*36c3 preroll music*
Herald: Our next talk's topic is the Large Hadron Collider infrastructure talk. You
probably know the Large Hadron Collider over at CERN. We heard quite a bit of it
in the recent talks. This time we will have a deep dive into the infrastructure.
You can assume our next speakers are doing a great job. Basically, it's pretty
obvious because we are not stucked into a great into a giant supermassive black
hole. So please welcome with a very warm applause, Severin and Stefan.
*Applause*
Severin: Yeah. Hello, everyone. Thanks for coming. So many people here, quite nice.
In the last couple of years we had several talks about, yeah, basically the physics
perspective of LHC, how physicists analyze data at LHC, how physicists store all the
data, et cetera. And we would like to give like more an engineering perspective of
the whole LHC. So three years ago we had a talk by Axel about how physicists analyze
massive big data and then last year we had a talk conquering large numbers at LHC by
Carsten and Stefanie. And we would, as I've mentioned already, we would like to
give like more an engineering perspective. We are Stefan and Severin. We're both
electrical engineers working at CERN. Stefan is working in the experimental
physics microelectronics section and he will give a second talk tomorrow about
designing high reliability digital electronics together with Szymon tomorrow
morning at 11:30. And I'm, as I mentioned already, also working at CERN. I'm working
in the electronics for machine protection section. I will describe briefly later. A
short disclaimer; the LHC is a pretty big machine and we try to explain it as good
as possible. 45 minutes is not really enough to talk about everything because I
think you can basically take one of the topics we are talking here about now and
talk for 45 minutes only about one specific topic, but we try to give an
overview as good as possible. So imagine you want to build an accelerator in your
backyard. OK, maybe not in your backyard because LHC is quite big, so 27 kilometers
in diameter is quite big, but basically we figured out three main challenges you have
to take. First of all, we have to accelerate particles because otherwise
it's not a particle accelerator. Second, we have to keep the particles on a
circular trajectory. And then third, we have to make sure that the particles which
are inside our beam tube or beam pipe don't collide with anything which is
there, for example, to beam pipe itself, air molecules, etc. And the solution we
adopted for LHC there is, that we accelerate the particles with a high power
radio frequency cavities. Then we have a beam control system which is quite
sophisticated using superconducting magnets and then we have the beam pipe
itself, which is evacuated, so it's under vacuum conditions to avoid any collisions
we have inside with gas molecules, etc. A brief overview about the location itself.
So probably many of you know already that CERN is next to Geneva. So it's in the
western-southern part of Switzerland. When we zoom in a little bit more, then we have
here an artificial like picture of LHC itself in the red circle there. To put it
a little bit in a perspective, we have a relatively big airport there. You can see
there, it's a 2200 metre long runway. We have Geneva Lake next to it. And that's
only one small part of Geneva Lake, but nevertheless, and what also quite nice, we
see Mont Blanc from LHC, er, from CERN. When we zoom in a little bit more, then we
basically have the big, circular collider there. That's LHC itself. And we have pre-
accelerators, I will talk in a few minutes about. Basically we have two main
campuses: we have Meyrin Site, which is in Switzerland, and we have to Prevessin
site, which is in France. Then at LHC itself, we have eight service points. We
also call it just points, to briefly go through all of them; we have point one
where we have the experiment called ATLAS, one of the big and major experiments at
LHC. Then at the exactly opposite side of ATLAS, we have CMS at point five. Then we
have a little bit smaller experiment, which is ALICE. It was basically
constructed for lead ion runs. We will talk about this later. And then we have
another relatively small experiment, it's called LHCb. And that's the only non-
symmetrical experiment at LHC. These are, I think, the four experiments you already
maybe heard of. Then there are four or three other smaller experiments. We have
LHCf at point one, it's a forward scattering experiment at point one. So
basically, they're taking data like scattered particles from ATLAS itself.
Then we have TOTEM. It's also a forward scattering experiment and point five, then
we have, sorry, we have MOEDAL, which is the experiment at point eight. They're
looking for magnetic mono-poles. Then we have TOTEM, sorry for that, at point five.
And then we have a relatively new experiment which is called PHASER. It's
actually under construction and it will be used, starting from 2021 and it's forward
scattering experiment, which, where they try to detect neutrinos. Then we have
point four, there we have the RF cavities to accelerate the particle beam itself. We
have the beam dump area. So when there is like a fault in a machine or we just want
to dump the beam, then we used the mean dump system at point six. And then we have
two more general service areas. It's point three and point seven. LHC would not be
possible without the pre-accelerator complex. So we have a relatively big one
and it's also sometimes relatively old. On the left hand side of the slide you can
see LINAC2, it's an old linear accelerator which was used until last year. It's not
now phased out. And now we have LINAC4, which is also a linear accelerator and it
has a little bit higher acceleration. Then we have the proton synchrotron booster.
It's the first circular collider. So you can see two pictures there. What is
relatively special about PSB is that we have there two, sorry, four beam pipes
instead of just one beam pipe. Then we have the proton synchrotron accelerator,
which is the next stage for acceleration. It then has only one one beam pipe. And
then we are going from PS we are going to SPS, which is the super proton synchrotron,
which is has circumferences of seven kilometers. There we basically accelerate
the particles the last time and then they are injected it in the LHC itself. We
mentioned a few accelerators already, basically all everything we just
highlighted here. But CERN is a little bit more. So CERN is famous for LHC, I would
say. But there is much more than only the LHC. So only about 15 percent of the
protons, which are accelerated in the pre- accelerator complex, are really going to
the LHC itself. So there is much more: there is material science, there is anti
matter research and all different other kinds of research going on. Of course,
everything has to be controlled. It's called a CCC, the CERN control center.
It's located at, the Prevessin site; looks like that. Basically, we have, four
Cs looking to each other and there the operators are sitting 24/7 and operate the
whole machine. So basically, the whole pre-accelerator complex, all the energy
cryogenics and LHC itself. Before you ask, everything is running on Scientific Linux.
So we have basically our own Linux distributed distribution, which is used
there and it of course, it's open source. Talking about the LHC beam itself, we have
two beams: one is running clockwise and the other one is to running anti-
clockwise because we don't have a fixed target experiment where we basically let
the accelerated particles colliding with like a fixed target, like metal or
something like that. We have controlled collisions at four points, we mentioned
before. Most of the year we have proton runs. So we have protons and protons
colliding each towards each other. And then we have at the end of the year,
nearly starting from November to December, we have lead ion run. The protons itself
is not really like a fixed, straight line of particles. We have something called
bunches. You can imagine a little bit like spaghetti. It's basically the same length
of a spaghetti, but it is much thinner than a spaghetti. And each bunch, when you
have a proton run, then each bunch consists of approximately 100 billion
protons. And when you have lead ion runs, then we have approximately 10 million lead
ions in LHC. And last year we operated with 2565(sic) bunches in the LHC itself.
The LHC tunnel. We already talked about the tunnel itself. It is 27 kilometers
long and you can see maybe a little bit on this graph, that we have some, we have
eight straight sections and we have eight arcs in the tunnel. Basically the straight
sections are always there, where we have like service cavities or we have areas and
also the experiments. Because it's not so good visible in this picture, I put the
picture here. Basically, that's a straight section of LHC. You can basically just see
the beam pipe itself, with aluminum foil around it, and there are also no magnets.
And when we look in the arc section of LHC, then you see here the arc itself and
I think it's quite famous picture of LHC itself because we have blue dipole magnets
there. The tunnel itself is an old tunnel, used previously by LEP, the large electron
proton collider. It has a diameter of 3.8 metres and the circumference is
approximately 27 kilometers. Inside the tunnel we have, first of all, cryogenics,
so we have big tubes, stainless steel tubes to carry all the cryogenic. So
liquid helium and gaseous helium. Then we have the magnet itself to bend the
particles and then we have electrical installations to carry like signals from
the magnets to have safety systems, electricity, etc, etc. Geography is a
little bit complicated in the area because we have in the western part of LFC we have
the Jura mountain range and this Jura mountain range has a relatively hard
material. It's not made out of, not made, but nature. I mean it's a limestone, so
it's relatively complicated to dig into this material, in comparison to all the
other areas at LHC. So when you would basically put a straight section of LHC,
then you have to dig much more into the relatively hard limestone. So that's why
it was decided that the LEP or LHC tunnel is tilted a little bit. So we have a tilt
angle of one 1.4 percent there. The depth is approximately between 50 meters at
point one or point eight, up to 170 metres deep at point four. We already talked a
little bit about magnets, but we would like to go a little bit more in the
details now. So why do we need magnets? Um, maybe you learned at school that when
you have a magnetic field and you have charged particles and you can bend
particles around an arc in a magnetic field. Depending on the charge of the
particles, you bend them around on the right side or left side, that's this
famous right hand and left side rule, you maybe learned during school. And at LHC we
cannot use a normal magnets like typical magnets. We have to use electromagnets
because normal magnets would not be strong enough to build in an electromagnetic
field which is feasible to bend the particles around the whole tunnel. At LHC
we use, in the dipole magnets, a magnetic field of 8.3 Tesla. And to do this we need
a current of 11850 amps. We have basically two different types of magnets. We have
bending magnets. So the dipole magnets I mentioned quite often already. And then we
have injection and extraction magnets. They are also dipole magnets there, but
they are a little bit differently constructed, because the injection and
extraction magnets have to be quite fast, because they have to basically be powered
up at full, the full magnetic field in several microseconds. Then we have higher
order magnets which are quadrupole pool, magnet, sextupole magnets and octupole
magnets, et cetera, et cetera. And they are used for focusing and defocusing the
beam itself. In total, we have 1200 dipole magnets at LHC. We have around 850
quadrupole magnets and we have 4800 higher order magnets. But they are normally quite
short or so, shorter than the the other magnets. The dipole magnets consist of two
apertures. They are used to bend to beam around, so I already said. In the middle
of the magnet itself we have a cold bore. So there is basically there are the
particles flying around. Then there is a metallic structure. You can see this in
the picture. It is just a shiny metallic sphere you see there. And then we have
next to the cold bore, we have the tool, the two apertures to bend the particle and
build the magnetic field itself. The dipole magnets have a length of 50 meters
and the manufacturing precision is plus minus one fine, one point five percent,
er, one point five millimeters. Then we have quadrupole magnets. They are used for
focusing and defocusing the beam. The problem is that we have bunches, were are
basically equally charged particles inside. And the Coloumb force tells us, that when
we have equally charged particles, then they are basically want to fade out from
each other and in the end they would just hit the beam pipe itself and we could
maybe destroy the beam or cannot do any collisions. So what we do is we use a
quadruple magnets as, yeah, similar to lenses, because we can focus and defocus
the beam. The quadrupole magnets, the name already suggested it, that we have
basically four apertures. So we have on the left and the right side two and then
we have on top and bottom we have also a few of them. To go a little bit into
detail about the focusing and defocusing scheme. In the beginning we have a
particle beam which is not focused, but we want to focus it. Then we go to the first
quadrupole magnet. So we focus the beam. And this is only done in one axis. That's
a little bit a problem. So, in the second axis we don't have any focus and we have a
defocusing effect there. And then we have to use a second quadrupole magnet for the
other axis, in this case the Y-axis to focus the beam even further. And you can
even see this here in the Z-axis, that's basically the cut off the beam itself. You
can also see that in the beginning we have on the left side, we have an unfocused
beam and then we focus it in one axis, so we have like a little bit more ellipse and
then we focus in the other direction, then we have a different ellipse. So we have to
use several quadruple magnets in a row to really focus the beam in the way we want
to have it. In the LHC magnets, we have quite high currents. We we need these
currents, because otherwise we cannot bend to the very high energetic particle beam
and to use normal conducting cable, it would not be possible to basically build a
magnet out of it. So what we do is, we use materials which are called superconducting
materials, because they're for very good effect. They go to basically zero
resistance at a specific temperature point. And after this point or when we
basically go lower, then the current can flow without any losses inside of it. But
to reach the state, we have to cool the magnets quite heavily, which is not so
easy, but it can be done. And on the right side you basically see a very historic
plot. That was 1911 in Denmark, a researcher called Heike Onnes detected for
the first time superconducting effect in mercury. And it was detected at 4.19
Kelvin. To show you a little bit the comparison between a normal conducting
cable and a superconducting cable, as we put the picture here. So that is basically
the same amount of cable you need to use to carry, the thirteen thousand amps and
to do the same or to transport the same amount of energy we also can use a very
small superconducting cable and I think it's quite obvious why we use here
superconducting cables. At the LHC we use Niobium tin(sic) as material. And this
material basically goes into a superconducting state at 10 Kelvin. But to
have a safe operation to LHC, we have to cool it down at 1.9 Kelvin.
Superconducting magnets have some benefits, but also some downsides, so
sometimes they change their state because there are small rigid vibrations and the
magnet or the temperature's not precise enough or the current is too high, then
they change that state and it's called "Quench". And, we basically can detect a
Quench when we measure the voltage across the magnet, because the resistance changes
at this point. So when there is a Quench then the resistance changes quite rapidly,
in milliseconds and we can detect this voltage rise with sophisticated
electronics. On the right side, you see a board I'm working on. So basically here,
we have a measuring system to measure the voltage across the magnet. And then we
have a detection logic implemented in FPGA to basically send triggers out and open an
Interlock loop. Interlock loop is a system at LHC. You can imagine that little bit
like a cable going around the whole tunnel and there are thousands of switches around
this Interlock loop. And as soon as one of the detection systems basically opens to
the interlock loop, then basically the whole machine will be switched off. And
what means switched off is basically, that we will power down the power converter,
but then the energy is still in the superconducting magnet and it has to be
taken out of the superconducting magnet. And therefore, we use dump resistors to
extract the energy. And here you can see a picture of such a dump resistor. It's
quite big. It's in a stainless steel tube, oil cooled. It's approximately three or
four meters long. And basically, when there was a Quench, and the energy was
extracted via these resistors, the whole resistor is heated up by several hundred
degrees and it needs several hours to cool it down again. Power converters; the power
converters are used to power the magnet itself. So they can produce a current of
approximately 13000 amps and a voltage of plus minus 190 volts. And you can see a
picture how here, how big it is. One downside with the power converters is that
they have to be, not downside but one difficulty is, that they have to be very
precise, because every instability in the current would have or has a direct effect
on the beam stability itself. So basically the beam would be not diverted in the
right amount of length. So that's why they have to be very precise and have to have a
very precise stability. So here I just pointed out, like in 24 hours, the power
converter is only allowed to have a deviation of 5 ppm. And in comparison, for
13000 amps we have a deviation of 65 milli amps. So the power converters have to be
very precise. And to do that, we had to develop our own ADC, because at the time
when LHC was built, there was no ADC on the market which was able to have this
precision and also the whole ADC is put into a super-precise temperature
controlled areas and it is calibrated quite regularly. Okay, cryogenics. We
already talked about that we have superconducting magnets and they have to
be cooled down quite low. So the superconducting magnets we have at LHC has
have to be cooled down to 1.9 Kelvin. And we are doing this when we like start the
LHC then we cool down on the first hand with liquid nitrogen. So approximately six
thousand tonnes of liquid nitrogen are put through the magnets to cool them down
to 18 Kelvin and afterwards we cool the magnets down with liquid helium. And
liquid helium is at 1.9 or 1.8 Kelvin. And to put it a little bit in a comparison,
outer space, so when we measure like the temperature of space, we have
approximately 2.7 Kelvin in outer space. So LHC is much colder than outer space.
The whole cooldown needs approximately one month and each dipole magnet, which is 15
meters long, shrinks several centimeters during that. Which also has to be taken
into account, because otherwise pipes would break. The cryogenic system is that
we have at each of the eight points at LHC we have compressors to cool down the
liquid helium or the helium itself. And then we compress the helium and pump it
down. We have one gaseous helium stream, which is at 15 Kelvin and we have liquid
helium stream at approximately 4.5 Kelvin. And then we pump it underground and then
we have something called Cold Compression System. And the Cold Compression System
even further reduces the pressure of the helium that we have in the end a helium,
which is at 1.8 Kelvin. So it can really cool down the magnet itself. And helium
has a very interesting effect because at 2.1 Kelvin, it becomes something called
superfluid. So it basically can run around like holes, for example, or walls. It can
basically flow against gravity, which is quite interesting. And it has also very
high thermal conductivity and that's also why we use superfluid helium here. And
that's why we cool down the whole magnets that low. And one other interesting effect
is also that the LHC tilt angle, which is 1.4 percent, has to be taken into account
because we have very low pressure inside all the tubes or all the system, at 16
millibars. But we have sometimes to pump the helium against gravity or going down.
So we also have to take into account the LHC tilt angle to not have wrong pressure
levels at the whole LHC itself. Okay. Stefan: All right! So, you probably
already got the idea, that what we've done in the last 20 minutes, was only solve the
first of the three challenges we had, which was actually bending the beam around
the circular trajectory. So I'm trying to go to the other challenges we have lined
up in the beginning. And the first one of that is the actual acceleration of the
particle beam. And large synchrotrons, e.g. like the LHC, they use radio
frequency or RF systems to do this acceleration. And I'm just going to do a
quick recap of the LHC beam and RF and how they interact. So Severin mentioned
already that the particles in LHC actually come in bunches. So in like packets that
contain about hundred billion protons and those bunches are spaced when they are
running around the LHC approximately 25 nanoseconds apart. And starting from that
the tasks of the RF system are basically twofold. It first has to ensure that these
bunches are kept tightly together in a process that we call longitudinal
focusing. And the second task is to care for the actual acceleration of the
particle bunches. So from their injection energy, when they come from one of the
pre-accelerators up to their final energy, that they are supposed to collide at
during the physics run. So in general, you can imagine RF as being a quickly
alternating electric and magnetic field components. And in the LHC, this RF energy
is basically injected into what is called a cavity, which is a resonant structure.
And there the particle beams travels through, while the field quickly
alternates and the RF signal, or the energy, basically interacts with the
particle beam. So perhaps you know that the protons are positively charged and
then a negative polarity of the field would attract these protons, while the
positive field location would basically move them away. And this has ... well,
after just injecting and with the frequency of this RF field being the same
as the speed that the particles actually go round the LHC, solves the first of the
two problems, which was the the focusing because actually the particles that are
too slow arrive only when the field is already changed to the opposite polarity
and actually get accelerated a bit, while the particles that are too fast, they are
actually being decelerated a bit. And this is a process that we call the longitudinal
focusing, which makes sure that the bunches stay neatly packed together. And
of course this would be relatively inefficient if we would only change the
polarity of this field once for each of the proton bunches that pass by. Which is
why we do it ten times. So the polarity basically changes ten times or the
frequency is ten times higher than the bunch crossing frequency. And by doing
that, we make sure that the change of this field is much faster and therefore the
particle bunches are packed much closer together and the focusing is better. So
here you can see these cavities that were shown in the previous picture as a
schematic, how they're actually placed in the tunnel. So eight of these huge
cavities are used per beam and they are the actual thing that is used to couple
the RF energy into the beam and transfer it to the particles. They are also
operating superconductively, so at cryogenic temperatures, to reduce the
thermal stress and the losses that would otherwise occur in their materials. And
these are actually – even though they are so big, similar to the magnets that had to
be very precisely manufactured – these also have very small manufacturing
tolerances and have to be precisely tuned to the RF frequency that is used to
inject. So and the second part of this, that actually produces this high power RF
signal. For that is used what we call Klystrons. So Klystrons are basically RF
amplifiers. They are built from high power RF vacuum tubes and they amplify this 400
MHz signal that is used to transfer energy to the particles. And each of those
Klystrons produces about 300 kW of power and you can probably imagine how much that
power for an individual unit that is, if you know that your microwave oven has like
2 or 3 kW. And of course, as we have eight cavities per beam and one Klystron always
feeds one cavity, we in total have 16 of those Klystrons and they are in principle
able to deliver a total energy of 4.8 MW into the LHC beam to accelerate it. But if
we take a small step back for now, we have only solved the first of the two problems,
which was to keep the bunches neatly focused. Because currently the particles
have been injected and the frequency is at some specific frequency and actually they
are only running basically in sync, the two. So what we do after all the particle
bunches from the pre-accelerators have been injected into LHC, is that we ever so
slightly increase the frequency, which of course also means that the particles need
to accelerate together with the RF signal. And this is the mechanism that we use to
accelerate them actually. And the change, that is required to do this, is very tiny,
actually. So it is less than a thousandth of a percent sometimes, that is used to
change the frequency to actually make them go so much faster. So from their
relatively low injection energy up to the top energy plateau that they need to have
to produce the actual physics collisions. And an interesting question to ask here is
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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