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Â >> During this third module, we are discussing techniques

Â for the acceleration and detection of elementary particles.

Â For this video, we are invited to hall SM18 of CERN, a hall where

Â the magnets of the Large Hadron Collider have been tested in the past and

Â spare magnets are still being checked out.

Â A part of this hall has been transformed into an exhibition.

Â At the end of this module, you will know how to describe the elements

Â of an accelerator and know how they look like in real life.

Â So, let us meet todayâ€™s host.

Â 0:58

For this video, we are accompanied by

Â Tobias Golling, who is one of my colleagues,

Â professor at University of Geneva, who dedicates 100% of his

Â research to the analysis of data from the ATLAS experiment at the LHC.

Â So, Tobis,

Â can you let us know what exactly you are doing in this domain?

Â >> Yes, with pleasure.

Â In short, I try to find new phenomena in the data,

Â 1:30

with my group.

Â Let me put this in context. The Standard Model is a great success,

Â it describes all visible matter, all forces, well almost all forces.

Â >> Except one.

Â >> Yes, except one.

Â The Higgs mechanism describes how all

Â particules acquire a mass, and the Higgs boson has been discovered

Â at CERN in 2012, so all of this is very, very well set.

Â But, there are weaknesses of the model.

Â For example, it cannot explain dark matter and it cannot explain

Â gravity, that his the missing force, and there are other phenomena, like, for example,

Â the famous hierarchy problem, i.e.

Â >> the difference in scaleâ€¦

Â >> exactly, between the Higgs mass and the scale of gravity.

Â So, there are open questions. What are we to do?

Â We can ask our theorist colleagues,

Â 2:26

and they formulate hypotheses, that there are particles,

Â that one must introduce particles beyond the Standard Model,

Â and the hypothesis is that one can create these new

Â particle in our collisions. So I dedicate

Â my research to finding new particles,

Â like particles predicted by supersymmetry, or theories with extra dimensions.

Â 2:57

>> So, for this, the predicted cross sections

Â are very, very small, are they not?

Â >> They are of the order of, or smaller than the cross sections

Â characterising the Standard Model, and thus

Â we evidently need an high performance accelerator, a collider.

Â >> So, what are the properties of the LHC,

Â which are crucial for your research?

Â >> We expect that these particles have a very large mass,

Â and that their production rate is very low.

Â So, the two most important parameters of the LHC for this

Â are the energy and the luminosity.

Â Higher energy allows us to produce more massive particles.

Â 3:58

>> The LHC is a sort of double accelerator and

Â storage ring, where the protons circulate in two directions.

Â So, in principle, it is two accelerators in one, isnâ€™t it?

Â >> Exactly. We have the principle of two-in-one, for all the magnets of the LHC

Â there are two coils.

Â >> Why does one need two accelerators?

Â >> Because there are two proton beams in opposite directions.

Â So, one needs two magnetic fields.

Â >> One beam circulates in one direction. >> Yes.

Â >> And the other in the opposite direction.

Â And they meet at the interaction points.

Â >> Yes.

Â We need the same Lorenz force towards the center of the orbit,

Â so we need one magnetic field pointing upwards, the other one downwards.

Â And for this one uses these dipole magnets.

Â >> So here, we are almost sitting on one of these dipole magnets,

Â Can you explain the elements of these magnets a little?

Â >> Exactly.

Â So here, one sees the two rings, the two vacuum tubes,

Â and one sees two coils, one and the other, which are exactly the same,

Â the only difference is that the current is reversed.

Â And here is the common yoke for the magnetic fields

Â and the cryostat is also in common.

Â >> Ok.

Â So, in one of these coils, there is an upward pointing magnetic field,

Â and in the other, the same dipole field, but pointing downward.

Â With means that we are deviating the two beam in the same direction,

Â even though they have opposite directions, simply

Â applying Lorentzâ€™ law, which gives the centripetal force,

Â which we need, q(v x B) in this case.

Â >> Exactly,

Â with the right hand rule.

Â >> Mh mh.

Â The two coils are in the same cryostat. Why?

Â Why does one not simply build two separate magnets?

Â >> One can reduce the cost this way,

Â having a common cryostat for the two coils.

Â >> So, for the enormous energies of the

Â 6:24

>> Exactly.

Â >> Ok.

Â And because of that, all are in one cryostat.

Â So, the coils themselves are made of copper, arenâ€™t they?

Â >> Maybe a few numbers about the magnetic system,

Â which show that it is really very important,

Â even essential for the LHC.

Â Of the 27 km of circumference, 85% are

Â occupied by magnets, the magnet systemâ€¦ >> Ok.

Â >> â€¦and 75% by dipoles, like this one.

Â >> So this is really the heart of the matter.

Â Magnets are practically everywhere.

Â >> There are magnets everywhere, right.

Â There are more than 1000, more than 1200 elements like this.

Â Each one weighs 30 tons,

Â has 15m length and a maximum magnetic field of 8.3 Tesla.

Â >> So what we see here is just a small segment of a dipole,

Â which in reality is much longer.

Â >> Ok.

Â So this makes a gigantic cryogenic system,

Â so, practically all the ring is at cryogenic temperature,

Â all around its length of 27km.

Â >> Yes.

Â So, later we will see the superconducting cables, there are 1200 tons of

Â superconducting cables and it needs 130 tons of superfluid helium

Â to cool down all this mass to 1.9 Kelvin.

Â >> Wow.

Â >> So I propose to move on to see

Â how these coils look and

Â also the focalising elements of the LHC.

Â 8:28

of the accelerator, Tobias?

Â >> So, here we come back to the luminosity, which we have already mentioned.

Â The luminosity is inversely proportional

Â to the common surface of the two proton beams,

Â and that is why one must focalise them, to reduce this surface.

Â And here we see very well

Â a section of a quadrupole magnet, which is responsible for the focalisation.

Â 9:01

One sees very well the four poles, here two north poles and two south poles, and then

Â here are two version of this magnet, one is responsible for

Â horizontal focalisation, the other one for the vertical one.

Â One alternates the twoâ€¦

Â >> Puts them in chains.

Â >> â€¦to have an optimum focalisation.

Â >> So if we look at the coils, the wires go effectively

Â in this direction, donâ€™t they?

Â >> Exactly. >> So here we have in fact

Â a section of such a coil. Can you

Â explain the ingredients of such a coil?

Â >> Yes.

Â All coils are composed of superconducting cables, they are very important,

Â as we said, there are 1200 tons of these cables.

Â They are in fact composed of these strands, with a diameter of roughly 1mm.

Â 10:21

>> And all of this is normally imbedded in a sort of aluminum stabiliser,

Â that is why there is this gray material.

Â >> Yes, yes.

Â >> OK. So this serves to wind the coils of the active part

Â of the magnets.

Â But also to transport current around the accelerator, doesnâ€™t it?

Â >> Exactly.

Â We have these cables all around the accelerator.

Â If one takes the total length of the filaments,

Â one arrives at an astronomical scale.

Â One can go five times to the sun an back with this.

Â >> It is really incredible, all this effort

Â which is made to produce the particles

Â you are searching for, isnâ€™t it?

Â >> In fact it serves this purpose.

Â >> So, we still need to discuss an element of the accelerator

Â which we have not covered yet.

Â These are the accelerating elements themselves.

Â So I propose that we move on to see what they look like in reality.

Â In this small video animation we see a cryostat which contains four

Â superconducting elements, i.e. radio frequency resonators.

Â The radio frequency wave enters through these wave guides

Â and establishes an electric field with the right polarity, such that

Â the beam is accelerated in its direction of motion.

Â So here is one of these elements.

Â Tobias, can you please explain these ingredients of this

Â radio frequency cavity?

Â >> Of course. Such a cavity has

Â as you said, the principle purpose to accelerate protons

Â from an energy of 450 GeV,

Â the energy with which they enter the LHC,

Â up to 7 TeV, which is their maximum energy.

Â Here we see the wave guides, the radio frequency

Â generated by the Klystron enter here and leaves there.

Â For the LHC, they have a frequency of 400 MHz.

Â >> OK.

Â So inside, a stationary wave is formed.

Â 12:36

And when the polarity is right at the right time of arrival of the protons,

Â we will have a net acceleration in the direction of the electric field.

Â So, how does one synchronise this wave with the beam passage?

Â >> It is very important to have mono-energetic beams

Â So we must equalise the proton energy.

Â So when the proton with the right energy

Â enters at the right moment, there will be no acceleration

Â if we are already at the maximum energy level.

Â However, if the protons arrive too early or too late,

Â i.e. when their energy is a little too large or too small, an acceleration is felt

Â the protons are decelerated or accelerated to approach the ideal energy.

Â This happens automatically, when the protons enter in the increasing

Â 13:37

phase of the electric field, as we have seen in video 3.2.

Â So, this accelerating cavity, the resonator and all what is around,

Â is made of copper and is emerged in a cryostat, which holds

Â it at liquid helium temperature, such that the copper is superconducting.

Â >> So why is it important that this structure

Â also is superconducting?

Â >> Yes, it is very important that there is a minimum

Â ohmic resistance, such that the power

Â of the wave is not attenuated.

Â >> Ok, this means that the radio frequency wave

Â arrives without loss from the klystron to the beam.

Â >> Exactly.

Â >> Ok.

Â So, thank you very much, Tobias, for this visit,

Â and for this demonstration of the LHC elements.

Â This concludes this video.

Â In the next video,

Â we will start to discuss particle detection.

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Â