LHC about to restart - some frequently asked questions

[mfb]

What is the main obstacle/bottleneck in terms of upping the eV's?

The field strength of the dipole magnets and the curvature of the tunnel.
Protons with more energy would hit the outer wall of the beam pipe as the magnets are not strong enough to keep them on track.

Tunnel:
For LEP, the limiting factor was acceleration versus synchrotron radiation, so the tunnel was built with 8 curves and 8 straight sections for acceleration. That design is not optimal for the LHC, but making the ring "more circular" to reach a higher energy would have needed a lot of time and money, so they decided to take the old geometry.

Field strength:
The dipole magnets are supposed to be strong enough to handle 7 TeV per proton. Unfortunately, those magnets need "training" - if you send too much current through them, they lose their superconductivity, this is called quench. A quench comes with mechanical changes of the precise geometry of the coils, and it is known that those quenches (usually) increase the current the magnets tolerate. That is repeated until the magnets are trained for the current you plan to run through them. For some magnets, training for 6.5 TeV took a long time, it is unclear if the design value can be reached at all (but they will certainly try).

 

[rootone]

That's interesting, does it mean that micro impreciseness in the construction of each dipole, it needs to be carefully adjusted for either in a hard or soft way, and that can be and is done, in order to get best quality results from the experiments?

 

[Vanadium 50]

"nearing", but no actual commercial viability yet.

You can buy commercial HTS leads today. They are a common way of powering superconducting magnets.

 

[rollingstein]

The training bit is very interesting? Is there a theory why the geometry of a magnet should adjust itself in this way?

 

[Vanadium 50]

Is there a theory why the geometry of a magnet should adjust itself in this way?

No. It probably has to do with mechanical stresses, but the phenomenon is not well understood.

 

[rollingstein]

There is a 1km HTS cable in Essen, Germany, as part of the local power grid (http://www.suptech.com/Cables_Oct_10.pdf). A pilot project, sure, but at least something.

Interesting indeed. HTS seems more realistic than I had thought.

Do these transmission wires need a liq. N2 pumping source continuously?

 

[mfb]

The training bit is very interesting? Is there a theory why the geometry of a magnet should adjust itself in this way?

You can find some articles with keywords like "magnet training superconductors", but as Vanadium said - it is not well understood. Training works, so it is done.

Do these transmission wires need a liq. N2 pumping source continuously?

They need constant cooling, I don't know details of it. While this requires constant power, using normal conducting cables needs power as well.

 

[QuasiParticle]

HTS magnet technology is not quite yet ready for high-field accelerator magnets, but development at CERN and elsewhere is ongoing.

Like mfb mentioned, there are no plans to increase the LHC energy. Instead they are studying the feasibility of a new 80-100 km accelerator, the FCC. The LHC will undergo a few updates in the future, for example to increase the luminosity, but the main dipoles will not be replaced.

The mentioned 15 tesla magnets would likely be possible to achieve with Nb3Sn technology (low-temperature superconductor). With HTS it's possible to go up to 20 T, but the cost per tesla increases rapidly at high fields (also true for LTS-only magnets).

That's interesting, does it mean that micro impreciseness in the construction of each dipole, it needs to be carefully adjusted for either in a hard or soft way, and that can be and is done, in order to get best quality results from the experiments?

The field quality requirements for the magnets are quite stringent and they are carefully designed and manufactured. There are also orbit corrector magnets in the LHC, which can adjust the beam positions if deviations are observed.

 

[mheslep]

You can buy commercial HTS leads today. They are a common way of powering superconducting magnets.

HTS has some current application as a way of powering the *leads* as you say, not yet the bulk magnet, as the additional HTS cost is worth the heat loss savings at the lead (per Sheahen). Still no nitrogen cooling in the case of HTS leads on GE MRIs.

 

[rollingstein]

Still no nitrogen cooling in the case of HTS leads on GE MRIs.

Interesting. How high a temperature can these leads remain superconducting at? At their typical currents.

 

[mheslep]

Hyperphysics - HTS
Wiki HTS
https://www.physicsforums.com/search/603685/?q=high+temperature+superconductors&o=relevance

 

[Vanadium 50]

6.5 TeV pilot bunches have circulated in both directions.

 

[Vanadium 50]

Experiments are seeing beam-gas collisions.

 

[mheslep]

Experiments are seeing beam-gas collisions.

Meaning beam - neutral collisions?

 

[mfb]

Collisions with remaining gas (mainly hydrogen) in the beam pipe. Those collisions are highly asymmetric and of low energy as they are basically fixed-target experiments.
Still useful to see if the detectors are working properly.

 

[bipin sonawane]

In this run of LHC ( 2015) , are we doing the investigation of spin and other related parameters of "the proton"?

 

[mfb]

No, the LHC experiments are not sensitive to properties of the proton as hadron - they see its components (quarks and gluons) only, those get studied.
Other experiments at much lower energy are used to study protons as effective particles.

 

[bipin sonawane]

Thanks mfb .
As per the theory is concerned the spin of the proton is the sum of the spins and angular momenta of its constituent particles that is quarks and gluon.But it is known that this sum is not equivalent to 1/2 ,which is the spin of the proton,something which contribute in the spin of the proton is missing. My question is - in this LHC run will we compute the spins of quarks and gluon? Do we collide polarized and unpolarized protons ?

 

[mfb]

The proton beams are unpolarized - it is not relevant for the main physics goals and polarizing them to a reasonable degree would be additional effort. I would guess that HERA data is much better even if the LHC had polarized protons.

 

[mfb]

Part 2 is there! To focus discussions to one thread, I closed this one.