mean[br]dump system at point six. And then we have
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two more general service areas. It's point[br]three and point seven. LHC would not be
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possible without the pre-accelerator[br]complex. So we have a relatively big one
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and it's also sometimes relatively old. On[br]the left hand side of the slide you can
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see LINAC2, it's an old linear accelerator[br]which was used until last year. It's not
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now phased out. And now we have LINAC4,[br]which is also a linear accelerator and it
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has a little bit higher acceleration. Then[br]we have the proton synchrotron booster.
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It's the first circular collider. So you[br]can see two pictures there. What is
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relatively special about PSB is that we[br]have there two, sorry, four beam pipes
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instead of just one beam pipe. Then we[br]have the proton synchrotron accelerator,
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which is the next stage for acceleration.[br]It then has only one one beam pipe. And
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then we are going from PS we are going to[br]SPS, which is the super proton synchrotron,
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which is has circumferences of seven[br]kilometers. There we basically accelerate
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the particles the last time and then they[br]are injected it in the LHC itself. We
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mentioned a few accelerators already,[br]basically all everything we just
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highlighted here. But CERN is a little bit[br]more. So CERN is famous for LHC, I would
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say. But there is much more than only the[br]LHC. So only about 15 percent of the
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protons, which are accelerated in the pre-[br]accelerator complex, are really going to
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the LHC itself. So there is much more:[br]there is material science, there is anti
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matter research and all different other[br]kinds of research going on. Of course,
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everything has to be controlled. It's[br]called a CCC, the CERN control center.
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It's located at, the Prevessin site;[br]looks like that. Basically, we have, four
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Cs looking to each other and there the[br]operators are sitting 24/7 and operate the
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whole machine. So basically, the whole[br]pre-accelerator complex, all the energy
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cryogenics and LHC itself. Before you ask,[br]everything is running on Scientific Linux.
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So we have basically our own Linux[br]distributed distribution, which is used
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there and it of course, it's open source.[br]Talking about the LHC beam itself, we have
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two beams: one is running clockwise and[br]the other one is to running anti-
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clockwise because we don't have a fixed[br]target experiment where we basically let
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the accelerated particles colliding with[br]like a fixed target, like metal or
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something like that. We have controlled[br]collisions at four points, we mentioned
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before. Most of the year we have proton[br]runs. So we have protons and protons
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colliding each towards each other. And[br]then we have at the end of the year,
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nearly starting from November to December,[br]we have lead ion run. The protons itself
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is not really like a fixed, straight line[br]of particles. We have something called
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bunches. You can imagine a little bit like[br]spaghetti. It's basically the same length
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of a spaghetti, but it is much thinner[br]than a spaghetti. And each bunch, when you
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have a proton run, then each bunch[br]consists of approximately 100 billion
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protons. And when you have lead ion runs,[br]then we have approximately 10 million lead
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ions in LHC. And last year we operated[br]with 2565(sic) bunches in the LHC itself.
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The LHC tunnel. We already talked about[br]the tunnel itself. It is 27 kilometers
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long and you can see maybe a little bit on[br]this graph, that we have some, we have
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eight straight sections and we have eight[br]arcs in the tunnel. Basically the straight
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sections are always there, where we have[br]like service cavities or we have areas and
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also the experiments. Because it's not so[br]good visible in this picture, I put the
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picture here. Basically, that's a straight[br]section of LHC. You can basically just see
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the beam pipe itself, with aluminum foil[br]around it, and there are also no magnets.
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And when we look in the arc section of[br]LHC, then you see here the arc itself and
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I think it's quite famous picture of LHC[br]itself because we have blue dipole magnets
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there. The tunnel itself is an old tunnel,[br]used previously by LEP, the large electron
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proton collider. It has a diameter of 3.8[br]metres and the circumference is
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approximately 27 kilometers. Inside the[br]tunnel we have, first of all, cryogenics,
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so we have big tubes, stainless steel[br]tubes to carry all the cryogenic. So
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liquid helium and gaseous helium. Then we[br]have the magnet itself to bend the
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particles and then we have electrical[br]installations to carry like signals from
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the magnets to have safety systems,[br]electricity, etc, etc. Geography is a
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little bit complicated in the area because[br]we have in the western part of LFC we have
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the Jura mountain range and this Jura[br]mountain range has a relatively hard
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material. It's not made out of, not made,[br]but nature. I mean it's a limestone, so
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it's relatively complicated to dig into[br]this material, in comparison to all the
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other areas at LHC. So when you would[br]basically put a straight section of LHC,
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then you have to dig much more into the[br]relatively hard limestone. So that's why
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it was decided that the LEP or LHC tunnel[br]is tilted a little bit. So we have a tilt
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angle of one 1.4 percent there. The depth[br]is approximately between 50 meters at
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point one or point eight, up to 170 metres[br]deep at point four. We already talked a
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little bit about magnets, but we would[br]like to go a little bit more in the
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details now. So why do we need magnets?[br]Um, maybe you learned at school that when
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you have a magnetic field and you have[br]charged particles and you can bend
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particles around an arc in a magnetic[br]field. Depending on the charge of the
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particles, you bend them around on the[br]right side or left side, that's this
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famous right hand and left side rule, you[br]maybe learned during school. And at LHC we
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cannot use a normal magnets like typical[br]magnets. We have to use electromagnets
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because normal magnets would not be strong[br]enough to build in an electromagnetic
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field which is feasible to bend the[br]particles around the whole tunnel. At LHC
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we use, in the dipole magnets, a magnetic[br]field of 8.3 Tesla. And to do this we need
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a current of 11850 amps. We have basically[br]two different types of magnets. We have
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bending magnets. So the dipole magnets I[br]mentioned quite often already. And then we
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have injection and extraction magnets.[br]They are also dipole magnets there, but
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they are a little bit differently[br]constructed, because the injection and
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extraction magnets have to be quite fast,[br]because they have to basically be powered
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up at full, the full magnetic field in[br]several microseconds. Then we have higher
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order magnets which are quadrupole pool,[br]magnet, sextupole magnets and octupole
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magnets, et cetera, et cetera. And they[br]are used for focusing and defocusing the
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beam itself. In total, we have 1200 dipole[br]magnets at LHC. We have around 850
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quadrupole magnets and we have 4800 higher[br]order magnets. But they are normally quite
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short or so, shorter than the the other[br]magnets. The dipole magnets consist of two
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apertures. They are used to bend to beam[br]around, so I already said. In the middle
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of the magnet itself we have a cold bore.[br]So there is basically there are the
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particles flying around. Then there is a[br]metallic structure. You can see this in
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the picture. It is just a shiny metallic[br]sphere you see there. And then we have
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next to the cold bore, we have the tool,[br]the two apertures to bend the particle and
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build the magnetic field itself. The[br]dipole magnets have a length of 50 meters
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and the manufacturing precision is plus[br]minus one fine, one point five percent,
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er, one point five millimeters. Then we[br]have quadrupole magnets. They are used for
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focusing and defocusing the beam. The[br]problem is that we have bunches, were are
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basically equally charged particles inside.[br]And the Coloumb force tells us, that when
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we have equally charged particles, then[br]they are basically want to fade out from
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each other and in the end they would just[br]hit the beam pipe itself and we could
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maybe destroy the beam or cannot do any[br]collisions. So what we do is we use a
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quadruple magnets as, yeah, similar to[br]lenses, because we can focus and defocus
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the beam. The quadrupole magnets, the name[br]already suggested it, that we have
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basically four apertures. So we have on[br]the left and the right side two and then
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we have on top and bottom we have also a[br]few of them. To go a little bit into
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detail about the focusing and defocusing[br]scheme. In the beginning we have a
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particle beam which is not focused, but we[br]want to focus it. Then we go to the first
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quadrupole magnet. So we focus the beam.[br]And this is only done in one axis. That's
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a little bit a problem. So, in the second[br]axis we don't have any focus and we have a
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defocusing effect there. And then we have[br]to use a second quadrupole magnet for the
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other axis, in this case the Y-axis to[br]focus the beam even further. And you can
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even see this here in the Z-axis, that's[br]basically the cut off the beam itself. You
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can also see that in the beginning we have[br]on the left side, we have an unfocused
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beam and then we focus it in one axis, so[br]we have like a little bit more ellipse and
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then we focus in the other direction, then[br]we have a different ellipse. So we have to
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use several quadruple magnets in a row to[br]really focus the beam in the way we want
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to have it. In the LHC magnets, we have[br]quite high currents. We we need these
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currents, because otherwise we cannot bend[br]to the very high energetic particle beam
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and to use normal conducting cable, it[br]would not be possible to basically build a
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magnet out of it. So what we do is, we use[br]materials which are called superconducting
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materials, because they're for very good[br]effect. They go to basically zero
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resistance at a specific temperature[br]point. And after this point or when we
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basically go lower, then the current can[br]flow without any losses inside of it. But
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to reach the state, we have to cool the[br]magnets quite heavily, which is not so
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easy, but it can be done. And on the right[br]side you basically see a very historic
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plot. That was 1911 in Denmark, a[br]researcher called Heike Onnes detected for
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the first time superconducting effect in[br]mercury. And it was detected at 4.19
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Kelvin. To show you a little bit the[br]comparison between a normal conducting
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cable and a superconducting cable, as we[br]put the picture here. So that is basically
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the same amount of cable you need to use[br]to carry, the thirteen thousand amps and
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to do the same or to transport the same[br]amount of energy we also can use a very
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small superconducting cable and I think[br]it's quite obvious why we use here
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superconducting cables. At the LHC we use[br]Niobium tin(sic) as material. And this
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material basically goes into a[br]superconducting state at 10 Kelvin. But to
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have a safe operation to LHC, we have to[br]cool it down at 1.9 Kelvin.
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Superconducting magnets have some[br]benefits, but also some downsides, so
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sometimes they change their state because[br]there are small rigid vibrations and the
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magnet or the temperature's not precise[br]enough or the current is too high, then
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they change that state and it's called[br]"Quench". And, we basically can detect a
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Quench when we measure the voltage across[br]the magnet, because the resistance changes
0:17:29.570,0:17:33.519
at this point. So when there is a Quench[br]then the resistance changes quite rapidly,
0:17:33.519,0:17:38.330
in milliseconds and we can detect this[br]voltage rise with sophisticated
0:17:38.330,0:17:42.530
electronics. On the right side, you see a[br]board I'm working on. So basically here,
0:17:42.530,0:17:47.610
we have a measuring system to measure the[br]voltage across the magnet. And then we
0:17:47.610,0:17:54.340
have a detection logic implemented in FPGA[br]to basically send triggers out and open an
0:17:54.340,0:17:58.549
Interlock loop. Interlock loop is a system[br]at LHC. You can imagine that little bit
0:17:58.549,0:18:03.269
like a cable going around the whole tunnel[br]and there are thousands of switches around
0:18:03.269,0:18:08.510
this Interlock loop. And as soon as one of[br]the detection systems basically opens to
0:18:08.510,0:18:13.669
the interlock loop, then basically the[br]whole machine will be switched off. And
0:18:13.669,0:18:16.530
what means switched off is basically, that[br]we will power down the power converter,
0:18:16.530,0:18:20.039
but then the energy is still in the[br]superconducting magnet and it has to be
0:18:20.039,0:18:24.190
taken out of the superconducting magnet.[br]And therefore, we use dump resistors to
0:18:24.190,0:18:30.600
extract the energy. And here you can see a[br]picture of such a dump resistor. It's
0:18:30.600,0:18:34.960
quite big. It's in a stainless steel tube,[br]oil cooled. It's approximately three or
0:18:34.960,0:18:41.470
four meters long. And basically, when[br]there was a Quench, and the energy was
0:18:41.470,0:18:44.480
extracted via these resistors, the whole[br]resistor is heated up by several hundred
0:18:44.480,0:18:50.179
degrees and it needs several hours to cool[br]it down again. Power converters; the power
0:18:50.179,0:18:54.679
converters are used to power the magnet[br]itself. So they can produce a current of
0:18:54.679,0:19:01.110
approximately 13000 amps and a voltage of[br]plus minus 190 volts. And you can see a
0:19:01.110,0:19:06.310
picture how here, how big it is. One[br]downside with the power converters is that
0:19:06.310,0:19:10.269
they have to be, not downside but one[br]difficulty is, that they have to be very
0:19:10.269,0:19:16.210
precise, because every instability in the[br]current would have or has a direct effect
0:19:16.210,0:19:20.290
on the beam stability itself. So basically[br]the beam would be not diverted in the
0:19:20.290,0:19:27.169
right amount of length. So that's why they[br]have to be very precise and have to have a
0:19:27.169,0:19:32.409
very precise stability. So here I just[br]pointed out, like in 24 hours, the power
0:19:32.409,0:19:36.700
converter is only allowed to have a[br]deviation of 5 ppm. And in comparison, for
0:19:36.700,0:19:41.970
13000 amps we have a deviation of 65 milli[br]amps. So the power converters have to be
0:19:41.970,0:19:46.600
very precise. And to do that, we had to[br]develop our own ADC, because at the time
0:19:46.600,0:19:51.270
when LHC was built, there was no ADC on[br]the market which was able to have this
0:19:51.270,0:19:55.900
precision and also the whole ADC is put[br]into a super-precise temperature
0:19:55.900,0:20:02.970
controlled areas and it is calibrated[br]quite regularly. Okay, cryogenics. We
0:20:02.970,0:20:05.520
already talked about that we have[br]superconducting magnets and they have to
0:20:05.520,0:20:11.340
be cooled down quite low. So the[br]superconducting magnets we have at LHC has
0:20:11.340,0:20:17.230
have to be cooled down to 1.9 Kelvin. And[br]we are doing this when we like start the
0:20:17.230,0:20:21.610
LHC then we cool down on the first hand[br]with liquid nitrogen. So approximately six
0:20:21.610,0:20:26.429
thousand tonnes of liquid nitrogen are put [br]through the magnets to cool them down
0:20:26.429,0:20:33.260
to 18 Kelvin and afterwards we cool the[br]magnets down with liquid helium. And
0:20:33.260,0:20:38.960
liquid helium is at 1.9 or 1.8 Kelvin. And[br]to put it a little bit in a comparison,
0:20:38.960,0:20:42.200
outer space, so when we measure like the[br]temperature of space, we have
0:20:42.200,0:20:47.799
approximately 2.7 Kelvin in outer space.[br]So LHC is much colder than outer space.
0:20:47.799,0:20:52.289
The whole cooldown needs approximately one[br]month and each dipole magnet, which is 15
0:20:52.289,0:20:57.010
meters long, shrinks several centimeters[br]during that. Which also has to be taken
0:20:57.010,0:21:02.870
into account, because otherwise pipes[br]would break. The cryogenic system is that
0:21:02.870,0:21:08.980
we have at each of the eight points at LHC[br]we have compressors to cool down the
0:21:08.980,0:21:13.860
liquid helium or the helium itself. And[br]then we compress the helium and pump it
0:21:13.860,0:21:17.970
down. We have one gaseous helium stream,[br]which is at 15 Kelvin and we have liquid
0:21:17.970,0:21:24.490
helium stream at approximately 4.5 Kelvin.[br]And then we pump it underground and then
0:21:24.490,0:21:28.580
we have something called Cold Compression[br]System. And the Cold Compression System
0:21:28.580,0:21:36.090
even further reduces the pressure of the[br]helium that we have in the end a helium,
0:21:36.090,0:21:40.980
which is at 1.8 Kelvin. So it can really[br]cool down the magnet itself. And helium
0:21:40.980,0:21:45.600
has a very interesting effect because at[br]2.1 Kelvin, it becomes something called
0:21:45.600,0:21:52.350
superfluid. So it basically can run around[br]like holes, for example, or walls. It can
0:21:52.350,0:21:57.679
basically flow against gravity, which is[br]quite interesting. And it has also very
0:21:57.679,0:22:03.860
high thermal conductivity and that's also[br]why we use superfluid helium here. And
0:22:03.860,0:22:08.370
that's why we cool down the whole magnets[br]that low. And one other interesting effect
0:22:08.370,0:22:13.570
is also that the LHC tilt angle, which is[br]1.4 percent, has to be taken into account
0:22:13.570,0:22:18.320
because we have very low pressure inside[br]all the tubes or all the system, at 16
0:22:18.320,0:22:24.539
millibars. But we have sometimes to pump[br]the helium against gravity or going down.
0:22:24.539,0:22:28.481
So we also have to take into account the[br]LHC tilt angle to not have wrong pressure
0:22:28.481,0:22:34.519
levels at the whole LHC itself. Okay.[br]Stefan: All right! So, you probably
0:22:34.519,0:22:38.649
already got the idea, that what we've done[br]in the last 20 minutes, was only solve the
0:22:38.649,0:22:42.250
first of the three challenges we had,[br]which was actually bending the beam around
0:22:42.250,0:22:49.269
the circular trajectory. So I'm trying to[br]go to the other challenges we have lined
0:22:49.269,0:22:54.350
up in the beginning. And the first one of[br]that is the actual acceleration of the
0:22:54.350,0:22:59.700
particle beam. And large synchrotrons,[br]e.g. like the LHC, they use radio
0:22:59.700,0:23:05.299
frequency or RF systems to do this[br]acceleration. And I'm just going to do a
0:23:05.299,0:23:10.510
quick recap of the LHC beam and RF and how[br]they interact. So Severin mentioned
0:23:10.510,0:23:15.600
already that the particles in LHC actually[br]come in bunches. So in like packets that
0:23:15.600,0:23:20.309
contain about hundred billion protons and[br]those bunches are spaced when they are
0:23:20.309,0:23:26.470
running around the LHC approximately 25[br]nanoseconds apart. And starting from that
0:23:26.470,0:23:31.250
the tasks of the RF system are basically[br]twofold. It first has to ensure that these
0:23:31.250,0:23:35.630
bunches are kept tightly together in a[br]process that we call longitudinal
0:23:35.630,0:23:39.700
focusing. And the second task is to care[br]for the actual acceleration of the
0:23:39.700,0:23:44.570
particle bunches. So from their injection[br]energy, when they come from one of the
0:23:44.570,0:23:49.039
pre-accelerators up to their final energy,[br]that they are supposed to collide at
0:23:49.039,0:23:55.549
during the physics run. So in general, you[br]can imagine RF as being a quickly
0:23:55.549,0:24:03.260
alternating electric and magnetic field[br]components. And in the LHC, this RF energy
0:24:03.260,0:24:07.700
is basically injected into what is called[br]a cavity, which is a resonant structure.
0:24:07.700,0:24:11.520
And there the particle beams travels[br]through, while the field quickly
0:24:11.520,0:24:16.690
alternates and the RF signal, or the[br]energy, basically interacts with the
0:24:16.690,0:24:21.750
particle beam. So perhaps you know that[br]the protons are positively charged and
0:24:21.750,0:24:26.210
then a negative polarity of the field[br]would attract these protons, while the
0:24:26.210,0:24:32.320
positive field location would basically[br]move them away. And this has ... well,
0:24:32.320,0:24:36.260
after just injecting and with the[br]frequency of this RF field being the same
0:24:36.260,0:24:41.390
as the speed that the particles actually[br]go round the LHC, solves the first of the
0:24:41.390,0:24:45.110
two problems, which was the the focusing[br]because actually the particles that are
0:24:45.110,0:24:49.370
too slow arrive only when the field is[br]already changed to the opposite polarity
0:24:49.370,0:24:52.990
and actually get accelerated a bit, while[br]the particles that are too fast, they are
0:24:52.990,0:24:58.000
actually being decelerated a bit. And this[br]is a process that we call the longitudinal
0:24:58.000,0:25:03.490
focusing, which makes sure that the[br]bunches stay neatly packed together. And
0:25:03.490,0:25:06.090
of course this would be relatively[br]inefficient if we would only change the
0:25:06.090,0:25:12.399
polarity of this field once for each of[br]the proton bunches that pass by. Which is
0:25:12.399,0:25:15.870
why we do it ten times. So the polarity[br]basically changes ten times or the
0:25:15.870,0:25:21.370
frequency is ten times higher than the[br]bunch crossing frequency. And by doing
0:25:21.370,0:25:25.960
that, we make sure that the change of this[br]field is much faster and therefore the
0:25:25.960,0:25:33.070
particle bunches are packed much closer[br]together and the focusing is better. So
0:25:33.070,0:25:35.389
here you can see these cavities that were[br]shown in the previous picture as a
0:25:35.389,0:25:39.899
schematic, how they're actually placed in[br]the tunnel. So eight of these huge
0:25:39.899,0:25:43.850
cavities are used per beam and they are[br]the actual thing that is used to couple
0:25:43.850,0:25:49.730
the RF energy into the beam and transfer[br]it to the particles. They are also
0:25:49.730,0:25:54.179
operating superconductively, so at[br]cryogenic temperatures, to reduce the
0:25:54.179,0:25:59.610
thermal stress and the losses that would[br]otherwise occur in their materials. And
0:25:59.610,0:26:01.270
these are actually – even though they are[br]so big, similar to the magnets that had to
0:26:01.270,0:26:05.820
be very precisely manufactured – these[br]also have very small manufacturing
0:26:05.820,0:26:12.070
tolerances and have to be precisely tuned[br]to the RF frequency that is used to
0:26:12.070,0:26:17.440
inject. So and the second part of this,[br]that actually produces this high power RF
0:26:17.440,0:26:22.420
signal. For that is used what we call[br]Klystrons. So Klystrons are basically RF
0:26:22.420,0:26:29.330
amplifiers. They are built from high power[br]RF vacuum tubes and they amplify this 400
0:26:29.330,0:26:34.289
MHz signal that is used to transfer energy[br]to the particles. And each of those
0:26:34.289,0:26:40.100
Klystrons produces about 300 kW of power[br]and you can probably imagine how much that
0:26:40.100,0:26:43.889
power for an individual unit that is, if[br]you know that your microwave oven has like
0:26:43.889,0:26:49.460
2 or 3 kW. And of course, as we have eight[br]cavities per beam and one Klystron always
0:26:49.460,0:26:54.840
feeds one cavity, we in total have 16 of[br]those Klystrons and they are in principle
0:26:54.840,0:27:03.659
able to deliver a total energy of 4.8 MW[br]into the LHC beam to accelerate it. But if
0:27:03.659,0:27:06.640
we take a small step back for now, we have[br]only solved the first of the two problems,
0:27:06.640,0:27:12.510
which was to keep the bunches neatly[br]focused. Because currently the particles
0:27:12.510,0:27:17.429
have been injected and the frequency is at[br]some specific frequency and actually they
0:27:17.429,0:27:22.400
are only running basically in sync, the[br]two. So what we do after all the particle
0:27:22.400,0:27:26.410
bunches from the pre-accelerators have[br]been injected into LHC, is that we ever so
0:27:26.410,0:27:30.700
slightly increase the frequency, which of[br]course also means that the particles need
0:27:30.700,0:27:35.399
to accelerate together with the RF signal.[br]And this is the mechanism that we use to
0:27:35.399,0:27:40.159
accelerate them actually. And the change,[br]that is required to do this, is very tiny,
0:27:40.159,0:27:44.419
actually. So it is less than a thousandth[br]of a percent sometimes, that is used to
0:27:44.419,0:27:48.379
change the frequency to actually make them[br]go so much faster. So from their
0:27:48.379,0:27:54.070
relatively low injection energy up to the[br]top energy plateau that they need to have
0:27:54.070,0:28:00.409
to produce the actual physics collisions.[br]And an interesting question to ask here is
0:28:00.409,0:28:03.990
where does this signal actually comes from[br]if it needs to be so precisely tuned to
0:28:03.990,0:28:10.250
some specific frequency? Who generates it[br]or who controls it? And that opens up the
0:28:10.250,0:28:15.669
whole complex of the timing of the LHC, of[br]the machine. So actually this first signal
0:28:15.669,0:28:21.460
that I mentioned, this RF signal, it[br]originates in a Faraday cage. So an
0:28:21.460,0:28:25.880
especially shielded area somewhere on the[br]Prévessin site of CERN. And from there it
0:28:25.880,0:28:33.440
is distributed to the low-level RF[br]subsystem with the Klystrons and the
0:28:33.440,0:28:38.690
cavities. But inside this room, there are[br]also a number of other signals generated.
0:28:38.690,0:28:42.720
The first one of that being this Bunch[br]Crossing Clock, which is the actual clock
0:28:42.720,0:28:48.240
that signals one pulse, basically every[br]time, it changes polarity one time a
0:28:48.240,0:28:54.640
proton bunch moves across a specific[br]location inside the LHC. And another one
0:28:54.640,0:28:59.470
is the so-called orbit clock, which always[br]indicates the start of the first or when
0:28:59.470,0:29:04.990
one proton bunch has basically re-arrived[br]at the same position and has completed one
0:29:04.990,0:29:10.820
orbit. And you may ask the question why[br]this is an important piece of information.
0:29:10.820,0:29:16.529
But if you think back to this image that[br]Severin has already shown, about the
0:29:16.529,0:29:21.789
accelerator complex, the big challenge[br]that all this brings is also the whole
0:29:21.789,0:29:24.909
synchronization of all these machines.[br]Because you have to imagine that while
0:29:24.909,0:29:29.790
these proton bunches run around the LHC[br]and new ones are supposed to be injected
0:29:29.790,0:29:34.000
from the outside, from another pre-[br]accelerator, this has to be very precisely
0:29:34.000,0:29:37.840
synchronized. So all these pre-accelerator[br]systems actually share a common
0:29:37.840,0:29:42.840
synchronized timing system that allows[br]them to precisely inject a new packet of
0:29:42.840,0:29:49.380
bunches at the right position, at the[br]right location into the LHC. And this a
0:29:49.380,0:29:53.309
bit how such a timing distribution system[br]looks like. It is only a very small
0:29:53.309,0:29:56.430
excerpt of what it looks like, but it[br]gives you an idea that somewhere
0:29:56.430,0:30:01.100
underground in the LHC there is rooms full[br]of equipment that is just used to
0:30:01.100,0:30:06.110
distribute timing signals between[br]different parts of the accelerator. And of
0:30:06.110,0:30:11.909
course, as CERN is forward-thinking and[br]realized that future colliders will need
0:30:11.909,0:30:15.400
quite a bit more of all this[br]synchronization and that the requirements
0:30:15.400,0:30:20.050
for how precisely everything needs to be[br]synchronized is ever growing, they
0:30:20.050,0:30:23.300
actually developed their own timing[br]distribution standard which is also
0:30:23.300,0:30:28.270
openly available and available for[br]everybody to use. So if you're interested,
0:30:28.270,0:30:34.320
look that up. But of course, not only the[br]accelerator itself is interested in this
0:30:34.320,0:30:40.309
information about what particles are where[br]and how quickly they interact or how
0:30:40.309,0:30:45.050
quickly they go around. But also all the[br]experiments need this information, because
0:30:45.050,0:30:50.070
in the end they want to know "Okay, has a[br]collision occurred at some specific time
0:30:50.070,0:30:54.940
in my experiment?" and actually providing[br]this timing information about when bunches
0:30:54.940,0:30:59.789
have crossed their experiment locations is[br]also vital for them to really time tag all
0:30:59.789,0:31:06.149
their collision data and basically track[br]which bunches were responsible for what
0:31:06.149,0:31:11.559
kind of event or what event throughout[br]their whole signal storage and processing
0:31:11.559,0:31:17.120
chain, let's say. Good. So that is[br]basically challenge 2 out of the way. So
0:31:17.120,0:31:19.889
that was the acceleration of the actual[br]particles and all the associated issues
0:31:19.889,0:31:25.899
with timing. And the third issue we[br]mentioned was that the particles need to,
0:31:25.899,0:31:31.330
let's say, be kept from colliding with[br]anything but themselves or the other beam.
0:31:31.330,0:31:36.210
And that is what we, why we need vacuum[br]systems for. So, again, it is not as
0:31:36.210,0:31:41.200
simple as just putting a vacuum somewhere.[br]Of course not. Because in fact, there is
0:31:41.200,0:31:45.740
not only one vacuum system at LHC, but[br]there are three. So, the first two of
0:31:45.740,0:31:51.090
those are perhaps a bit less interesting[br]to most of us. They are mainly insulation
0:31:51.090,0:31:56.919
vacuum systems that are used for the[br]cryogenic magnets. So they isolate,
0:31:56.919,0:32:02.789
basically thermally isolate the magnets at[br]those very cool temperatures from the
0:32:02.789,0:32:08.789
surrounding air to avoid them getting more[br]heat load than they need to. And there is
0:32:08.789,0:32:11.830
an insulation vacuum also for the helium[br]distribution lines that are actually
0:32:11.830,0:32:16.299
distributing, delivering the helium to[br]these magnets. And then the third one,
0:32:16.299,0:32:19.820
which is perhaps the most interesting one,[br]is the beam vacuum. So the one where
0:32:19.820,0:32:24.999
actually the beam circulates inside the[br]LHC. And this is a cross section of what
0:32:24.999,0:32:30.242
this beam vacuum typically looks like. So[br]it is approximately this size, so a very
0:32:30.242,0:32:37.470
... handful, let's say. And the question[br]you may ask "OK, if I want to keep all the
0:32:37.470,0:32:41.590
like the particles in my particle beam[br]from colliding with anything they are not
0:32:41.590,0:32:47.340
supposed to, for example, rest molecules[br]of remaining air there, how many molecules
0:32:47.340,0:32:51.240
can there still be?" So somebody has to[br]make up that number. And typically you
0:32:51.240,0:32:55.880
express this as a quantity called the[br]beam lifetime, which basically says if you
0:32:55.880,0:33:00.870
were only to keep those particles[br]circulating in the accelerator, how long
0:33:00.870,0:33:04.620
would it take until they have all[br]dispersed and lost their energy due to
0:33:04.620,0:33:10.129
colliding with rest gas molecules? And it[br]was decided that this should be at a value
0:33:10.129,0:33:15.230
of 100 hours, is what the beams should[br]basically be able to circulate without
0:33:15.230,0:33:19.340
collisions, without being lost. And this[br]gave the requirement for pressures down to
0:33:19.340,0:33:24.700
about 100 femtobar, which is a very small,[br]very, very tiny fraction of the
0:33:24.700,0:33:29.679
atmospheric pressure we have here, which[br]is about 1 bar. And to actually get to
0:33:29.679,0:33:34.019
this level of vacuum, it requires multiple[br]stages and multiple components to actually
0:33:34.019,0:33:42.399
get there. So the initial vacuum inside[br]these beam tubes, which are basically
0:33:42.399,0:33:48.610
going throughout the whole LHC tunnel, has[br]the volume of approximately the Notre-Dame
0:33:48.610,0:33:53.800
cathedral. So the first step of getting[br]all the air out of these beam tubes is
0:33:53.800,0:34:00.179
using turbomolecular pumps. And then there[br]needs to be more mechanisms to reduce the
0:34:00.179,0:34:04.000
pressure even further, because these pumps[br]are not able to reduce the pressure to the
0:34:04.000,0:34:09.231
levels required. And they actually use a[br]relatively clever trick to do that, which
0:34:09.231,0:34:16.169
is the use of cryopumping. So the, ... I[br]cannot show that? Okay. So the outer wall
0:34:16.169,0:34:20.380
of this beam pipe cross section that you[br]see here is actually also where the very
0:34:20.380,0:34:27.230
cold helium inside the magnets is outside[br]of. And what that does is, it leads to an
0:34:27.230,0:34:31.589
effect called cryopumping. So actually any[br]rest gas molecule that hits this wall
0:34:31.589,0:34:35.990
actually condenses there. And as the[br]molecules condense there, they are of
0:34:35.990,0:34:40.510
course removed from the atmosphere inside[br]this beam pipe, which removes them from
0:34:40.510,0:34:44.750
the atmosphere and increases the quality[br]of the vacuum. And with the use of this
0:34:44.750,0:34:47.760
and then the warm sections, the use of[br]getter coatings, which are basically able
0:34:47.760,0:34:53.609
to trap gas molecules, you are able to[br]reach the crazy vacuum levels that are
0:34:53.609,0:34:59.020
required to make this happen. But they[br]realized also during the design that one
0:34:59.020,0:35:05.430
big problem – for the first time in an[br]accelerator – another effect will create a
0:35:05.430,0:35:08.540
significant problem for the vacuum, which[br]is the generation of synchrotron
0:35:08.540,0:35:15.240
radiation. So synchrotron radiation is a[br]byproduct of when you do bend a particle
0:35:15.240,0:35:20.220
beam, it results in a phenomenon called[br]synchrotron radiation. And when this
0:35:20.220,0:35:24.369
synchrotron radiation, as it goes straight[br]on and is not bent, hits the walls of this
0:35:24.369,0:35:30.170
vacuum system, or in this case of the beam[br]pipe, it actually liberates molecules from
0:35:30.170,0:35:33.810
there and reintroduces them into the[br]vacuum, which of course then makes the
0:35:33.810,0:35:40.230
vacuum worse again. An additional problem[br]that gives the synchrotron radiation is,
0:35:40.230,0:35:44.960
that it also gives a significant heat[br]load, and if you need to dissipate all
0:35:44.960,0:35:49.660
this heat that is generated through the[br]very cold helium, this is not a very
0:35:49.660,0:35:53.820
efficient process. Because making this[br]helium so cool, is actually a very energy
0:35:53.820,0:35:58.590
intensive process. And just removing a[br]single watt of thermal power through the
0:35:58.590,0:36:03.230
superfluid helium costs about 1 kW of[br]energy. So that is not the most efficient
0:36:03.230,0:36:07.770
part. And this is why the cross-section[br]you have just seen includes another large
0:36:07.770,0:36:11.200
component, which also technically belongs[br]to the vacuum system, which is called the
0:36:11.200,0:36:15.940
beam screen. And this beam screen is[br]basically another tube running inside the
0:36:15.940,0:36:20.760
beam pipe, of which we have, of course,[br]two, which run inside the magnet cold
0:36:20.760,0:36:25.200
bores. And it shields the synchrotron[br]radiation heat load from the outer walls,
0:36:25.200,0:36:30.550
which are at 1.8 Kelvin, while this pipe[br]itself is actively cooled to only about 20
0:36:30.550,0:36:36.240
Kelvin of temperature, which is much more[br]efficient to dissipate this heat. So it is
0:36:36.240,0:36:39.440
basically a steel tube about one[br]millimeter thick. It has these pumping
0:36:39.440,0:36:46.920
holes, where hydrogen gas molecules can go[br]out of, and on the inside it has a copper
0:36:46.920,0:36:51.970
coating, which is used to reduce its[br]electrical resistance, which is required
0:36:51.970,0:36:55.530
because the beam, while it circulates,[br]also induces current that would otherwise
0:36:55.530,0:37:00.180
flow inside this tube, which is really, if[br]you think about it, only a simple tube and
0:37:00.180,0:37:03.950
it would increase the heat load again. So[br]a lot of engineering already has to go
0:37:03.950,0:37:11.579
into a very simple piece of ... a thing[br]like that. So after having spoken so much
0:37:11.579,0:37:16.450
about all the things required to just make[br]a beam circulate and accelerate and so on,
0:37:16.450,0:37:20.590
now it's probably also time to talk a[br]little bit about the beam itself and how
0:37:20.590,0:37:27.220
to control it and how to instrument, how[br]to measure things about this beam. Even
0:37:27.220,0:37:32.080
without going yet about collisions and[br]doing actual physics experiments. So the
0:37:32.080,0:37:36.760
first important bit that is able to[br]basically control or influence the beam
0:37:36.760,0:37:41.540
here is what's called the beam cleaning or[br]collimation system. So typically such a
0:37:41.540,0:37:47.050
particle beam is not very clean. It always[br]travels associated with what is called
0:37:47.050,0:37:51.640
halo of particles around this core area[br]that is less than a millimeter wide where
0:37:51.640,0:37:57.140
most of the intensity is focused. And[br]these particles outside we want to remove,
0:37:57.140,0:38:00.260
because they otherwise would be lost[br]inside the magnets and for example, would
0:38:00.260,0:38:05.640
lead to quenches of the superconducting[br]magnets. And for collimation, we basically
0:38:05.640,0:38:10.819
use small slits that are adjustable and[br]are located at two main locations of the
0:38:10.819,0:38:15.100
LHC. So they have collimation systems[br]there, with vertical and horizontal slits
0:38:15.100,0:38:21.590
that can be adjusted in width, in order to[br]scrape off all the particles that they do
0:38:21.590,0:38:25.319
want to get rid of and extract out of the[br]beam, while only the core part can
0:38:25.319,0:38:30.490
circulate and produce clean collisions[br]without any background, that otherwise
0:38:30.490,0:38:36.110
would need to be accounted for. And then[br]there is a whole other open topic of beam
0:38:36.110,0:38:39.710
instrumentation. So when you run a[br]particle accelerator, you want to measure
0:38:39.710,0:38:45.220
various quantities and performance figures[br]of such a beam. And that is crucial for a
0:38:45.220,0:38:48.730
correct operation and for the highest[br]performance, getting the highest
0:38:48.730,0:38:52.331
performance from an accelerator. And there[br]are a lot of different types of those, and
0:38:52.331,0:38:58.920
I want to go quickly about ... over why we[br]have them and what we do with them. So the
0:38:58.920,0:39:03.730
first and most basic measurement you want[br]to do, is the beam current measurement. So
0:39:03.730,0:39:08.550
the beam current is a basic accelerator[br]beam intensity measurement. So it gives
0:39:08.550,0:39:13.260
you an idea of how strong the beam that is[br]running inside your accelerator is. And it
0:39:13.260,0:39:18.000
is measured using these DCCTs or DC[br]current transformers. And their basic
0:39:18.000,0:39:22.230
principle of operation is, that while the[br]particles move through this torus, which
0:39:22.230,0:39:26.069
is actually a coil or a transformer,[br]induces a voltage there that you can
0:39:26.069,0:39:30.579
measure and then use to quantify the[br]intensity of this beam. And the big
0:39:30.579,0:39:34.760
challenge here is that the dynamic range,[br]this instrument needs to capture, is
0:39:34.760,0:39:39.930
really large, because it has to operate[br]from the lowest intensity pilot injection
0:39:39.930,0:39:46.119
beams up to the full energy, full number[br]of bunches running inside the LHC. So it
0:39:46.119,0:39:51.580
has to cover six orders of magnitude of[br]measurement dynamic range. Then the second
0:39:51.580,0:39:57.130
thing when talking about collisions is the[br]luminosity measurement. So luminosity is a
0:39:57.130,0:40:02.210
quantity basically said to measure the[br]rate of interaction of the particle beams.
0:40:02.210,0:40:06.710
So to give you an idea of how often[br]interactions happen inside the experiments
0:40:06.710,0:40:11.280
or where you want them to happen. And this[br]measurement is used to first of all,
0:40:11.280,0:40:14.461
adjust this interaction rate to a target[br]value, which is optimal for the
0:40:14.461,0:40:19.109
experiments to function and to equalize[br]the interaction rates in different
0:40:19.109,0:40:23.970
experiments. So different experiments also[br]are specified to have the same interaction
0:40:23.970,0:40:29.650
rate so they can get the same let's say[br]statistical quality of their data. So it's
0:40:29.650,0:40:33.869
used to equalize those. And then as a[br]third thing, this system is also used to
0:40:33.869,0:40:37.510
measure the crossing angle of the beam. So[br]as you may know, at some point, when the
0:40:37.510,0:40:42.610
beams are collided, they collide at an[br]angle, that is very small. And this angle
0:40:42.610,0:40:47.559
is actually measured also very precisely[br]in order to adjust it correctly. And it is
0:40:47.559,0:40:50.190
measured to less than a thousandth of a[br]degree, which is again a very impressive
0:40:50.190,0:40:54.640
feat, given that the detection principle[br]of this measurement is only measurement of
0:40:54.640,0:40:58.700
some neutral particles that are a result[br]of the particle interaction of the beam
0:40:58.700,0:41:05.080
... of the collision. Okay, so that is[br]number two. Then number three that we have
0:41:05.080,0:41:09.610
is the beam position monitor. Because[br]along the LHC, you also always want to
0:41:09.610,0:41:14.940
know, where the beam is at any given time.[br]So you want to measure the position of the
0:41:14.940,0:41:19.190
beam inside the beam pipe in order to[br]optimally adjust it to the position you
0:41:19.190,0:41:23.740
want to have it. And for that we use these[br]beam position monitors of which we have
0:41:23.740,0:41:27.910
more than a thousand installed along the[br]LHC. So they are typically capacitive
0:41:27.910,0:41:31.780
probes or electromagnetic strip lines. As[br]you can see on top and bottom
0:41:31.780,0:41:36.790
respectively. And they basically are[br]distributed along the LHC and provide
0:41:36.790,0:41:40.300
position of the particle beam along the[br]accelerator, which can then be used to
0:41:40.300,0:41:47.640
tune, for example, the magnets. All right.[br]Then we have beam profile. So after the
0:41:47.640,0:41:51.059
position, that gives you an idea where the[br]beam is, you also want to know its
0:41:51.059,0:41:57.329
intensity distribution. Basically when you[br]do a cut through the beam pipe somewhere,
0:41:57.329,0:42:02.480
you want to know how the intensity profile[br]looks like. And for that we have basically
0:42:02.480,0:42:07.380
two measurement systems. One measures the[br]profile in X and Y directions. So if you
0:42:07.380,0:42:10.400
really would do a cut and it gives you[br]something like this and it's, for example,
0:42:10.400,0:42:14.950
done with wire scanners, which is literal,[br]very thin wire that is moved through the
0:42:14.950,0:42:20.290
beam. And then the current that the beam[br]moving through this wire, generates is
0:42:20.290,0:42:25.280
used to generate such a profile map, when[br]scanning with this wire. The other one is
0:42:25.280,0:42:29.750
the longitudinal profile, which gives you[br]an idea about the quality of your RF
0:42:29.750,0:42:34.040
system and there you want to know how the[br]intensity profile of your beam looks like.
0:42:34.040,0:42:37.690
If you were looking only at one spot of[br]the accelerator and the beam would pass
0:42:37.690,0:42:43.230
by, and you would basically see over time[br]how the intensity looks like. And then as
0:42:43.230,0:42:47.980
the last bit of beam instrumentation,[br]there is the beam loss monitors. So they
0:42:47.980,0:42:53.170
are these yellow tubes, that are located[br]on the outside of mostly all the magnets,
0:42:53.170,0:42:57.951
of the dipole and quadrupole magnets and[br]so on. Again, there's more than a thousand
0:42:57.951,0:43:03.230
of those. And the idea here is that you[br]need a lot of detectors that are basically
0:43:03.230,0:43:07.819
small ionization chambers, which detect[br]any showers of secondary particles that
0:43:07.819,0:43:12.720
are generated when one of the high energy[br]protons are lost somewhere in the magnet
0:43:12.720,0:43:16.760
materials. So these are really used for[br]protection of the system, because if a
0:43:16.760,0:43:21.109
specific threshold of energy loss is[br]detected, then the accelerator needs to be
0:43:21.109,0:43:26.220
quickly shut down. Which is why they have[br]to react in a matter of nanoseconds in
0:43:26.220,0:43:31.260
order to keep the accelerator safe.[br]Because any interaction of the particle
0:43:31.260,0:43:36.059
beam with for example, the magnets could[br]just destroy huge amounts of money and of
0:43:36.059,0:43:41.870
time that would be needed to rebuild. And[br]as a last and final thing, we have spoken
0:43:41.870,0:43:46.260
one or two times already about shutting[br]down the LHC. Which sounds also trivial at
0:43:46.260,0:43:51.799
first, but really is not. So, the last[br]thing here is, what we call the Beam Dump.
0:43:51.799,0:43:56.589
So the energy content that is contained in[br]those particular beams, it can be used,
0:43:56.589,0:44:02.950
could be used if it were shot on a copper[br]target, it could just melt 1000 kilograms
0:44:02.950,0:44:07.599
or one ton of copper instantly. So during[br]beam extraction, the process of getting
0:44:07.599,0:44:12.359
the particle beam outside out of the LHC,[br]this energy needs to be dissipated
0:44:12.359,0:44:16.990
somehow. And for that, this special Beam[br]Dump area is constructed. So there are
0:44:16.990,0:44:20.670
fast kicker magnets, that are used to ...,[br]that are able to ramp up in a really,
0:44:20.670,0:44:24.930
really short amount of time of[br]microseconds. And then the beam is
0:44:24.930,0:44:30.309
carefully and in a controlled manner[br]directed into a set of concrete blocks,
0:44:30.309,0:44:35.339
that is basically big enough to dissipate[br]all this energy, when required. And in the
0:44:35.339,0:44:38.800
process of doing so, it also heats up to[br]about 800 degrees Celsius, and then of
0:44:38.800,0:44:45.530
course, also needs the associated time to[br]cool down again. Good. So as you may or
0:44:45.530,0:44:50.040
may not know, currently the LHC is not in[br]operation. So LHC currently is undergoing
0:44:50.040,0:44:56.099
its second long shutdown phase, or LS2.[br]But what we do when the LHC is in
0:44:56.099,0:44:59.961
operation, is that we have these status[br]dashboards, that you can see here, that
0:44:59.961,0:45:04.970
are distributed all around CERN, and can[br]be used by anyone, any passer-by, to
0:45:04.970,0:45:09.980
basically monitor what the current[br]operation mode or the current situation of
0:45:09.980,0:45:15.609
the accelerator is. And can be used also[br]to quickly see if like an operator needs
0:45:15.609,0:45:19.990
to go somewhere or is needed, or how the[br]shift planning for the next shift works
0:45:19.990,0:45:24.430
out and so on. And on the right side you[br]would see what this currently looks like.
0:45:24.430,0:45:30.990
So basically black screen saying next beam[br]expected in spring 2021. And the good
0:45:30.990,0:45:33.980
thing about these status pages is that you[br]can actually see them from your home,
0:45:33.980,0:45:40.390
because they're also openly available, as[br]most of the stuff we do at CERN. So if you
0:45:40.390,0:45:44.510
are interested, then perhaps in a year[br]from now or a bit longer than a year, it
0:45:44.510,0:45:47.670
would be quite interesting to follow all[br]the commissioning process of when they are
0:45:47.670,0:45:54.130
trying to start the LHC back up, and[br]follow that process from your home.
0:45:54.130,0:45:58.109
Otherwise, if you now feel the urge to[br]maybe visit CERN, pay some of the things
0:45:58.109,0:46:01.980
we talked about a visit, or are just[br]generally interested, CERN offers a
0:46:01.980,0:46:06.970
variety of tours free of charge. So if[br]you're interested in that, visit that web
0:46:06.970,0:46:10.819
site and we would be happy to welcome you[br]there. And with that, thank you very much
0:46:10.819,0:46:14.569
for your attention.
0:46:14.569,0:46:17.530
Applause
0:46:17.530,0:46:24.470
Severin: Punktlandung.[br]Herald: Thank you, Stefan and Severin. If
0:46:24.470,0:46:29.869
you have questions, there are six[br]microphones in the room. Please make a
0:46:29.869,0:46:34.660
queue, and we start with the Signal Angel.[br]Signal Angel, please, first question.
0:46:34.660,0:46:39.630
Signal Angel: There is said to be a master[br]red button for shutting down the whole
0:46:39.630,0:46:45.859
system in case of heavy problems. How[br]often did you push it yet?
0:46:45.859,0:46:49.059
Stefan: Master red button?[br]Severin: Master button ...
0:46:49.059,0:46:55.160
Signal Angel: Like a shut down button.[br]Severin: I cannot really understand you. I
0:46:55.160,0:46:59.270
think the question was about how often[br]basically we used the Beam Dump system to
0:46:59.270,0:47:01.050
basically get rid of the beam, is that[br]correct?
0:47:01.050,0:47:04.280
Signal Angel: I guess so.[br]Stefan: He said there is a master button.
0:47:04.280,0:47:06.280
Signal Angel: I guess so.[br]Stefan: I think there's a master button in
0:47:06.280,0:47:08.280
the...[br]Severin: There is not only one master
0:47:08.280,0:47:11.790
button, there are several master buttons.[br]These are switches, called beam interlock
0:47:11.790,0:47:17.530
switch. Basically, at every operator's[br]screen, there is basically one beam
0:47:17.530,0:47:23.690
interlock switch. I don't know. I think[br]sometimes they get rid of the beam just
0:47:23.690,0:47:29.579
because, I mean. When we have LHC at full[br]operation – Stefan talked about the
0:47:29.579,0:47:33.799
luminosity – so what is happening, that in[br]the beginning we have a very high amount
0:47:33.799,0:47:39.920
of luminosity, So many particles collide[br]on each other. But over time, like after
0:47:39.920,0:47:45.230
12 or 15 hours or whatever, basically the[br]luminosity ..., so the amount of particles
0:47:45.230,0:47:49.119
which collide with each other, is going[br]down and down. So the luminosity
0:47:49.119,0:47:53.770
decreases. And then at some point in time,[br]basically the operators decide, that they
0:47:53.770,0:47:58.950
will now get rid of the actual beam, which[br]is inside LHC and basically will recover
0:47:58.950,0:48:02.290
the whole machine and then restart the[br]machine again. And this is done sometimes,
0:48:02.290,0:48:08.170
I don't know, every 12 hours, sometimes[br]after 24 hours. Something like that, yes.
0:48:08.170,0:48:11.380
Herald: Cool. And microphone number four,[br]I think.
0:48:11.380,0:48:17.559
Q: Yes. So where's the energy coming from?[br]So do you have your own power plant, or
0:48:17.559,0:48:20.119
so?[br]Severin: So, no, not really, not really.
0:48:20.119,0:48:24.349
Basically, we get all the power from the[br]French grid. So we have relatively big
0:48:24.349,0:48:32.640
power trails coming from the French grid.[br]So we get 450 kV of power there. So
0:48:32.640,0:48:35.390
basically the voltage is quite high and[br]then we have our own transformers on site.
0:48:35.390,0:48:39.609
And I think only, ... a little bit smaller[br]fraction of the energy is coming from the
0:48:39.609,0:48:43.940
Swiss grid. So basically we use most of[br]the energy which is coming from the French
0:48:43.940,0:48:46.940
grid.[br]Q: Okay. Thank you.
0:48:46.940,0:48:48.940
Herald: Thank you for your question. And[br]microphone number one, please.
0:48:48.940,0:48:57.280
Q: Hi. Thank you for your presentation. If[br]I'm not wrong, you say the beam can warm a
0:48:57.280,0:49:04.599
block of concrete to 800 Celsius. Would it[br]be possible to use it as a weapon?
0:49:04.599,0:49:09.960
Stefan: laughs Very likely not. And CERN[br]very much condemns these actions in any
0:49:09.960,0:49:13.690
form, I guess. So CERN operates in a[br]purely peaceful mission and would never
0:49:13.690,0:49:18.000
think about using their particle beams as[br]a weapon. And even if they could, it is
0:49:18.000,0:49:21.720
probably not the most practical thing to[br]do, I guess. laughs
0:49:21.720,0:49:26.710
Herald: But if your telephone is again[br]hanging up, you can destroy it, right?
0:49:26.710,0:49:29.090
Stefan: laughs[br]Herald: And microphone number six, I
0:49:29.090,0:49:33.390
think.[br]Q: Yes. So you said, you can stop in
0:49:33.390,0:49:39.980
nanoseconds, but just the light would go[br]just 30 centimeters, you know, a
0:49:39.980,0:49:45.110
nanosecond. How will you be able to stop[br]in this small time?
0:49:45.110,0:49:49.220
Stefan: Ah, no, no. So what I was talking[br]about is that these magnets that are used
0:49:49.220,0:49:54.770
to extract the beam out of the LHC, they[br]have reaction times, or ramp up times that
0:49:54.770,0:50:01.020
are in the order of 1, 2, 3 microseconds.[br]So not nanoseconds, but microseconds. And
0:50:01.020,0:50:06.329
really only then basically the particles[br]still circulate, worst case one full turn,
0:50:06.329,0:50:10.430
and only then moving outside of the[br]accelerator.
0:50:10.430,0:50:17.349
Herald: And microphone number one again.[br]Q: So do you have any photos of the front
0:50:17.349,0:50:22.609
of the dump block? It has to look like[br]it's got hit a lot.
0:50:22.609,0:50:26.110
laughter[br]Severin: No, not really. I think it's one
0:50:26.110,0:50:31.819
of the only pictures we could find about,[br]the Beam Dump system. And these areas, I
0:50:31.819,0:50:37.059
think it's not really opened any more. So[br]since operation of LHC, which was in
0:50:37.059,0:50:43.329
basically LHC started in 2008, and since[br]then, the Beam Dump system was not opened
0:50:43.329,0:50:48.860
again because it's completely sealed in[br]stainless steel. And that's why it wasn't
0:50:48.860,0:50:52.339
opened anymore.[br]Heral: Cool. Question from the interwebs.
0:50:52.339,0:50:59.329
Signal: Regarding power supply. How do you[br]switch or fine-control the currents? Are
0:50:59.329,0:51:03.460
you using classic silicone transistors,[br]off-the-shelf IGBTs?
0:51:03.460,0:51:07.230
Sverin: Um, yes. laughs[br]laughter
0:51:07.230,0:51:12.530
Severin: Yes. Uh, the system was developed[br]at CERN. And I think that's quite common
0:51:12.530,0:51:16.280
at CERN that we basically developed all[br]the technology at CERN or try to develop
0:51:16.280,0:51:20.250
nearly everything at CERN. But then[br]production, for example, is put into
0:51:20.250,0:51:25.750
industry. And yes, these are relatively[br]classical power converters. The
0:51:25.750,0:51:29.609
interesting or like challenging part about[br]the current power converters is really
0:51:29.609,0:51:32.859
that the current has to be measured quite[br]precisely and also controlled quite
0:51:32.859,0:51:39.430
precisely so there we use also DCC TS.[br]Which we have also mentioned before. But
0:51:39.430,0:51:42.290
basically all this controlled mechanism[br]there. That's one of the big challenges
0:51:42.290,0:51:47.980
there.[br]Herald: Cool. Microphone number one again.
0:51:47.980,0:51:55.609
Q: You talked about the orbit clock that[br]detects when the bunch is completed one
0:51:55.609,0:52:00.200
round. How is it possible to detect which[br]is the first bunch?
0:52:00.200,0:52:03.980
Stefan: Yeah. So it is it is actually not[br]detected, but this clock is actually
0:52:03.980,0:52:07.470
something that is constructed. So we[br]basically what we do is, we count these
0:52:07.470,0:52:13.250
cycles of the of the RF cycle. Maybe I can[br]open this slide. So somewhere there is a
0:52:13.250,0:52:18.510
counter that basically knows how many 40[br]MHz clock cycles a full rotation takes.
0:52:18.510,0:52:21.770
And then at some point decides this is[br]number one. And that's also where they
0:52:21.770,0:52:26.180
start counting when they inject bunches[br]into the LHC. So there's no marker, let's
0:52:26.180,0:52:29.510
say. But there is a certain structure to[br]the beam. So you could potentially do
0:52:29.510,0:52:33.320
that. So, for example, for these longer[br]periods where the kicker magnets need to
0:52:33.320,0:52:36.910
ramp up, they have something they call the[br]abort gap. So a number of bunches that are
0:52:36.910,0:52:41.350
never filled but are always kept empty. So[br]the magnets have enough time to deflect
0:52:41.350,0:52:44.460
the beam when the next bunch comes around.[br]So you could probably measure that, but
0:52:44.460,0:52:47.260
it's much easier to do it the other way[br]around.
0:52:47.260,0:52:54.570
Herald: Microphone number four, please.[br]Q: You said you had quite tight needs for
0:52:54.570,0:53:01.230
the timing clock. Is it tight enough? That[br]the speed of light was the limit with the
0:53:01.230,0:53:03.589
distances between locations or that was[br]not a concern?
0:53:03.589,0:53:08.680
Stefan: No, it is a concern. So because[br]just distributing a cable for 27
0:53:08.680,0:53:14.869
kilometers produces like just considerable[br]run times of electrical signals. All the
0:53:14.869,0:53:18.150
delays of all the cables need to be[br]measured precisely for their delay and
0:53:18.150,0:53:22.849
then calibrated out so all the experiments[br]get their clocks at the right time,
0:53:22.849,0:53:27.400
shifted, compensated for the delay time[br]that it just takes to get the signal
0:53:27.400,0:53:33.000
there.[br]Herald: And again, the interwebs.
0:53:33.000,0:53:40.210
Signal Angel: Is it too dangerous to stand[br]near the concrete cooling blocks, like
0:53:40.210,0:53:46.980
radioactive wise or, I don't know.[br]Severin: Yes.
0:53:46.980,0:53:49.090
laughter[br]Stefan: Not recommended.
0:53:49.090,0:53:54.859
Severin: Not recommended. We have a very[br]good interlock system. Also, the doors,
0:53:54.859,0:53:58.270
all the doors have switches. So basically[br]when one door is basically like opened
0:53:58.270,0:54:03.059
then basically the whole machine will be[br]shut down. So we have very critical and
0:54:03.059,0:54:11.319
safety related access system at LHC. Maybe[br]you watch Angels and Demons. This
0:54:11.319,0:54:16.760
Hollywood movie that we have, the eye[br]scanners are shown. It's a little bit. I
0:54:16.760,0:54:22.430
mean, it's Hollywood. But, we have eye[br]scanners. So iris scanners. So every time
0:54:22.430,0:54:25.780
like we want to go to the tunnel, for[br]example, then we have to let also our iris
0:54:25.780,0:54:30.000
be scanned because otherwise we will not[br]be able to go to the tunnel. So there's a
0:54:30.000,0:54:33.890
very sophisticated access system to really[br]go to the tunnel. So when there is
0:54:33.890,0:54:36.810
operation, the whole tunnel access is[br]completely blocked.
0:54:36.810,0:54:40.510
Herald: Good, microphone number one,[br]please.
0:54:40.510,0:54:47.780
Q: What is the exact reason to have each[br]of the experiments, every side. I mean, so
0:54:47.780,0:54:52.080
far apart on the LHC. I mean, on[br]opposite sides.
0:54:52.080,0:54:57.609
Severin: Um, basically, you are talking[br]about Atlas and CMS. The reason for that
0:54:57.609,0:55:02.060
is because when, these two experiments[br]were constructed, there was a little bit
0:55:02.060,0:55:10.060
of fear that particles basically interact[br]at the two experiments. So that they
0:55:10.060,0:55:13.670
really are like the most far away. We like[br]to have a very big distance from each
0:55:13.670,0:55:17.930
other. So there is no interaction between[br]them. That's why we basically put them at
0:55:17.930,0:55:20.869
point one and point five. That's the[br]reason why.
0:55:20.869,0:55:25.549
Herald: If I can see it correctly.[br]Microphone number five.
0:55:25.549,0:55:32.030
Q: Yes, hello. I've seen that you're also[br]using the CAN bus. What are you using the
0:55:32.030,0:55:37.190
CAN bus for in CERN?[br]Stefan: I know of at least one use, but it
0:55:37.190,0:55:42.539
is inside an experiment. So there are, as[br]far as I know, investigations under way to
0:55:42.539,0:55:47.610
use the CAN bus to do the actual control[br]of the detectors of one experiment. I
0:55:47.610,0:55:51.059
don't know if there is a use inside the[br]accelerator itself. So apart from the
0:55:51.059,0:55:55.790
experiments. But perhaps if you come by[br]afterwards we can find one.
0:55:55.790,0:55:59.539
Q: Thank you.[br]Herald: Microphone number one.
0:55:59.539,0:56:05.829
Q: Do you have any official data about how[br]many tons of duct taper used in
0:56:05.829,0:56:08.480
daily operations?[br]laughter
0:56:08.480,0:56:13.750
Severin: No. No.[br]Herald: What about zip ties?
0:56:13.750,0:56:21.460
Severin: Many. Yeah. Millions. Billions.[br]Herald: Okay. As far as I can see... Ah,
0:56:21.460,0:56:25.470
the intercepts again with a question.[br]Signal Angel: Do you know your monthly
0:56:25.470,0:56:27.470
power bill?[br]laughter
0:56:27.470,0:56:31.470
Severin: No, not no. No, sorry.[br]Stefan: No. But it is, I think, in fact
0:56:31.470,0:56:37.960
the contribution of France, which is the[br]main contributor in terms of energy. That
0:56:37.960,0:56:41.440
it is part of their contribution to[br]contribute the electricity bill basically
0:56:41.440,0:56:44.440
instead of paying money to CERN. That's as[br]far as I know.
0:56:44.440,0:56:48.480
Severin: Yes. And also we shut down the[br]LHC and the accelerator complex through
0:56:48.480,0:56:52.859
like the wintertime. And one of the[br]reasons for that is because electricity is
0:56:52.859,0:56:56.940
more expensive during wintertime in France[br]than in summer.
0:56:56.940,0:57:03.529
Herald: In this case, I can't see any[br]other questions. I have a maybe stupid
0:57:03.529,0:57:09.780
question. You said earlier you have to[br]focus and defocus the beam. But as we
0:57:09.780,0:57:12.529
know, you accelerated already the[br]particles. Why do we have to focus the
0:57:12.529,0:57:17.020
beam?[br]Severin: Because every time when we have a
0:57:17.020,0:57:21.910
dipole magnet, then basically we bend the[br]particle around an arc. But then we also
0:57:21.910,0:57:24.720
defocus a little bit. And also, the[br]coulomb force is still a problem because
0:57:24.720,0:57:29.770
we have equally charged particles in the[br]bunch or in the whole beam itself. So they
0:57:29.770,0:57:33.539
will by themselves basically go out of[br]each other. And if you would not focus it
0:57:33.539,0:57:36.299
again, then basically we would lose the[br]whole beam in the end.
0:57:36.299,0:57:44.630
Herald: Oh, thank you. I don't see any[br]questions. Internet? In this case thank
0:57:44.630,0:57:49.730
you very, very much, Stefan and Severin.[br]Please. With a warm applause. The Large
0:57:49.730,0:57:50.730
Hadron infrastructure talk.
0:57:50.730,0:57:51.730
Applause
0:57:51.730,0:57:52.989
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