Taking a closer look at LHC

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 LHC
 


LHC is the most powerful particle accelerator in the world and it´s located in CERN on the France-Swiss border.

It uses part of the same structure as the former accelerator (LEP), and it has a circumference of 27 km long and runs 100 m underground.

L A R G E :

The size of an accelerator is related to the maximum energy obtainable. In the case of a collider or storage ring, this is a function of the radius of the machine and the strength of the dipole magnetic field that keeps particles on their orbits. The LHC uses some of the most powerful dipoles and radiofrequency cavities in existence. The size of the tunnel, magnets, cavities and other essential elements of the machine, represent the main constraints that determine the design energy of 7 TeV per proton.
 

 

There are eight elevators leading down into the tunnel, and although the ride is only one stop, it takes a whole minute. To move between the eight access points, maintenance and security people use bicycles to move around the tunnel – sometimes for several kilometres. The LHC is automatically operated from a central control centre, so once the experiments have started, engineers and technicians will only have to access the tunnel for maintenance.

H A D R O N:

The LHC accelerates two beams of particles of the same kind, either protons or lead ions, which are hadrons.

A hadron, in particle physics, is any strongly interacting composite subatomic particle. All hadrons are  composed of quarks  (i.e.: protons and neutrons).

 

C O L L I D E R:

A collider (that is a machine where counter-circulating beams collide) has a big advantage over accelerators where a beam collides with a stationary target.

When two beams collide, the energy of the collision is the sum of the energies of the two beams:

E=2Ebeam

 

As it's been said before, the LHC was built into a tunnel that already existed.  The tunnel had a diameter of 3.0 m. We can make the approximation that a very thin circular tunnel is the same as a tunnel of the same diameter but straight.


Let’s calculate the volume of the tunnel:

 V = π·r2 ·L  ⇒  V = π·1,52·27000 ⇒  V = 191000 m3

The mass of the rock that was dug out to form the tunnel, assuming an average density of 5000 kg/m3,

M= d·V     ⇒  M = 5000·191000  ⇒  M = 10 6 tonnes

So, 76 olympic pools could be filled with this amount of rock.


In the pipes where protons travel a high vacuum is required. The pressure in some parts is over 10-9 Pa.

The two beams are made up of cylinder-like bunches 7,48 cm long  and ~16x16 micras SECTION (1 millimetre wide when they are far from a collision point).
 
Between each consecutive bunch there are 7,5 m. So, with a circumference of 27 km there should be:

26659 / 7,5 ~ 3550  bunches.

To allow a correct sequence of bunches injected into the ring, and to be able to insert new bunches when non-useful ones are extracted it´s necessary to allow enough space for that.
For a more complete discussion click here...

The effective number of bunches is 2808.

So the rate of "bunches with protons" is: f = (2808/3550) ~ 0,8

Four snap-shots of beam profile


With 11245 crosses per second we get:

11245 x 2808 ~ 32 millions crosses , the "average crossing rate".

And the  number of collisions  per second  ~ 600 millions collision/s

If we consider 3550 bunches: 11245 x 3550 = 40 millions crosses  ⇒ 40 MHz





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