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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