I am very glad to finally have found the time to reinitiate my series of post ‘Live from CERN’. Everything comes with patience (the next item on my to-do list is to continue my quantum mechanics lecture series).

For those who where not there 3-4 months ago, in this ‘Live from CERN’ series of posts, I am discussing **some fresh news about what is going on at CERN** those days. 

I decided today to discuss some developments **targeting the future of CERN**. An by future, I mean, the **post-LHC era.**

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## A FUTURE CIRCULAR COLLIDER ##

<div class="pull-left">

<center> ![](https://s8.postimg.org/zavaa8b91/tlep.png)
<sub>[image credits: [FCC-ee @ CERN](http://tlep.web.cern.ch/)]</sub> 
</center> 

</div>

Long ago, I introduced on Steemit (in [this post](https://steemit.com/science/@lemouth/the-future-of-particle-physics-from-the-large-hadron-collider-to-future-circular-colliders)) one of the future options particle physicists could consider as a machine to be built after the LHC, in roughly 2035-2040

This option is called the FCC, an acronym that stands for a **future circular collider**. 

This name actually speaks for itself. It is a machine to be built in 15-20 years from now, that will be circular and that will collide some particles. 

Two possible sites are currently being discussed, one being at CERN, and the second one in China.


The CERN site is sketched on the above picture. One can notice a bunch of circles. Black circles are existing colliders, the big black circle being the LHC. The blue circle shows where the FCC tunnel would be. 

<div class="pull-right">

<center> ![](http://www.steemimg.com/images/2017/01/20/fcc-cern49bca.png)
<sub>[image credits: [CERN](https://fcc.web.cern.ch/Pages/Hadron-Collider.aspx)]</sub> 
</center> 

</div>
This is also depicted on the map on the right.



The high-energy physics community is for the moment working on many different aspects of this project. One of them concerns the **physics goals that could be reached at the FCC**. I briefly presented some of my research work last week, [here](https://steemit.com/science/@lemouth/higgs-bosons-beauties-taus-and-other-beasts-particle-physics-again-around-on-steemit), discussing how the production of three Higgs bosons could help to understand better how nature works at the fundamental level.

There are also other on-going developments, in particular on the **machine side**.

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##  HIGHLY-ENERGETIC BEAMS  ##

The FCC machine is expected to collide very energetic particles. Whilst this may sound cool in terms of physics opportunities, this also consists of a challenge technologically speaking.

Focusing on the proton-proton collisions, **the beams that will circulate in the accelerator will contain a huge amount of energy**: 8.4 GJ, or 8.400.000.000 J. To make the picture clearer about what this number means, let us compare it to better known stuff. 

8.4 GJ is twice the average annual energy usage of a fridge, or the energy released by the explosion of 2 tons of TNT. Please do not worry: we will not collide fridges…

This energy is really, really huge, and it is thus important to add **protection systems** to the machine  so that one can safely take care of the beams in the event of a failure or at the end of an experimental run.

In other words, we need to be able to  **safely smash the beam into an absorber, or a beam dump**, when one does need it anymore.

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## BEAM EXTRACTION ##


Beam extraction is achieved by making use of a two magnets, as illustrated by the picture on the left below.

<div class="pull-left">

<center> 
![](https://s21.postimg.org/4opx46oaf/magnets.png)
<sub>[image credits: [CAS 2009 school](https://arxiv.org/abs/1103.1583)]</sub> 
</center> 

</div>

The first magnet, called the **kicker magnet** (right hand-side of the picture), has the simple task of kicking the beam into the second magnet by means of a fast-pulsed weak field. 

The second magnet, called the **septum magnet** (middle-upper part of the picture), is instead slow-pulsed, but generates a very strong magnetic field that allows for the final deflection of the beam. 

After having been deflected, the beam finally hits the external absorber. 

The problem is that on the one hand, one needs a strong field in the septum magnet to deflect the beam to be extracted. On the other hand, the field outside cannot affect the circulating beam. It must hence be very weak

**We thus need a sharp transition in space from a strong to a weak field that is very well located.**

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## SUSHI TO EXTRACT PARTICLE BEAMS ##

<div class="pull-right">

<center> 
![](https://s4.postimg.org/sx8jq5cy5/sushi.png)
<sub>[image credits: [CERN](http://home.cern/cern-people/updates/2017/06/how-will-sushi-protect-fcc)]</sub> 
</center> 

</div>

The SuShi device is illustrated on the right. 

The name is once again an acronym and stands for  a **Superconducting Shield** septum magnet. This device is aiming for getting this sharp transition zone from a weak to a strong field in the septum magnet that I have mentioned above.

The idea relies on a **passive superconducting shield** that is required to create a zero-field zone inside the strong field in the septum. 

This no-field zone is expected to be created by means of persistent induced Foucault (or Eddy) currents at the surface of the shield. 

More precisely, the superconductor is cooled down in zero magnetic field. Then, when the superconductor will be exposed to a changing magnetic field, Foucault currents are induced on its surface in such a way that the change in the field is compensated (or at least tamed).

While usually, the resistivity of the conductors make the Foucault currents quickly decaying, superconductors have zero resistance so that Foucault currents have a very long lifetime of days or weeks.

**The Foucault currents can hence be arranged to cancel the field where it needs to be canceled and during a long enough period**. 

And the technology works: **SuShi prototypes are currently being tested at CERN and they do 2.5 times better than at the LHC**.

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## TAKE-HOME MESSAGE AND REFERENCES ##


In this article, I describe one of the numerous technological developments that are being undertaken at CERN in the aim of maybe building, in a couple of decades, the next generation of particle colliders. I discussed some device that must be invented to be able to properly extract the beam when a danger occurs or when an experimental run is over.

SuShi is a piece of technology allowing to do it, accounting for the problem originating form the fact that the beams will be extremely energetic.

More information can be found v clicking on the following links:
 * The CERN FCC official webpage can be found [here](https://fcc.web.cern.ch/Pages/default.aspx).
 * The CERN news about SuShi and its role for the FCC can be found [here](http://home.cern/cern-people/updates/2017/06/how-will-sushi-protect-fcc).
 * The official SuShi webpage is [here](http://sushi-septum-project.web.cern.ch/).
 * More on Eddy/Foucault currents can be found on [Wikipedia](https://en.wikipedia.org/wiki/Eddy_current). The page is very well detailed.