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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
00:17:54.340 --> 00:17:58.549
Interlock loop. Interlock loop is a system
at LHC. You can imagine that little bit
00:17:58.549 --> 00:18:03.269
like a cable going around the whole tunnel
and there are thousands of switches around
00:18:03.269 --> 00:18:08.510
this Interlock loop. And as soon as one of
the detection systems basically opens to
00:18:08.510 --> 00:18:13.669
the interlock loop, then basically the
whole machine will be switched off. And
00:18:13.669 --> 00:18:16.530
what means switched off is basically, that
we will power down the power converter,
00:18:16.530 --> 00:18:20.039
but then the energy is still in the
superconducting magnet and it has to be
00:18:20.039 --> 00:18:24.190
taken out of the superconducting magnet.
And therefore, we use dump resistors to
00:18:24.190 --> 00:18:30.600
extract the energy. And here you can see a
picture of such a dump resistor. It's
00:18:30.600 --> 00:18:34.960
quite big. It's in a stainless steel tube,
oil cooled. It's approximately three or
00:18:34.960 --> 00:18:41.470
four meters long. And basically, when
there was a Quench, and the energy was
00:18:41.470 --> 00:18:44.480
extracted via these resistors, the whole
resistor is heated up by several hundred
00:18:44.480 --> 00:18:50.179
degrees and it needs several hours to cool
it down again. Power converters; the power
00:18:50.179 --> 00:18:54.679
converters are used to power the magnet
itself. So they can produce a current of
00:18:54.679 --> 00:19:01.110
approximately 13000 amps and a voltage of
plus minus 190 volts. And you can see a
00:19:01.110 --> 00:19:06.310
picture how here, how big it is. One
downside with the power converters is that
00:19:06.310 --> 00:19:10.269
they have to be, not downside but one
difficulty is, that they have to be very
00:19:10.269 --> 00:19:16.210
precise, because every instability in the
current would have or has a direct effect
00:19:16.210 --> 00:19:20.290
on the beam stability itself. So basically
the beam would be not diverted in the
00:19:20.290 --> 00:19:27.169
right amount of length. So that's why they
have to be very precise and have to have a
00:19:27.169 --> 00:19:32.409
very precise stability. So here I just
pointed out, like in 24 hours, the power
00:19:32.409 --> 00:19:36.700
converter is only allowed to have a
deviation of 5 ppm. And in comparison, for
00:19:36.700 --> 00:19:41.970
13000 amps we have a deviation of 65 milli
amps. So the power converters have to be
00:19:41.970 --> 00:19:46.600
very precise. And to do that, we had to
develop our own ADC, because at the time
00:19:46.600 --> 00:19:51.270
when LHC was built, there was no ADC on
the market which was able to have this
00:19:51.270 --> 00:19:55.900
precision and also the whole ADC is put
into a super-precise temperature
00:19:55.900 --> 00:20:02.970
controlled areas and it is calibrated
quite regularly. Okay, cryogenics. We
00:20:02.970 --> 00:20:05.520
already talked about that we have
superconducting magnets and they have to
00:20:05.520 --> 00:20:11.340
be cooled down quite low. So the
superconducting magnets we have at LHC has
00:20:11.340 --> 00:20:17.230
have to be cooled down to 1.9 Kelvin. And
we are doing this when we like start the
00:20:17.230 --> 00:20:21.610
LHC then we cool down on the first hand
with liquid nitrogen. So approximately six
00:20:21.610 --> 00:20:26.429
thousand tonnes of liquid nitrogen are put
through the magnets to cool them down
00:20:26.429 --> 00:20:33.260
to 18 Kelvin and afterwards we cool the
magnets down with liquid helium. And
00:20:33.260 --> 00:20:38.960
liquid helium is at 1.9 or 1.8 Kelvin. And
to put it a little bit in a comparison,
00:20:38.960 --> 00:20:42.200
outer space, so when we measure like the
temperature of space, we have
00:20:42.200 --> 00:20:47.799
approximately 2.7 Kelvin in outer space.
So LHC is much colder than outer space.
00:20:47.799 --> 00:20:52.289
The whole cooldown needs approximately one
month and each dipole magnet, which is 15
00:20:52.289 --> 00:20:57.010
meters long, shrinks several centimeters
during that. Which also has to be taken
00:20:57.010 --> 00:21:02.870
into account, because otherwise pipes
would break. The cryogenic system is that
00:21:02.870 --> 00:21:08.980
we have at each of the eight points at LHC
we have compressors to cool down the
00:21:08.980 --> 00:21:13.860
liquid helium or the helium itself. And
then we compress the helium and pump it
00:21:13.860 --> 00:21:17.970
down. We have one gaseous helium stream,
which is at 15 Kelvin and we have liquid
00:21:17.970 --> 00:21:24.490
helium stream at approximately 4.5 Kelvin.
And then we pump it underground and then
00:21:24.490 --> 00:21:28.580
we have something called Cold Compression
System. And the Cold Compression System
00:21:28.580 --> 00:21:36.090
even further reduces the pressure of the
helium that we have in the end a helium,
00:21:36.090 --> 00:21:40.980
which is at 1.8 Kelvin. So it can really
cool down the magnet itself. And helium
00:21:40.980 --> 00:21:45.600
has a very interesting effect because at
2.1 Kelvin, it becomes something called
00:21:45.600 --> 00:21:52.350
superfluid. So it basically can run around
like holes, for example, or walls. It can
00:21:52.350 --> 00:21:57.679
basically flow against gravity, which is
quite interesting. And it has also very
00:21:57.679 --> 00:22:03.860
high thermal conductivity and that's also
why we use superfluid helium here. And
00:22:03.860 --> 00:22:08.370
that's why we cool down the whole magnets
that low. And one other interesting effect
00:22:08.370 --> 00:22:13.570
is also that the LHC tilt angle, which is
1.4 percent, has to be taken into account
00:22:13.570 --> 00:22:18.320
because we have very low pressure inside
all the tubes or all the system, at 16
00:22:18.320 --> 00:22:24.539
millibars. But we have sometimes to pump
the helium against gravity or going down.
00:22:24.539 --> 00:22:28.481
So we also have to take into account the
LHC tilt angle to not have wrong pressure
00:22:28.481 --> 00:22:34.519
levels at the whole LHC itself. Okay.
Stefan: All right! So, you probably
00:22:34.519 --> 00:22:38.649
already got the idea, that what we've done
in the last 20 minutes, was only solve the
00:22:38.649 --> 00:22:42.250
first of the three challenges we had,
which was actually bending the beam around
00:22:42.250 --> 00:22:49.269
the circular trajectory. So I'm trying to
go to the other challenges we have lined
00:22:49.269 --> 00:22:54.350
up in the beginning. And the first one of
that is the actual acceleration of the
00:22:54.350 --> 00:22:59.700
particle beam. And large synchrotrons,
e.g. like the LHC, they use radio
00:22:59.700 --> 00:23:05.299
frequency or RF systems to do this
acceleration. And I'm just going to do a
00:23:05.299 --> 00:23:10.510
quick recap of the LHC beam and RF and how
they interact. So Severin mentioned
00:23:10.510 --> 00:23:15.600
already that the particles in LHC actually
come in bunches. So in like packets that
00:23:15.600 --> 00:23:20.309
contain about hundred billion protons and
those bunches are spaced when they are
00:23:20.309 --> 00:23:26.470
running around the LHC approximately 25
nanoseconds apart. And starting from that
00:23:26.470 --> 00:23:31.250
the tasks of the RF system are basically
twofold. It first has to ensure that these
00:23:31.250 --> 00:23:35.630
bunches are kept tightly together in a
process that we call longitudinal
00:23:35.630 --> 00:23:39.700
focusing. And the second task is to care
for the actual acceleration of the
00:23:39.700 --> 00:23:44.570
particle bunches. So from their injection
energy, when they come from one of the
00:23:44.570 --> 00:23:49.039
pre-accelerators up to their final energy,
that they are supposed to collide at
00:23:49.039 --> 00:23:55.549
during the physics run. So in general, you
can imagine RF as being a quickly
00:23:55.549 --> 00:24:03.260
alternating electric and magnetic field
components. And in the LHC, this RF energy
00:24:03.260 --> 00:24:07.700
is basically injected into what is called
a cavity, which is a resonant structure.
00:24:07.700 --> 00:24:11.520
And there the particle beams travels
through, while the field quickly
00:24:11.520 --> 00:24:16.690
alternates and the RF signal, or the
energy, basically interacts with the
00:24:16.690 --> 00:24:21.750
particle beam. So perhaps you know that
the protons are positively charged and
00:24:21.750 --> 00:24:26.210
then a negative polarity of the field
would attract these protons, while the
00:24:26.210 --> 00:24:32.320
positive field location would basically
move them away. And this has ... well,
00:24:32.320 --> 00:24:36.260
after just injecting and with the
frequency of this RF field being the same
00:24:36.260 --> 00:24:41.390
as the speed that the particles actually
go round the LHC, solves the first of the
00:24:41.390 --> 00:24:45.110
two problems, which was the the focusing
because actually the particles that are
00:24:45.110 --> 00:24:49.370
too slow arrive only when the field is
already changed to the opposite polarity
00:24:49.370 --> 00:24:52.990
and actually get accelerated a bit, while
the particles that are too fast, they are
00:24:52.990 --> 00:24:58.000
actually being decelerated a bit. And this
is a process that we call the longitudinal
00:24:58.000 --> 00:25:03.490
focusing, which makes sure that the
bunches stay neatly packed together. And
00:25:03.490 --> 00:25:06.090
of course this would be relatively
inefficient if we would only change the
00:25:06.090 --> 00:25:12.399
polarity of this field once for each of
the proton bunches that pass by. Which is
00:25:12.399 --> 00:25:15.870
why we do it ten times. So the polarity
basically changes ten times or the
00:25:15.870 --> 00:25:21.370
frequency is ten times higher than the
bunch crossing frequency. And by doing
00:25:21.370 --> 00:25:25.960
that, we make sure that the change of this
field is much faster and therefore the
00:25:25.960 --> 00:25:33.070
particle bunches are packed much closer
together and the focusing is better. So
00:25:33.070 --> 00:25:35.389
here you can see these cavities that were
shown in the previous picture as a
00:25:35.389 --> 00:25:39.899
schematic, how they're actually placed in
the tunnel. So eight of these huge
00:25:39.899 --> 00:25:43.850
cavities are used per beam and they are
the actual thing that is used to couple
00:25:43.850 --> 00:25:49.730
the RF energy into the beam and transfer
it to the particles. They are also
00:25:49.730 --> 00:25:54.179
operating superconductively, so at
cryogenic temperatures, to reduce the
00:25:54.179 --> 00:25:59.610
thermal stress and the losses that would
otherwise occur in their materials. And
00:25:59.610 --> 00:26:01.270
these are actually – even though they are
so big, similar to the magnets that had to
00:26:01.270 --> 00:26:05.820
be very precisely manufactured – these
also have very small manufacturing
00:26:05.820 --> 00:26:12.070
tolerances and have to be precisely tuned
to the RF frequency that is used to
00:26:12.070 --> 00:26:17.440
inject. So and the second part of this,
that actually produces this high power RF
00:26:17.440 --> 00:26:22.420
signal. For that is used what we call
Klystrons. So Klystrons are basically RF
00:26:22.420 --> 00:26:29.330
amplifiers. They are built from high power
RF vacuum tubes and they amplify this 400
00:26:29.330 --> 00:26:34.289
MHz signal that is used to transfer energy
to the particles. And each of those
00:26:34.289 --> 00:26:40.100
Klystrons produces about 300 kW of power
and you can probably imagine how much that
00:26:40.100 --> 00:26:43.889
power for an individual unit that is, if
you know that your microwave oven has like
00:26:43.889 --> 00:26:49.460
2 or 3 kW. And of course, as we have eight
cavities per beam and one Klystron always
00:26:49.460 --> 00:26:54.840
feeds one cavity, we in total have 16 of
those Klystrons and they are in principle
00:26:54.840 --> 00:27:03.659
able to deliver a total energy of 4.8 MW
into the LHC beam to accelerate it. But if
00:27:03.659 --> 00:27:06.640
we take a small step back for now, we have
only solved the first of the two problems,
00:27:06.640 --> 00:27:12.510
which was to keep the bunches neatly
focused. Because currently the particles
00:27:12.510 --> 00:27:17.429
have been injected and the frequency is at
some specific frequency and actually they
00:27:17.429 --> 00:27:22.400
are only running basically in sync, the
two. So what we do after all the particle
00:27:22.400 --> 00:27:26.410
bunches from the pre-accelerators have
been injected into LHC, is that we ever so
00:27:26.410 --> 00:27:30.700
slightly increase the frequency, which of
course also means that the particles need
00:27:30.700 --> 00:27:35.399
to accelerate together with the RF signal.
And this is the mechanism that we use to
00:27:35.399 --> 00:27:40.159
accelerate them actually. And the change,
that is required to do this, is very tiny,
00:27:40.159 --> 00:27:44.419
actually. So it is less than a thousandth
of a percent sometimes, that is used to
00:27:44.419 --> 00:27:48.379
change the frequency to actually make them
go so much faster. So from their
00:27:48.379 --> 00:27:54.070
relatively low injection energy up to the
top energy plateau that they need to have
00:27:54.070 --> 00:28:00.409
to produce the actual physics collisions.
And an interesting question to ask here is
00:28:00.409 --> 00:28:03.990
where does this signal actually comes from
if it needs to be so precisely tuned to
00:28:03.990 --> 00:28:10.250
some specific frequency? Who generates it
or who controls it? And that opens up the
00:28:10.250 --> 00:28:15.669
whole complex of the timing of the LHC, of
the machine. So actually this first signal
00:28:15.669 --> 00:28:21.460
that I mentioned, this RF signal, it
originates in a Faraday cage. So an
00:28:21.460 --> 00:28:25.880
especially shielded area somewhere on the
Prévessin site of CERN. And from there it
00:28:25.880 --> 00:28:33.440
is distributed to the low-level RF
subsystem with the Klystrons and the
00:28:33.440 --> 00:28:38.690
cavities. But inside this room, there are
also a number of other signals generated.
00:28:38.690 --> 00:28:42.720
The first one of that being this Bunch
Crossing Clock, which is the actual clock
00:28:42.720 --> 00:28:48.240
that signals one pulse, basically every
time, it changes polarity one time a
00:28:48.240 --> 00:28:54.640
proton bunch moves across a specific
location inside the LHC. And another one
00:28:54.640 --> 00:28:59.470
is the so-called orbit clock, which always
indicates the start of the first or when
00:28:59.470 --> 00:29:04.990
one proton bunch has basically re-arrived
at the same position and has completed one
00:29:04.990 --> 00:29:10.820
orbit. And you may ask the question why
this is an important piece of information.
00:29:10.820 --> 00:29:16.529
But if you think back to this image that
Severin has already shown, about the
00:29:16.529 --> 00:29:21.789
accelerator complex, the big challenge
that all this brings is also the whole
00:29:21.789 --> 00:29:24.909
synchronization of all these machines.
Because you have to imagine that while
00:29:24.909 --> 00:29:29.790
these proton bunches run around the LHC
and new ones are supposed to be injected
00:29:29.790 --> 00:29:34.000
from the outside, from another pre-
accelerator, this has to be very precisely
00:29:34.000 --> 00:29:37.840
synchronized. So all these pre-accelerator
systems actually share a common
00:29:37.840 --> 00:29:42.840
synchronized timing system that allows
them to precisely inject a new packet of
00:29:42.840 --> 00:29:49.380
bunches at the right position, at the
right location into the LHC. And this a
00:29:49.380 --> 00:29:53.309
bit how such a timing distribution system
looks like. It is only a very small
00:29:53.309 --> 00:29:56.430
excerpt of what it looks like, but it
gives you an idea that somewhere
00:29:56.430 --> 00:30:01.100
underground in the LHC there is rooms full
of equipment that is just used to
00:30:01.100 --> 00:30:06.110
distribute timing signals between
different parts of the accelerator. And of
00:30:06.110 --> 00:30:11.909
course, as CERN is forward-thinking and
realized that future colliders will need
00:30:11.909 --> 00:30:15.400
quite a bit more of all this
synchronization and that the requirements
00:30:15.400 --> 00:30:20.050
for how precisely everything needs to be
synchronized is ever growing, they
00:30:20.050 --> 00:30:23.300
actually developed their own timing
distribution standard which is also
00:30:23.300 --> 00:30:28.270
openly available and available for
everybody to use. So if you're interested,
00:30:28.270 --> 00:30:34.320
look that up. But of course, not only the
accelerator itself is interested in this
00:30:34.320 --> 00:30:40.309
information about what particles are where
and how quickly they interact or how
00:30:40.309 --> 00:30:45.050
quickly they go around. But also all the
experiments need this information, because
00:30:45.050 --> 00:30:50.070
in the end they want to know "Okay, has a
collision occurred at some specific time
00:30:50.070 --> 00:30:54.940
in my experiment?" and actually providing
this timing information about when bunches
00:30:54.940 --> 00:30:59.789
have crossed their experiment locations is
also vital for them to really time tag all
00:30:59.789 --> 00:31:06.149
their collision data and basically track
which bunches were responsible for what
00:31:06.149 --> 00:31:11.559
kind of event or what event throughout
their whole signal storage and processing
00:31:11.559 --> 00:31:17.120
chain, let's say. Good. So that is
basically challenge 2 out of the way. So
00:31:17.120 --> 00:31:19.889
that was the acceleration of the actual
particles and all the associated issues
00:31:19.889 --> 00:31:25.899
with timing. And the third issue we
mentioned was that the particles need to,
00:31:25.899 --> 00:31:31.330
let's say, be kept from colliding with
anything but themselves or the other beam.
00:31:31.330 --> 00:31:36.210
And that is what we, why we need vacuum
systems for. So, again, it is not as
00:31:36.210 --> 00:31:41.200
simple as just putting a vacuum somewhere.
Of course not. Because in fact, there is
00:31:41.200 --> 00:31:45.740
not only one vacuum system at LHC, but
there are three. So, the first two of
00:31:45.740 --> 00:31:51.090
those are perhaps a bit less interesting
to most of us. They are mainly insulation
00:31:51.090 --> 00:31:56.919
vacuum systems that are used for the
cryogenic magnets. So they isolate,
00:31:56.919 --> 00:32:02.789
basically thermally isolate the magnets at
those very cool temperatures from the
00:32:02.789 --> 00:32:08.789
surrounding air to avoid them getting more
heat load than they need to. And there is
00:32:08.789 --> 00:32:11.830
an insulation vacuum also for the helium
distribution lines that are actually
00:32:11.830 --> 00:32:16.299
distributing, delivering the helium to
these magnets. And then the third one,
00:32:16.299 --> 00:32:19.820
which is perhaps the most interesting one,
is the beam vacuum. So the one where
00:32:19.820 --> 00:32:24.999
actually the beam circulates inside the
LHC. And this is a cross section of what
00:32:24.999 --> 00:32:30.242
this beam vacuum typically looks like. So
it is approximately this size, so a very
00:32:30.242 --> 00:32:37.470
... handful, let's say. And the question
you may ask "OK, if I want to keep all the
00:32:37.470 --> 00:32:41.590
like the particles in my particle beam
from colliding with anything they are not
00:32:41.590 --> 00:32:47.340
supposed to, for example, rest molecules
of remaining air there, how many molecules
00:32:47.340 --> 00:32:51.240
can there still be?" So somebody has to
make up that number. And typically you
00:32:51.240 --> 00:32:55.880
express this as a quantity called the
beam lifetime, which basically says if you
00:32:55.880 --> 00:33:00.870
were only to keep those particles
circulating in the accelerator, how long
00:33:00.870 --> 00:33:04.620
would it take until they have all
dispersed and lost their energy due to
00:33:04.620 --> 00:33:10.129
colliding with rest gas molecules? And it
was decided that this should be at a value
00:33:10.129 --> 00:33:15.230
of 100 hours, is what the beams should
basically be able to circulate without
00:33:15.230 --> 00:33:19.340
collisions, without being lost. And this
gave the requirement for pressures down to
00:33:19.340 --> 00:33:24.700
about 100 femtobar, which is a very small,
very, very tiny fraction of the
00:33:24.700 --> 00:33:29.679
atmospheric pressure we have here, which
is about 1 bar. And to actually get to
00:33:29.679 --> 00:33:34.019
this level of vacuum, it requires multiple
stages and multiple components to actually
00:33:34.019 --> 00:33:42.399
get there. So the initial vacuum inside
these beam tubes, which are basically
00:33:42.399 --> 00:33:48.610
going throughout the whole LHC tunnel, has
the volume of approximately the Notre-Dame
00:33:48.610 --> 00:33:53.800
cathedral. So the first step of getting
all the air out of these beam tubes is
00:33:53.800 --> 00:34:00.179
using turbomolecular pumps. And then there
needs to be more mechanisms to reduce the
00:34:00.179 --> 00:34:04.000
pressure even further, because these pumps
are not able to reduce the pressure to the
00:34:04.000 --> 00:34:09.231
levels required. And they actually use a
relatively clever trick to do that, which
00:34:09.231 --> 00:34:16.169
is the use of cryopumping. So the, ... I
cannot show that? Okay. So the outer wall
00:34:16.169 --> 00:34:20.380
of this beam pipe cross section that you
see here is actually also where the very
00:34:20.380 --> 00:34:27.230
cold helium inside the magnets is outside
of. And what that does is, it leads to an
00:34:27.230 --> 00:34:31.589
effect called cryopumping. So actually any
rest gas molecule that hits this wall
00:34:31.589 --> 00:34:35.990
actually condenses there. And as the
molecules condense there, they are of
00:34:35.990 --> 00:34:40.510
course removed from the atmosphere inside
this beam pipe, which removes them from
00:34:40.510 --> 00:34:44.750
the atmosphere and increases the quality
of the vacuum. And with the use of this
00:34:44.750 --> 00:34:47.760
and then the warm sections, the use of
getter coatings, which are basically able
00:34:47.760 --> 00:34:53.609
to trap gas molecules, you are able to
reach the crazy vacuum levels that are
00:34:53.609 --> 00:34:59.020
required to make this happen. But they
realized also during the design that one
00:34:59.020 --> 00:35:05.430
big problem – for the first time in an
accelerator – another effect will create a
00:35:05.430 --> 00:35:08.540
significant problem for the vacuum, which
is the generation of synchrotron
00:35:08.540 --> 00:35:15.240
radiation. So synchrotron radiation is a
byproduct of when you do bend a particle
00:35:15.240 --> 00:35:20.220
beam, it results in a phenomenon called
synchrotron radiation. And when this
00:35:20.220 --> 00:35:24.369
synchrotron radiation, as it goes straight
on and is not bent, hits the walls of this
00:35:24.369 --> 00:35:30.170
vacuum system, or in this case of the beam
pipe, it actually liberates molecules from
00:35:30.170 --> 00:35:33.810
there and reintroduces them into the
vacuum, which of course then makes the
00:35:33.810 --> 00:35:40.230
vacuum worse again. An additional problem
that gives the synchrotron radiation is,
00:35:40.230 --> 00:35:44.960
that it also gives a significant heat
load, and if you need to dissipate all
00:35:44.960 --> 00:35:49.660
this heat that is generated through the
very cold helium, this is not a very
00:35:49.660 --> 00:35:53.820
efficient process. Because making this
helium so cool, is actually a very energy
00:35:53.820 --> 00:35:58.590
intensive process. And just removing a
single watt of thermal power through the
00:35:58.590 --> 00:36:03.230
superfluid helium costs about 1 kW of
energy. So that is not the most efficient
00:36:03.230 --> 00:36:07.770
part. And this is why the cross-section
you have just seen includes another large
00:36:07.770 --> 00:36:11.200
component, which also technically belongs
to the vacuum system, which is called the
00:36:11.200 --> 00:36:15.940
beam screen. And this beam screen is
basically another tube running inside the
00:36:15.940 --> 00:36:20.760
beam pipe, of which we have, of course,
two, which run inside the magnet cold
00:36:20.760 --> 00:36:25.200
bores. And it shields the synchrotron
radiation heat load from the outer walls,
00:36:25.200 --> 00:36:30.550
which are at 1.8 Kelvin, while this pipe
itself is actively cooled to only about 20
00:36:30.550 --> 00:36:36.240
Kelvin of temperature, which is much more
efficient to dissipate this heat. So it is
00:36:36.240 --> 00:36:39.440
basically a steel tube about one
millimeter thick. It has these pumping
00:36:39.440 --> 00:36:46.920
holes, where hydrogen gas molecules can go
out of, and on the inside it has a copper
00:36:46.920 --> 00:36:51.970
coating, which is used to reduce its
electrical resistance, which is required
00:36:51.970 --> 00:36:55.530
because the beam, while it circulates,
also induces current that would otherwise
00:36:55.530 --> 00:37:00.180
flow inside this tube, which is really, if
you think about it, only a simple tube and
00:37:00.180 --> 00:37:03.950
it would increase the heat load again. So
a lot of engineering already has to go
00:37:03.950 --> 00:37:11.579
into a very simple piece of ... a thing
like that. So after having spoken so much
00:37:11.579 --> 00:37:16.450
about all the things required to just make
a beam circulate and accelerate and so on,
00:37:16.450 --> 00:37:20.590
now it's probably also time to talk a
little bit about the beam itself and how
00:37:20.590 --> 00:37:27.220
to control it and how to instrument, how
to measure things about this beam. Even
00:37:27.220 --> 00:37:32.080
without going yet about collisions and
doing actual physics experiments. So the
00:37:32.080 --> 00:37:36.760
first important bit that is able to
basically control or influence the beam
00:37:36.760 --> 00:37:41.540
here is what's called the beam cleaning or
collimation system. So typically such a
00:37:41.540 --> 00:37:47.050
particle beam is not very clean. It always
travels associated with what is called
00:37:47.050 --> 00:37:51.640
halo of particles around this core area
that is less than a millimeter wide where
00:37:51.640 --> 00:37:57.140
most of the intensity is focused. And
these particles outside we want to remove,
00:37:57.140 --> 00:38:00.260
because they otherwise would be lost
inside the magnets and for example, would
00:38:00.260 --> 00:38:05.640
lead to quenches of the superconducting
magnets. And for collimation, we basically
00:38:05.640 --> 00:38:10.819
use small slits that are adjustable and
are located at two main locations of the
00:38:10.819 --> 00:38:15.100
LHC. So they have collimation systems
there, with vertical and horizontal slits
00:38:15.100 --> 00:38:21.590
that can be adjusted in width, in order to
scrape off all the particles that they do
00:38:21.590 --> 00:38:25.319
want to get rid of and extract out of the
beam, while only the core part can
00:38:25.319 --> 00:38:30.490
circulate and produce clean collisions
without any background, that otherwise
00:38:30.490 --> 00:38:36.110
would need to be accounted for. And then
there is a whole other open topic of beam
00:38:36.110 --> 00:38:39.710
instrumentation. So when you run a
particle accelerator, you want to measure
00:38:39.710 --> 00:38:45.220
various quantities and performance figures
of such a beam. And that is crucial for a
00:38:45.220 --> 00:38:48.730
correct operation and for the highest
performance, getting the highest
00:38:48.730 --> 00:38:52.331
performance from an accelerator. And there
are a lot of different types of those, and
00:38:52.331 --> 00:38:58.920
I want to go quickly about ... over why we
have them and what we do with them. So the
00:38:58.920 --> 00:39:03.730
first and most basic measurement you want
to do, is the beam current measurement. So
00:39:03.730 --> 00:39:08.550
the beam current is a basic accelerator
beam intensity measurement. So it gives
00:39:08.550 --> 00:39:13.260
you an idea of how strong the beam that is
running inside your accelerator is. And it
00:39:13.260 --> 00:39:18.000
is measured using these DCCTs or DC
current transformers. And their basic
00:39:18.000 --> 00:39:22.230
principle of operation is, that while the
particles move through this torus, which
00:39:22.230 --> 00:39:26.069
is actually a coil or a transformer,
induces a voltage there that you can
00:39:26.069 --> 00:39:30.579
measure and then use to quantify the
intensity of this beam. And the big
00:39:30.579 --> 00:39:34.760
challenge here is that the dynamic range,
this instrument needs to capture, is
00:39:34.760 --> 00:39:39.930
really large, because it has to operate
from the lowest intensity pilot injection
00:39:39.930 --> 00:39:46.119
beams up to the full energy, full number
of bunches running inside the LHC. So it
00:39:46.119 --> 00:39:51.580
has to cover six orders of magnitude of
measurement dynamic range. Then the second
00:39:51.580 --> 00:39:57.130
thing when talking about collisions is the
luminosity measurement. So luminosity is a
00:39:57.130 --> 00:40:02.210
quantity basically said to measure the
rate of interaction of the particle beams.
00:40:02.210 --> 00:40:06.710
So to give you an idea of how often
interactions happen inside the experiments
00:40:06.710 --> 00:40:11.280
or where you want them to happen. And this
measurement is used to first of all,
00:40:11.280 --> 00:40:14.461
adjust this interaction rate to a target
value, which is optimal for the
00:40:14.461 --> 00:40:19.109
experiments to function and to equalize
the interaction rates in different
00:40:19.109 --> 00:40:23.970
experiments. So different experiments also
are specified to have the same interaction
00:40:23.970 --> 00:40:29.650
rate so they can get the same let's say
statistical quality of their data. So it's
00:40:29.650 --> 00:40:33.869
used to equalize those. And then as a
third thing, this system is also used to
00:40:33.869 --> 00:40:37.510
measure the crossing angle of the beam. So
as you may know, at some point, when the
00:40:37.510 --> 00:40:42.610
beams are collided, they collide at an
angle, that is very small. And this angle
00:40:42.610 --> 00:40:47.559
is actually measured also very precisely
in order to adjust it correctly. And it is
00:40:47.559 --> 00:40:50.190
measured to less than a thousandth of a
degree, which is again a very impressive
00:40:50.190 --> 00:40:54.640
feat, given that the detection principle
of this measurement is only measurement of
00:40:54.640 --> 00:40:58.700
some neutral particles that are a result
of the particle interaction of the beam
00:40:58.700 --> 00:41:05.080
... of the collision. Okay, so that is
number two. Then number three that we have
00:41:05.080 --> 00:41:09.610
is the beam position monitor. Because
along the LHC, you also always want to
00:41:09.610 --> 00:41:14.940
know, where the beam is at any given time.
So you want to measure the position of the
00:41:14.940 --> 00:41:19.190
beam inside the beam pipe in order to
optimally adjust it to the position you
00:41:19.190 --> 00:41:23.740
want to have it. And for that we use these
beam position monitors of which we have
00:41:23.740 --> 00:41:27.910
more than a thousand installed along the
LHC. So they are typically capacitive
00:41:27.910 --> 00:41:31.780
probes or electromagnetic strip lines. As
you can see on top and bottom
00:41:31.780 --> 00:41:36.790
respectively. And they basically are
distributed along the LHC and provide
00:41:36.790 --> 00:41:40.300
position of the particle beam along the
accelerator, which can then be used to
00:41:40.300 --> 00:41:47.640
tune, for example, the magnets. All right.
Then we have beam profile. So after the
00:41:47.640 --> 00:41:51.059
position, that gives you an idea where the
beam is, you also want to know its
00:41:51.059 --> 00:41:57.329
intensity distribution. Basically when you
do a cut through the beam pipe somewhere,
00:41:57.329 --> 00:42:02.480
you want to know how the intensity profile
looks like. And for that we have basically
00:42:02.480 --> 00:42:07.380
two measurement systems. One measures the
profile in X and Y directions. So if you
00:42:07.380 --> 00:42:10.400
really would do a cut and it gives you
something like this and it's, for example,
00:42:10.400 --> 00:42:14.950
done with wire scanners, which is literal,
very thin wire that is moved through the
00:42:14.950 --> 00:42:20.290
beam. And then the current that the beam
moving through this wire, generates is
00:42:20.290 --> 00:42:25.280
used to generate such a profile map, when
scanning with this wire. The other one is
00:42:25.280 --> 00:42:29.750
the longitudinal profile, which gives you
an idea about the quality of your RF
00:42:29.750 --> 00:42:34.040
system and there you want to know how the
intensity profile of your beam looks like.
00:42:34.040 --> 00:42:37.690
If you were looking only at one spot of
the accelerator and the beam would pass
00:42:37.690 --> 00:42:43.230
by, and you would basically see over time
how the intensity looks like. And then as
00:42:43.230 --> 00:42:47.980
the last bit of beam instrumentation,
there is the beam loss monitors. So they
00:42:47.980 --> 00:42:53.170
are these yellow tubes, that are located
on the outside of mostly all the magnets,
00:42:53.170 --> 00:42:57.951
of the dipole and quadrupole magnets and
so on. Again, there's more than a thousand
00:42:57.951 --> 00:43:03.230
of those. And the idea here is that you
need a lot of detectors that are basically
00:43:03.230 --> 00:43:07.819
small ionization chambers, which detect
any showers of secondary particles that
00:43:07.819 --> 00:43:12.720
are generated when one of the high energy
protons are lost somewhere in the magnet
00:43:12.720 --> 00:43:16.760
materials. So these are really used for
protection of the system, because if a
00:43:16.760 --> 00:43:21.109
specific threshold of energy loss is
detected, then the accelerator needs to be
00:43:21.109 --> 00:43:26.220
quickly shut down. Which is why they have
to react in a matter of nanoseconds in
00:43:26.220 --> 00:43:31.260
order to keep the accelerator safe.
Because any interaction of the particle
00:43:31.260 --> 00:43:36.059
beam with for example, the magnets could
just destroy huge amounts of money and of
00:43:36.059 --> 00:43:41.870
time that would be needed to rebuild. And
as a last and final thing, we have spoken
00:43:41.870 --> 00:43:46.260
one or two times already about shutting
down the LHC. Which sounds also trivial at
00:43:46.260 --> 00:43:51.799
first, but really is not. So, the last
thing here is, what we call the Beam Dump.
00:43:51.799 --> 00:43:56.589
So the energy content that is contained in
those particular beams, it can be used,
00:43:56.589 --> 00:44:02.950
could be used if it were shot on a copper
target, it could just melt 1000 kilograms
00:44:02.950 --> 00:44:07.599
or one ton of copper instantly. So during
beam extraction, the process of getting
00:44:07.599 --> 00:44:12.359
the particle beam outside out of the LHC,
this energy needs to be dissipated
00:44:12.359 --> 00:44:16.990
somehow. And for that, this special Beam
Dump area is constructed. So there are
00:44:16.990 --> 00:44:20.670
fast kicker magnets, that are used to ...,
that are able to ramp up in a really,
00:44:20.670 --> 00:44:24.930
really short amount of time of
microseconds. And then the beam is
00:44:24.930 --> 00:44:30.309
carefully and in a controlled manner
directed into a set of concrete blocks,
00:44:30.309 --> 00:44:35.339
that is basically big enough to dissipate
all this energy, when required. And in the
00:44:35.339 --> 00:44:38.800
process of doing so, it also heats up to
about 800 degrees Celsius, and then of
00:44:38.800 --> 00:44:45.530
course, also needs the associated time to
cool down again. Good. So as you may or
00:44:45.530 --> 00:44:50.040
may not know, currently the LHC is not in
operation. So LHC currently is undergoing
00:44:50.040 --> 00:44:56.099
its second long shutdown phase, or LS2.
But what we do when the LHC is in
00:44:56.099 --> 00:44:59.961
operation, is that we have these status
dashboards, that you can see here, that
00:44:59.961 --> 00:45:04.970
are distributed all around CERN, and can
be used by anyone, any passer-by, to
00:45:04.970 --> 00:45:09.980
basically monitor what the current
operation mode or the current situation of
00:45:09.980 --> 00:45:15.609
the accelerator is. And can be used also
to quickly see if like an operator needs
00:45:15.609 --> 00:45:19.990
to go somewhere or is needed, or how the
shift planning for the next shift works
00:45:19.990 --> 00:45:24.430
out and so on. And on the right side you
would see what this currently looks like.
00:45:24.430 --> 00:45:30.990
So basically black screen saying next beam
expected in spring 2021. And the good
00:45:30.990 --> 00:45:33.980
thing about these status pages is that you
can actually see them from your home,
00:45:33.980 --> 00:45:40.390
because they're also openly available, as
most of the stuff we do at CERN. So if you
00:45:40.390 --> 00:45:44.510
are interested, then perhaps in a year
from now or a bit longer than a year, it
00:45:44.510 --> 00:45:47.670
would be quite interesting to follow all
the commissioning process of when they are
00:45:47.670 --> 00:45:54.130
trying to start the LHC back up, and
follow that process from your home.
00:45:54.130 --> 00:45:58.109
Otherwise, if you now feel the urge to
maybe visit CERN, pay some of the things
00:45:58.109 --> 00:46:01.980
we talked about a visit, or are just
generally interested, CERN offers a
00:46:01.980 --> 00:46:06.970
variety of tours free of charge. So if
you're interested in that, visit that web
00:46:06.970 --> 00:46:10.819
site and we would be happy to welcome you
there. And with that, thank you very much
00:46:10.819 --> 00:46:14.569
for your attention.
00:46:14.569 --> 00:46:17.530
Applause
00:46:17.530 --> 00:46:24.470
Severin: Punktlandung.
Herald: Thank you, Stefan and Severin. If
00:46:24.470 --> 00:46:29.869
you have questions, there are six
microphones in the room. Please make a
00:46:29.869 --> 00:46:34.660
queue, and we start with the Signal Angel.
Signal Angel, please, first question.
00:46:34.660 --> 00:46:39.630
Signal Angel: There is said to be a master
red button for shutting down the whole
00:46:39.630 --> 00:46:45.859
system in case of heavy problems. How
often did you push it yet?
00:46:45.859 --> 00:46:49.059
Stefan: Master red button?
Severin: Master button ...
00:46:49.059 --> 00:46:55.160
Signal Angel: Like a shut down button.
Severin: I cannot really understand you. I
00:46:55.160 --> 00:46:59.270
think the question was about how often
basically we used the Beam Dump system to
00:46:59.270 --> 00:47:01.050
basically get rid of the beam, is that
correct?
00:47:01.050 --> 00:47:04.280
Signal Angel: I guess so.
Stefan: He said there is a master button.
00:47:04.280 --> 00:47:06.280
Signal Angel: I guess so.
Stefan: I think there's a master button in
00:47:06.280 --> 00:47:08.280
the...
Severin: There is not only one master
00:47:08.280 --> 00:47:11.790
button, there are several master buttons.
These are switches, called beam interlock
00:47:11.790 --> 00:47:17.530
switch. Basically, at every operator's
screen, there is basically one beam
00:47:17.530 --> 00:47:23.690
interlock switch. I don't know. I think
sometimes they get rid of the beam just
00:47:23.690 --> 00:47:29.579
because, I mean. When we have LHC at full
operation – Stefan talked about the
00:47:29.579 --> 00:47:33.799
luminosity – so what is happening, that in
the beginning we have a very high amount
00:47:33.799 --> 00:47:39.920
of luminosity, So many particles collide
on each other. But over time, like after
00:47:39.920 --> 00:47:45.230
12 or 15 hours or whatever, basically the
luminosity ..., so the amount of particles
00:47:45.230 --> 00:47:49.119
which collide with each other, is going
down and down. So the luminosity
00:47:49.119 --> 00:47:53.770
decreases. And then at some point in time,
basically the operators decide, that they
00:47:53.770 --> 00:47:58.950
will now get rid of the actual beam, which
is inside LHC and basically will recover
00:47:58.950 --> 00:48:02.290
the whole machine and then restart the
machine again. And this is done sometimes,
00:48:02.290 --> 00:48:08.170
I don't know, every 12 hours, sometimes
after 24 hours. Something like that, yes.
00:48:08.170 --> 00:48:11.380
Herald: Cool. And microphone number four,
I think.
00:48:11.380 --> 00:48:17.559
Q: Yes. So where's the energy coming from?
So do you have your own power plant, or
00:48:17.559 --> 00:48:20.119
so?
Severin: So, no, not really, not really.
00:48:20.119 --> 00:48:24.349
Basically, we get all the power from the
French grid. So we have relatively big
00:48:24.349 --> 00:48:32.640
power trails coming from the French grid.
So we get 450 kV of power there. So
00:48:32.640 --> 00:48:35.390
basically the voltage is quite high and
then we have our own transformers on site.
00:48:35.390 --> 00:48:39.609
And I think only, ... a little bit smaller
fraction of the energy is coming from the
00:48:39.609 --> 00:48:43.940
Swiss grid. So basically we use most of
the energy which is coming from the French
00:48:43.940 --> 00:48:46.940
grid.
Q: Okay. Thank you.
00:48:46.940 --> 00:48:48.940
Herald: Thank you for your question. And
microphone number one, please.
00:48:48.940 --> 00:48:57.280
Q: Hi. Thank you for your presentation. If
I'm not wrong, you say the beam can warm a
00:48:57.280 --> 00:49:04.599
block of concrete to 800 Celsius. Would it
be possible to use it as a weapon?
00:49:04.599 --> 00:49:09.960
Stefan: laughs Very likely not. And CERN
very much condemns these actions in any
00:49:09.960 --> 00:49:13.690
form, I guess. So CERN operates in a
purely peaceful mission and would never
00:49:13.690 --> 00:49:18.000
think about using their particle beams as
a weapon. And even if they could, it is
00:49:18.000 --> 00:49:21.720
probably not the most practical thing to
do, I guess. laughs
00:49:21.720 --> 00:49:26.710
Herald: But if your telephone is again
hanging up, you can destroy it, right?
00:49:26.710 --> 00:49:29.090
Stefan: laughs
Herald: And microphone number six, I
00:49:29.090 --> 00:49:33.390
think.
Q: Yes. So you said, you can stop in
00:49:33.390 --> 00:49:39.980
nanoseconds, but just the light would go
just 30 centimeters, you know, a
00:49:39.980 --> 00:49:45.110
nanosecond. How will you be able to stop
in this small time?
00:49:45.110 --> 00:49:49.220
Stefan: Ah, no, no. So what I was talking
about is that these magnets that are used
00:49:49.220 --> 00:49:54.770
to extract the beam out of the LHC, they
have reaction times, or ramp up times that
00:49:54.770 --> 00:50:01.020
are in the order of 1, 2, 3 microseconds.
So not nanoseconds, but microseconds. And
00:50:01.020 --> 00:50:06.329
really only then basically the particles
still circulate, worst case one full turn,
00:50:06.329 --> 00:50:10.430
and only then moving outside of the
accelerator.
00:50:10.430 --> 00:50:17.349
Herald: And microphone number one again.
Q: So do you have any photos of the front
00:50:17.349 --> 00:50:22.609
of the dump block? It has to look like
it's got hit a lot.
00:50:22.609 --> 00:50:26.110
laughter
Severin: No, not really. I think it's one
00:50:26.110 --> 00:50:31.819
of the only pictures we could find about,
the Beam Dump system. And these areas, I
00:50:31.819 --> 00:50:37.059
think it's not really opened any more. So
since operation of LHC, which was in
00:50:37.059 --> 00:50:43.329
basically LHC started in 2008, and since
then, the Beam Dump system was not opened
00:50:43.329 --> 00:50:48.860
again because it's completely sealed in
stainless steel. And that's why it wasn't
00:50:48.860 --> 00:50:52.339
opened anymore.
Heral: Cool. Question from the interwebs.
00:50:52.339 --> 00:50:59.329
Signal: Regarding power supply. How do you
switch or fine-control the currents? Are
00:50:59.329 --> 00:51:03.460
you using classic silicone transistors,
off-the-shelf IGBTs?
00:51:03.460 --> 00:51:07.230
Sverin: Um, yes. laughs
laughter
00:51:07.230 --> 00:51:12.530
Severin: Yes. Uh, the system was developed
at CERN. And I think that's quite common
00:51:12.530 --> 00:51:16.280
at CERN that we basically developed all
the technology at CERN or try to develop
00:51:16.280 --> 00:51:20.250
nearly everything at CERN. But then
production, for example, is put into
00:51:20.250 --> 00:51:25.750
industry. And yes, these are relatively
classical power converters. The
00:51:25.750 --> 00:51:29.609
interesting or like challenging part about
the current power converters is really
00:51:29.609 --> 00:51:32.859
that the current has to be measured quite
precisely and also controlled quite
00:51:32.859 --> 00:51:39.430
precisely so there we use also DCC TS.
Which we have also mentioned before. But
00:51:39.430 --> 00:51:42.290
basically all this controlled mechanism
there. That's one of the big challenges
00:51:42.290 --> 00:51:47.980
there.
Herald: Cool. Microphone number one again.
00:51:47.980 --> 00:51:55.609
Q: You talked about the orbit clock that
detects when the bunch is completed one
00:51:55.609 --> 00:52:00.200
round. How is it possible to detect which
is the first bunch?
00:52:00.200 --> 00:52:03.980
Stefan: Yeah. So it is it is actually not
detected, but this clock is actually
00:52:03.980 --> 00:52:07.470
something that is constructed. So we
basically what we do is, we count these
00:52:07.470 --> 00:52:13.250
cycles of the of the RF cycle. Maybe I can
open this slide. So somewhere there is a
00:52:13.250 --> 00:52:18.510
counter that basically knows how many 40
MHz clock cycles a full rotation takes.
00:52:18.510 --> 00:52:21.770
And then at some point decides this is
number one. And that's also where they
00:52:21.770 --> 00:52:26.180
start counting when they inject bunches
into the LHC. So there's no marker, let's
00:52:26.180 --> 00:52:29.510
say. But there is a certain structure to
the beam. So you could potentially do
00:52:29.510 --> 00:52:33.320
that. So, for example, for these longer
periods where the kicker magnets need to
00:52:33.320 --> 00:52:36.910
ramp up, they have something they call the
abort gap. So a number of bunches that are
00:52:36.910 --> 00:52:41.350
never filled but are always kept empty. So
the magnets have enough time to deflect
00:52:41.350 --> 00:52:44.460
the beam when the next bunch comes around.
So you could probably measure that, but
00:52:44.460 --> 00:52:47.260
it's much easier to do it the other way
around.
00:52:47.260 --> 00:52:54.570
Herald: Microphone number four, please.
Q: You said you had quite tight needs for
00:52:54.570 --> 00:53:01.230
the timing clock. Is it tight enough? That
the speed of light was the limit with the
00:53:01.230 --> 00:53:03.589
distances between locations or that was
not a concern?
00:53:03.589 --> 00:53:08.680
Stefan: No, it is a concern. So because
just distributing a cable for 27
00:53:08.680 --> 00:53:14.869
kilometers produces like just considerable
run times of electrical signals. All the
00:53:14.869 --> 00:53:18.150
delays of all the cables need to be
measured precisely for their delay and
00:53:18.150 --> 00:53:22.849
then calibrated out so all the experiments
get their clocks at the right time,
00:53:22.849 --> 00:53:27.400
shifted, compensated for the delay time
that it just takes to get the signal
00:53:27.400 --> 00:53:33.000
there.
Herald: And again, the interwebs.
00:53:33.000 --> 00:53:40.210
Signal Angel: Is it too dangerous to stand
near the concrete cooling blocks, like
00:53:40.210 --> 00:53:46.980
radioactive wise or, I don't know.
Severin: Yes.
00:53:46.980 --> 00:53:49.090
laughter
Stefan: Not recommended.
00:53:49.090 --> 00:53:54.859
Severin: Not recommended. We have a very
good interlock system. Also, the doors,
00:53:54.859 --> 00:53:58.270
all the doors have switches. So basically
when one door is basically like opened
00:53:58.270 --> 00:54:03.059
then basically the whole machine will be
shut down. So we have very critical and
00:54:03.059 --> 00:54:11.319
safety related access system at LHC. Maybe
you watch Angels and Demons. This
00:54:11.319 --> 00:54:16.760
Hollywood movie that we have, the eye
scanners are shown. It's a little bit. I
00:54:16.760 --> 00:54:22.430
mean, it's Hollywood. But, we have eye
scanners. So iris scanners. So every time
00:54:22.430 --> 00:54:25.780
like we want to go to the tunnel, for
example, then we have to let also our iris
00:54:25.780 --> 00:54:30.000
be scanned because otherwise we will not
be able to go to the tunnel. So there's a
00:54:30.000 --> 00:54:33.890
very sophisticated access system to really
go to the tunnel. So when there is
00:54:33.890 --> 00:54:36.810
operation, the whole tunnel access is
completely blocked.
00:54:36.810 --> 00:54:40.510
Herald: Good, microphone number one,
please.
00:54:40.510 --> 00:54:47.780
Q: What is the exact reason to have each
of the experiments, every side. I mean, so
00:54:47.780 --> 00:54:52.080
far apart on the LHC. I mean, on
opposite sides.
00:54:52.080 --> 00:54:57.609
Severin: Um, basically, you are talking
about Atlas and CMS. The reason for that
00:54:57.609 --> 00:55:02.060
is because when, these two experiments
were constructed, there was a little bit
00:55:02.060 --> 00:55:10.060
of fear that particles basically interact
at the two experiments. So that they
00:55:10.060 --> 00:55:13.670
really are like the most far away. We like
to have a very big distance from each
00:55:13.670 --> 00:55:17.930
other. So there is no interaction between
them. That's why we basically put them at
00:55:17.930 --> 00:55:20.869
point one and point five. That's the
reason why.
00:55:20.869 --> 00:55:25.549
Herald: If I can see it correctly.
Microphone number five.
00:55:25.549 --> 00:55:32.030
Q: Yes, hello. I've seen that you're also
using the CAN bus. What are you using the
00:55:32.030 --> 00:55:37.190
CAN bus for in CERN?
Stefan: I know of at least one use, but it
00:55:37.190 --> 00:55:42.539
is inside an experiment. So there are, as
far as I know, investigations under way to
00:55:42.539 --> 00:55:47.610
use the CAN bus to do the actual control
of the detectors of one experiment. I
00:55:47.610 --> 00:55:51.059
don't know if there is a use inside the
accelerator itself. So apart from the
00:55:51.059 --> 00:55:55.790
experiments. But perhaps if you come by
afterwards we can find one.
00:55:55.790 --> 00:55:59.539
Q: Thank you.
Herald: Microphone number one.
00:55:59.539 --> 00:56:05.829
Q: Do you have any official data about how
many tons of duct taper used in
00:56:05.829 --> 00:56:08.480
daily operations?
laughter
00:56:08.480 --> 00:56:13.750
Severin: No. No.
Herald: What about zip ties?
00:56:13.750 --> 00:56:21.460
Severin: Many. Yeah. Millions. Billions.
Herald: Okay. As far as I can see... Ah,
00:56:21.460 --> 00:56:25.470
the intercepts again with a question.
Signal Angel: Do you know your monthly
00:56:25.470 --> 00:56:27.470
power bill?
laughter
00:56:27.470 --> 00:56:31.470
Severin: No, not no. No, sorry.
Stefan: No. But it is, I think, in fact
00:56:31.470 --> 00:56:37.960
the contribution of France, which is the
main contributor in terms of energy. That
00:56:37.960 --> 00:56:41.440
it is part of their contribution to
contribute the electricity bill basically
00:56:41.440 --> 00:56:44.440
instead of paying money to CERN. That's as
far as I know.
00:56:44.440 --> 00:56:48.480
Severin: Yes. And also we shut down the
LHC and the accelerator complex through
00:56:48.480 --> 00:56:52.859
like the wintertime. And one of the
reasons for that is because electricity is
00:56:52.859 --> 00:56:56.940
more expensive during wintertime in France
than in summer.
00:56:56.940 --> 00:57:03.529
Herald: In this case, I can't see any
other questions. I have a maybe stupid
00:57:03.529 --> 00:57:09.780
question. You said earlier you have to
focus and defocus the beam. But as we
00:57:09.780 --> 00:57:12.529
know, you accelerated already the
particles. Why do we have to focus the
00:57:12.529 --> 00:57:17.020
beam?
Severin: Because every time when we have a
00:57:17.020 --> 00:57:21.910
dipole magnet, then basically we bend the
particle around an arc. But then we also
00:57:21.910 --> 00:57:24.720
defocus a little bit. And also, the
coulomb force is still a problem because
00:57:24.720 --> 00:57:29.770
we have equally charged particles in the
bunch or in the whole beam itself. So they
00:57:29.770 --> 00:57:33.539
will by themselves basically go out of
each other. And if you would not focus it
00:57:33.539 --> 00:57:36.299
again, then basically we would lose the
whole beam in the end.
00:57:36.299 --> 00:57:44.630
Herald: Oh, thank you. I don't see any
questions. Internet? In this case thank
00:57:44.630 --> 00:57:49.730
you very, very much, Stefan and Severin.
Please. With a warm applause. The Large
00:57:49.730 --> 00:57:50.730
Hadron infrastructure talk.
00:57:50.730 --> 00:57:51.730
Applause
00:57:51.730 --> 00:57:52.989
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