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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
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precision and also the whole ADC is put
into a super-precise temperature
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controlled areas and it is calibrated
quite regularly. Okay, cryogenics. We
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already talked about that we have
superconducting magnets and they have to
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be cooled down quite low. So the
superconducting magnets we have at LHC has
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have to be cooled down to 1.9 Kelvin. And
we are doing this when we like start the
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LHC then we cool down on the first hand
with liquid nitrogen. So approximately six
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thousand tonnes of liquid nitrogen are put
through the magnets to cool them down
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to 18 Kelvin and afterwards we cool the
magnets down with liquid helium. And
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liquid helium is at 1.9 or 1.8 Kelvin. And
to put it a little bit in a comparison,
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outer space, so when we measure like the
temperature of space, we have
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approximately 2.7 Kelvin in outer space.
So LHC is much colder than outer space.
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The whole cooldown needs approximately one
month and each dipole magnet, which is 15
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meters long, shrinks several centimeters
during that. Which also has to be taken
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into account, because otherwise pipes
would break. The cryogenic system is that
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we have at each of the eight points at LHC
we have compressors to cool down the
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liquid helium or the helium itself. And
then we compress the helium and pump it
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down. We have one gaseous helium stream,
which is at 15 Kelvin and we have liquid
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helium stream at approximately 4.5 Kelvin.
And then we pump it underground and then
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we have something called Cold Compression
System. And the Cold Compression System
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even further reduces the pressure of the
helium that we have in the end a helium,
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which is at 1.8 Kelvin. So it can really
cool down the magnet itself. And helium
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has a very interesting effect because at
2.1 Kelvin, it becomes something called
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superfluid. So it basically can run around
like holes, for example, or walls. It can
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basically flow against gravity, which is
quite interesting. And it has also very
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high thermal conductivity and that's also
why we use superfluid helium here. And
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that's why we cool down the whole magnets
that low. And one other interesting effect
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is also that the LHC tilt angle, which is
1.4 percent, has to be taken into account
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because we have very low pressure inside
all the tubes or all the system, at 16
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millibars. But we have sometimes to pump
the helium against gravity or going down.
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So we also have to take into account the
LHC tilt angle to not have wrong pressure
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levels at the whole LHC itself. Okay.
Stefan: All right! So, you probably
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already got the idea, that what we've done
in the last 20 minutes, was only solve the
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first of the three challenges we had,
which was actually bending the beam around
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the circular trajectory. So I'm trying to
go to the other challenges we have lined
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up in the beginning. And the first one of
that is the actual acceleration of the
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particle beam. And large synchrotrons,
e.g. like the LHC, they use radio
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frequency or RF systems to do this
acceleration. And I'm just going to do a
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quick recap of the LHC beam and RF and how
they interact. So Severin mentioned
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already that the particles in LHC actually
come in bunches. So in like packets that
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contain about hundred billion protons and
those bunches are spaced when they are
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running around the LHC approximately 25
nanoseconds apart. And starting from that
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the tasks of the RF system are basically
twofold. It first has to ensure that these
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bunches are kept tightly together in a
process that we call longitudinal
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focusing. And the second task is to care
for the actual acceleration of the
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particle bunches. So from their injection
energy, when they come from one of the
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pre-accelerators up to their final energy,
that they are supposed to collide at
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during the physics run. So in general, you
can imagine RF as being a quickly
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alternating electric and magnetic field
components. And in the LHC, this RF energy
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is basically injected into what is called
a cavity, which is a resonant structure.
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And there the particle beams travels
through, while the field quickly
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alternates and the RF signal, or the
energy, basically interacts with the
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particle beam. So perhaps you know that
the protons are positively charged and
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then a negative polarity of the field
would attract these protons, while the
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positive field location would basically
move them away. And this has ... well,
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after just injecting and with the
frequency of this RF field being the same
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as the speed that the particles actually
go round the LHC, solves the first of the
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two problems, which was the the focusing
because actually the particles that are
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too slow arrive only when the field is
already changed to the opposite polarity
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and actually get accelerated a bit, while
the particles that are too fast, they are
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actually being decelerated a bit. And this
is a process that we call the longitudinal
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focusing, which makes sure that the
bunches stay neatly packed together. And
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of course this would be relatively
inefficient if we would only change the
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polarity of this field once for each of
the proton bunches that pass by. Which is
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why we do it ten times. So the polarity
basically changes ten times or the
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frequency is ten times higher than the
bunch crossing frequency. And by doing
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that, we make sure that the change of this
field is much faster and therefore the
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particle bunches are packed much closer
together and the focusing is better. So
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here you can see these cavities that were
shown in the previous picture as a
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schematic, how they're actually placed in
the tunnel. So eight of these huge
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cavities are used per beam and they are
the actual thing that is used to couple
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the RF energy into the beam and transfer
it to the particles. They are also
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operating superconductively, so at
cryogenic temperatures, to reduce the
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thermal stress and the losses that would
otherwise occur in their materials. And
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these are actually – even though they are
so big, similar to the magnets that had to
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be very precisely manufactured – these
also have very small manufacturing
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tolerances and have to be precisely tuned
to the RF frequency that is used to
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inject. So and the second part of this,
that actually produces this high power RF
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signal. For that is used what we call
Klystrons. So Klystrons are basically RF
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amplifiers. They are built from high power
RF vacuum tubes and they amplify this 400
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MHz signal that is used to transfer energy
to the particles. And each of those
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Klystrons produces about 300 kW of power
and you can probably imagine how much that
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power for an individual unit that is, if
you know that your microwave oven has like
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2 or 3 kW. And of course, as we have eight
cavities per beam and one Klystron always
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feeds one cavity, we in total have 16 of
those Klystrons and they are in principle
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able to deliver a total energy of 4.8 MW
into the LHC beam to accelerate it. But if
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we take a small step back for now, we have
only solved the first of the two problems,
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which was to keep the bunches neatly
focused. Because currently the particles
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have been injected and the frequency is at
some specific frequency and actually they
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are only running basically in sync, the
two. So what we do after all the particle
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bunches from the pre-accelerators have
been injected into LHC, is that we ever so
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slightly increase the frequency, which of
course also means that the particles need
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to accelerate together with the RF signal.
And this is the mechanism that we use to
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accelerate them actually. And the change,
that is required to do this, is very tiny,
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actually. So it is less than a thousandth
of a percent sometimes, that is used to
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change the frequency to actually make them
go so much faster. So from their
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relatively low injection energy up to the
top energy plateau that they need to have
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to produce the actual physics collisions.
And an interesting question to ask here is
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where does this signal actually comes from
if it needs to be so precisely tuned to
-
some specific frequency? Who generates it
or who controls it? And that opens up the
-
whole complex of the timing of the LHC, of
the machine. So actually this first signal
-
that I mentioned, this RF signal, it
originates in a Faraday cage. So an
-
especially shielded area somewhere on the
Prévessin site of CERN. And from there it
-
is distributed to the low-level RF
subsystem with the Klystrons and the
-
cavities. But inside this room, there are
also a number of other signals generated.
-
The first one of that being this Bunch
Crossing Clock, which is the actual clock
-
that signals one pulse, basically every
time, it changes polarity one time a
-
proton bunch moves across a specific
location inside the LHC. And another one
-
is the so-called orbit clock, which always
indicates the start of the first or when
-
one proton bunch has basically re-arrived
at the same position and has completed one
-
orbit. And you may ask the question why
this is an important piece of information.
-
But if you think back to this image that
Severin has already shown, about the
-
accelerator complex, the big challenge
that all this brings is also the whole
-
synchronization of all these machines.
Because you have to imagine that while
-
these proton bunches run around the LHC
and new ones are supposed to be injected
-
from the outside, from another pre-
accelerator, this has to be very precisely
-
synchronized. So all these pre-accelerator
systems actually share a common
-
synchronized timing system that allows
them to precisely inject a new packet of
-
bunches at the right position, at the
right location into the LHC. And this a
-
bit how such a timing distribution system
looks like. It is only a very small
-
excerpt of what it looks like, but it
gives you an idea that somewhere
-
underground in the LHC there is rooms full
of equipment that is just used to
-
distribute timing signals between
different parts of the accelerator. And of
-
course, as CERN is forward-thinking and
realized that future colliders will need
-
quite a bit more of all this
synchronization and that the requirements
-
for how precisely everything needs to be
synchronized is ever growing, they
-
actually developed their own timing
distribution standard which is also
-
openly available and available for
everybody to use. So if you're interested,
-
look that up. But of course, not only the
accelerator itself is interested in this
-
information about what particles are where
and how quickly they interact or how
-
quickly they go around. But also all the
experiments need this information, because
-
in the end they want to know "Okay, has a
collision occurred at some specific time
-
in my experiment?" and actually providing
this timing information about when bunches
-
have crossed their experiment locations is
also vital for them to really time tag all
-
their collision data and basically track
which bunches were responsible for what
-
kind of event or what event throughout
their whole signal storage and processing
-
chain, let's say. Good. So that is
basically challenge 2 out of the way. So
-
that was the acceleration of the actual
particles and all the associated issues
-
with timing. And the third issue we
mentioned was that the particles need to,
-
let's say, be kept from colliding with
anything but themselves or the other beam.
-
And that is what we, why we need vacuum
systems for. So, again, it is not as
-
simple as just putting a vacuum somewhere.
Of course not. Because in fact, there is
-
not only one vacuum system at LHC, but
there are three. So, the first two of
-
those are perhaps a bit less interesting
to most of us. They are mainly insulation
-
vacuum systems that are used for the
cryogenic magnets. So they isolate,
-
basically thermally isolate the magnets at
those very cool temperatures from the
-
surrounding air to avoid them getting more
heat load than they need to. And there is
-
an insulation vacuum also for the helium
distribution lines that are actually
-
distributing, delivering the helium to
these magnets. And then the third one,
-
which is perhaps the most interesting one,
is the beam vacuum. So the one where
-
actually the beam circulates inside the
LHC. And this is a cross section of what
-
this beam vacuum typically looks like. So
it is approximately this size, so a very
-
... handful, let's say. And the question
you may ask "OK, if I want to keep all the
-
like the particles in my particle beam
from colliding with anything they are not
-
supposed to, for example, rest molecules
of remaining air there, how many molecules
-
can there still be?" So somebody has to
make up that number. And typically you
-
express this as a quantity called the
beam lifetime, which basically says if you
-
were only to keep those particles
circulating in the accelerator, how long
-
would it take until they have all
dispersed and lost their energy due to
-
colliding with rest gas molecules? And it
was decided that this should be at a value
-
of 100 hours, is what the beams should
basically be able to circulate without
-
collisions, without being lost. And this
gave the requirement for pressures down to
-
about 100 femtobar, which is a very small,
very, very tiny fraction of the
-
atmospheric pressure we have here, which
is about 1 bar. And to actually get to
-
this level of vacuum, it requires multiple
stages and multiple components to actually
-
get there. So the initial vacuum inside
these beam tubes, which are basically
-
going throughout the whole LHC tunnel, has
the volume of approximately the Notre-Dame
-
cathedral. So the first step of getting
all the air out of these beam tubes is
-
using turbomolecular pumps. And then there
needs to be more mechanisms to reduce the
-
pressure even further, because these pumps
are not able to reduce the pressure to the
-
levels required. And they actually use a
relatively clever trick to do that, which
-
is the use of cryopumping. So the, ... I
cannot show that? Okay. So the outer wall
-
of this beam pipe cross section that you
see here is actually also where the very
-
cold helium inside the magnets is outside
of. And what that does is, it leads to an
-
effect called cryopumping. So actually any
rest gas molecule that hits this wall
-
actually condenses there. And as the
molecules condense there, they are of
-
course removed from the atmosphere inside
this beam pipe, which removes them from
-
the atmosphere and increases the quality
of the vacuum. And with the use of this
-
and then the warm sections, the use of
getter coatings, which are basically able
-
to trap gas molecules, you are able to
reach the crazy vacuum levels that are
-
required to make this happen. But they
realized also during the design that one
-
big problem – for the first time in an
accelerator – another effect will create a
-
significant problem for the vacuum, which
is the generation of synchrotron
-
radiation. So synchrotron radiation is a
byproduct of when you do bend a particle
-
beam, it results in a phenomenon called
synchrotron radiation. And when this
-
synchrotron radiation, as it goes straight
on and is not bent, hits the walls of this
-
vacuum system, or in this case of the beam
pipe, it actually liberates molecules from
-
there and reintroduces them into the
vacuum, which of course then makes the
-
vacuum worse again. An additional problem
that gives the synchrotron radiation is,
-
that it also gives a significant heat
load, and if you need to dissipate all
-
this heat that is generated through the
very cold helium, this is not a very
-
efficient process. Because making this
helium so cool, is actually a very energy
-
intensive process. And just removing a
single watt of thermal power through the
-
superfluid helium costs about 1 kW of
energy. So that is not the most efficient
-
part. And this is why the cross-section
you have just seen includes another large
-
component, which also technically belongs
to the vacuum system, which is called the
-
beam screen. And this beam screen is
basically another tube running inside the
-
beam pipe, of which we have, of course,
two, which run inside the magnet cold
-
bores. And it shields the synchrotron
radiation heat load from the outer walls,
-
which are at 1.8 Kelvin, while this pipe
itself is actively cooled to only about 20
-
Kelvin of temperature, which is much more
efficient to dissipate this heat. So it is
-
basically a steel tube about one
millimeter thick. It has these pumping
-
holes, where hydrogen gas molecules can go
out of, and on the inside it has a copper
-
coating, which is used to reduce its
electrical resistance, which is required
-
because the beam, while it circulates,
also induces current that would otherwise
-
flow inside this tube, which is really, if
you think about it, only a simple tube and
-
it would increase the heat load again. So
a lot of engineering already has to go
-
into a very simple piece of ... a thing
like that. So after having spoken so much
-
about all the things required to just make
a beam circulate and accelerate and so on,
-
now it's probably also time to talk a
little bit about the beam itself and how
-
to control it and how to instrument, how
to measure things about this beam. Even
-
without going yet about collisions and
doing actual physics experiments. So the
-
first important bit that is able to
basically control or influence the beam
-
here is what's called the beam cleaning or
collimation system. So typically such a
-
particle beam is not very clean. It always
travels associated with what is called
-
halo of particles around this core area
that is less than a millimeter wide where
-
most of the intensity is focused. And
these particles outside we want to remove,
-
because they otherwise would be lost
inside the magnets and for example, would
-
lead to quenches of the superconducting
magnets. And for collimation, we basically
-
use small slits that are adjustable and
are located at two main locations of the
-
LHC. So they have collimation systems
there, with vertical and horizontal slits
-
that can be adjusted in width, in order to
scrape off all the particles that they do
-
want to get rid of and extract out of the
beam, while only the core part can
-
circulate and produce clean collisions
without any background, that otherwise
-
would need to be accounted for. And then
there is a whole other open topic of beam
-
instrumentation. So when you run a
particle accelerator, you want to measure
-
various quantities and performance figures
of such a beam. And that is crucial for a
-
correct operation and for the highest
performance, getting the highest
-
performance from an accelerator. And there
are a lot of different types of those, and
-
I want to go quickly about ... over why we
have them and what we do with them. So the
-
first and most basic measurement you want
to do, is the beam current measurement. So
-
the beam current is a basic accelerator
beam intensity measurement. So it gives
-
you an idea of how strong the beam that is
running inside your accelerator is. And it
-
is measured using these DCCTs or DC
current transformers. And their basic
-
principle of operation is, that while the
particles move through this torus, which
-
is actually a coil or a transformer,
induces a voltage there that you can
-
measure and then use to quantify the
intensity of this beam. And the big
-
challenge here is that the dynamic range,
this instrument needs to capture, is
-
really large, because it has to operate
from the lowest intensity pilot injection
-
beams up to the full energy, full number
of bunches running inside the LHC. So it
-
has to cover six orders of magnitude of
measurement dynamic range. Then the second
-
thing when talking about collisions is the
luminosity measurement. So luminosity is a
-
quantity basically said to measure the
rate of interaction of the particle beams.
-
So to give you an idea of how often
interactions happen inside the experiments
-
or where you want them to happen. And this
measurement is used to first of all,
-
adjust this interaction rate to a target
value, which is optimal for the
-
experiments to function and to equalize
the interaction rates in different
-
experiments. So different experiments also
are specified to have the same interaction
-
rate so they can get the same let's say
statistical quality of their data. So it's
-
used to equalize those. And then as a
third thing, this system is also used to
-
measure the crossing angle of the beam. So
as you may know, at some point, when the
-
beams are collided, they collide at an
angle, that is very small. And this angle
-
is actually measured also very precisely
in order to adjust it correctly. And it is
-
measured to less than a thousandth of a
degree, which is again a very impressive
-
feat, given that the detection principle
of this measurement is only measurement of
-
some neutral particles that are a result
of the particle interaction of the beam
-
... of the collision. Okay, so that is
number two. Then number three that we have
-
is the beam position monitor. Because
along the LHC, you also always want to
-
know, where the beam is at any given time.
So you want to measure the position of the
-
beam inside the beam pipe in order to
optimally adjust it to the position you
-
want to have it. And for that we use these
beam position monitors of which we have
-
more than a thousand installed along the
LHC. So they are typically capacitive
-
probes or electromagnetic strip lines. As
you can see on top and bottom
-
respectively. And they basically are
distributed along the LHC and provide
-
position of the particle beam along the
accelerator, which can then be used to
-
tune, for example, the magnets. All right.
Then we have beam profile. So after the
-
position, that gives you an idea where the
beam is, you also want to know its
-
intensity distribution. Basically when you
do a cut through the beam pipe somewhere,
-
you want to know how the intensity profile
looks like. And for that we have basically
-
two measurement systems. One measures the
profile in X and Y directions. So if you
-
really would do a cut and it gives you
something like this and it's, for example,
-
done with wire scanners, which is literal,
very thin wire that is moved through the
-
beam. And then the current that the beam
moving through this wire, generates is
-
used to generate such a profile map, when
scanning with this wire. The other one is
-
the longitudinal profile, which gives you
an idea about the quality of your RF
-
system and there you want to know how the
intensity profile of your beam looks like.
-
If you were looking only at one spot of
the accelerator and the beam would pass
-
by, and you would basically see over time
how the intensity looks like. And then as
-
the last bit of beam instrumentation,
there is the beam loss monitors. So they
-
are these yellow tubes, that are located
on the outside of mostly all the magnets,
-
of the dipole and quadrupole magnets and
so on. Again, there's more than a thousand
-
of those. And the idea here is that you
need a lot of detectors that are basically
-
small ionization chambers, which detect
any showers of secondary particles that
-
are generated when one of the high energy
protons are lost somewhere in the magnet
-
materials. So these are really used for
protection of the system, because if a
-
specific threshold of energy loss is
detected, then the accelerator needs to be
-
quickly shut down. Which is why they have
to react in a matter of nanoseconds in
-
order to keep the accelerator safe.
Because any interaction of the particle
-
beam with for example, the magnets could
just destroy huge amounts of money and of
-
time that would be needed to rebuild. And
as a last and final thing, we have spoken
-
one or two times already about shutting
down the LHC. Which sounds also trivial at
-
first, but really is not. So, the last
thing here is, what we call the Beam Dump.
-
So the energy content that is contained in
those particular beams, it can be used,
-
could be used if it were shot on a copper
target, it could just melt 1000 kilograms
-
or one ton of copper instantly. So during
beam extraction, the process of getting
-
the particle beam outside out of the LHC,
this energy needs to be dissipated
-
somehow. And for that, this special Beam
Dump area is constructed. So there are
-
fast kicker magnets, that are used to ...,
that are able to ramp up in a really,
-
really short amount of time of
microseconds. And then the beam is
-
carefully and in a controlled manner
directed into a set of concrete blocks,
-
that is basically big enough to dissipate
all this energy, when required. And in the
-
process of doing so, it also heats up to
about 800 degrees Celsius, and then of
-
course, also needs the associated time to
cool down again. Good. So as you may or
-
may not know, currently the LHC is not in
operation. So LHC currently is undergoing
-
its second long shutdown phase, or LS2.
But what we do when the LHC is in
-
operation, is that we have these status
dashboards, that you can see here, that
-
are distributed all around CERN, and can
be used by anyone, any passer-by, to
-
basically monitor what the current
operation mode or the current situation of
-
the accelerator is. And can be used also
to quickly see if like an operator needs
-
to go somewhere or is needed, or how the
shift planning for the next shift works
-
out and so on. And on the right side you
would see what this currently looks like.
-
So basically black screen saying next beam
expected in spring 2021. And the good
-
thing about these status pages is that you
can actually see them from your home,
-
because they're also openly available, as
most of the stuff we do at CERN. So if you
-
are interested, then perhaps in a year
from now or a bit longer than a year, it
-
would be quite interesting to follow all
the commissioning process of when they are
-
trying to start the LHC back up, and
follow that process from your home.
-
Otherwise, if you now feel the urge to
maybe visit CERN, pay some of the things
-
we talked about a visit, or are just
generally interested, CERN offers a
-
variety of tours free of charge. So if
you're interested in that, visit that web
-
site and we would be happy to welcome you
there. And with that, thank you very much
-
for your attention.
-
Applause
-
Severin: Punktlandung.
Herald: Thank you, Stefan and Severin. If
-
you have questions, there are six
microphones in the room. Please make a
-
queue, and we start with the Signal Angel.
Signal Angel, please, first question.
-
Signal Angel: There is said to be a master
red button for shutting down the whole
-
system in case of heavy problems. How
often did you push it yet?
-
Stefan: Master red button?
Severin: Master button ...
-
Signal Angel: Like a shut down button.
Severin: I cannot really understand you. I
-
think the question was about how often
basically we used the Beam Dump system to
-
basically get rid of the beam, is that
correct?
-
Signal Angel: I guess so.
Stefan: He said there is a master button.
-
Signal Angel: I guess so.
Stefan: I think there's a master button in
-
the...
Severin: There is not only one master
-
button, there are several master buttons.
These are switches, called beam interlock
-
switch. Basically, at every operator's
screen, there is basically one beam
-
interlock switch. I don't know. I think
sometimes they get rid of the beam just
-
because, I mean. When we have LHC at full
operation – Stefan talked about the
-
luminosity – so what is happening, that in
the beginning we have a very high amount
-
of luminosity, So many particles collide
on each other. But over time, like after
-
12 or 15 hours or whatever, basically the
luminosity ..., so the amount of particles
-
which collide with each other, is going
down and down. So the luminosity
-
decreases. And then at some point in time,
basically the operators decide, that they
-
will now get rid of the actual beam, which
is inside LHC and basically will recover
-
the whole machine and then restart the
machine again. And this is done sometimes,
-
I don't know, every 12 hours, sometimes
after 24 hours. Something like that, yes.
-
Herald: Cool. And microphone number four,
I think.
-
Q: Yes. So where's the energy coming from?
So do you have your own power plant, or
-
so?
Severin: So, no, not really, not really.
-
Basically, we get all the power from the
French grid. So we have relatively big
-
power trails coming from the French grid.
So we get 450 kV of power there. So
-
basically the voltage is quite high and
then we have our own transformers on site.
-
And I think only, ... a little bit smaller
fraction of the energy is coming from the
-
Swiss grid. So basically we use most of
the energy which is coming from the French
-
grid.
Q: Okay. Thank you.
-
Herald: Thank you for your question. And
microphone number one, please.
-
Q: Hi. Thank you for your presentation. If
I'm not wrong, you say the beam can warm a
-
block of concrete to 800 Celsius. Would it
be possible to use it as a weapon?
-
Stefan: laughs Very likely not. And CERN
very much condemns these actions in any
-
form, I guess. So CERN operates in a
purely peaceful mission and would never
-
think about using their particle beams as
a weapon. And even if they could, it is
-
probably not the most practical thing to
do, I guess. laughs
-
Herald: But if your telephone is again
hanging up, you can destroy it, right?
-
Stefan: laughs
Herald: And microphone number six, I
-
think.
Q: Yes. So you said, you can stop in
-
nanoseconds, but just the light would go
just 30 centimeters, you know, a
-
nanosecond. How will you be able to stop
in this small time?
-
Stefan: Ah, no, no. So what I was talking
about is that these magnets that are used
-
to extract the beam out of the LHC, they
have reaction times, or ramp up times that
-
are in the order of 1, 2, 3 microseconds.
So not nanoseconds, but microseconds. And
-
really only then basically the particles
still circulate, worst case one full turn,
-
and only then moving outside of the
accelerator.
-
Herald: And microphone number one again.
Q: So do you have any photos of the front
-
of the dump block? It has to look like
it's got hit a lot.
-
laughter
Severin: No, not really. I think it's one
-
of the only pictures we could find about,
the Beam Dump system. And these areas, I
-
think it's not really opened any more. So
since operation of LHC, which was in
-
basically LHC started in 2008, and since
then, the Beam Dump system was not opened
-
again because it's completely sealed in
stainless steel. And that's why it wasn't
-
opened anymore.
Heral: Cool. Question from the interwebs.
-
Signal: Regarding power supply. How do you
switch or fine-control the currents? Are
-
you using classic silicone transistors,
off-the-shelf IGBTs?
-
Sverin: Um, yes. laughs
laughter
-
Severin: Yes. Uh, the system was developed
at CERN. And I think that's quite common
-
at CERN that we basically developed all
the technology at CERN or try to develop
-
nearly everything at CERN. But then
production, for example, is put into
-
industry. And yes, these are relatively
classical power converters. The
-
interesting or like challenging part about
the current power converters is really
-
that the current has to be measured quite
precisely and also controlled quite
-
precisely so there we use also DCC TS.
Which we have also mentioned before. But
-
basically all this controlled mechanism
there. That's one of the big challenges
-
there.
Herald: Cool. Microphone number one again.
-
Q: You talked about the orbit clock that
detects when the bunch is completed one
-
round. How is it possible to detect which
is the first bunch?
-
Stefan: Yeah. So it is it is actually not
detected, but this clock is actually
-
something that is constructed. So we
basically what we do is, we count these
-
cycles of the of the RF cycle. Maybe I can
open this slide. So somewhere there is a
-
counter that basically knows how many 40
MHz clock cycles a full rotation takes.
-
And then at some point decides this is
number one. And that's also where they
-
start counting when they inject bunches
into the LHC. So there's no marker, let's
-
say. But there is a certain structure to
the beam. So you could potentially do
-
that. So, for example, for these longer
periods where the kicker magnets need to
-
ramp up, they have something they call the
abort gap. So a number of bunches that are
-
never filled but are always kept empty. So
the magnets have enough time to deflect
-
the beam when the next bunch comes around.
So you could probably measure that, but
-
it's much easier to do it the other way
around.
-
Herald: Microphone number four, please.
Q: You said you had quite tight needs for
-
the timing clock. Is it tight enough? That
the speed of light was the limit with the
-
distances between locations or that was
not a concern?
-
Stefan: No, it is a concern. So because
just distributing a cable for 27
-
kilometers produces like just considerable
run times of electrical signals. All the
-
delays of all the cables need to be
measured precisely for their delay and
-
then calibrated out so all the experiments
get their clocks at the right time,
-
shifted, compensated for the delay time
that it just takes to get the signal
-
there.
Herald: And again, the interwebs.
-
Signal Angel: Is it too dangerous to stand
near the concrete cooling blocks, like
-
radioactive wise or, I don't know.
Severin: Yes.
-
laughter
Stefan: Not recommended.
-
Severin: Not recommended. We have a very
good interlock system. Also, the doors,
-
all the doors have switches. So basically
when one door is basically like opened
-
then basically the whole machine will be
shut down. So we have very critical and
-
safety related access system at LHC. Maybe
you watch Angels and Demons. This
-
Hollywood movie that we have, the eye
scanners are shown. It's a little bit. I
-
mean, it's Hollywood. But, we have eye
scanners. So iris scanners. So every time
-
like we want to go to the tunnel, for
example, then we have to let also our iris
-
be scanned because otherwise we will not
be able to go to the tunnel. So there's a
-
very sophisticated access system to really
go to the tunnel. So when there is
-
operation, the whole tunnel access is
completely blocked.
-
Herald: Good, microphone number one,
please.
-
Q: What is the exact reason to have each
of the experiments, every side. I mean, so
-
far apart on the LHC. I mean, on
opposite sides.
-
Severin: Um, basically, you are talking
about Atlas and CMS. The reason for that
-
is because when, these two experiments
were constructed, there was a little bit
-
of fear that particles basically interact
at the two experiments. So that they
-
really are like the most far away. We like
to have a very big distance from each
-
other. So there is no interaction between
them. That's why we basically put them at
-
point one and point five. That's the
reason why.
-
Herald: If I can see it correctly.
Microphone number five.
-
Q: Yes, hello. I've seen that you're also
using the CAN bus. What are you using the
-
CAN bus for in CERN?
Stefan: I know of at least one use, but it
-
is inside an experiment. So there are, as
far as I know, investigations under way to
-
use the CAN bus to do the actual control
of the detectors of one experiment. I
-
don't know if there is a use inside the
accelerator itself. So apart from the
-
experiments. But perhaps if you come by
afterwards we can find one.
-
Q: Thank you.
Herald: Microphone number one.
-
Q: Do you have any official data about how
many tons of duct taper used in
-
daily operations?
laughter
-
Severin: No. No.
Herald: What about zip ties?
-
Severin: Many. Yeah. Millions. Billions.
Herald: Okay. As far as I can see... Ah,
-
the intercepts again with a question.
Signal Angel: Do you know your monthly
-
power bill?
laughter
-
Severin: No, not no. No, sorry.
Stefan: No. But it is, I think, in fact
-
the contribution of France, which is the
main contributor in terms of energy. That
-
it is part of their contribution to
contribute the electricity bill basically
-
instead of paying money to CERN. That's as
far as I know.
-
Severin: Yes. And also we shut down the
LHC and the accelerator complex through
-
like the wintertime. And one of the
reasons for that is because electricity is
-
more expensive during wintertime in France
than in summer.
-
Herald: In this case, I can't see any
other questions. I have a maybe stupid
-
question. You said earlier you have to
focus and defocus the beam. But as we
-
know, you accelerated already the
particles. Why do we have to focus the
-
beam?
Severin: Because every time when we have a
-
dipole magnet, then basically we bend the
particle around an arc. But then we also
-
defocus a little bit. And also, the
coulomb force is still a problem because
-
we have equally charged particles in the
bunch or in the whole beam itself. So they
-
will by themselves basically go out of
each other. And if you would not focus it
-
again, then basically we would lose the
whole beam in the end.
-
Herald: Oh, thank you. I don't see any
questions. Internet? In this case thank
-
you very, very much, Stefan and Severin.
Please. With a warm applause. The Large
-
Hadron infrastructure talk.
-
Applause
-
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