Proton source

Taking a closer look at LHC

The proton sourcis where it all starts at CERN.

The current proton source is already working in Linac4 after the Long Shutdown 2 2019-2022. Its operation is explained in the Linac4 section. In this case, this new source provides H(-) ions. The Linac4 section explains how this proton source works.

For historical reasons, we explain here what the process was like before Linac4 started operating, when Linac2 was the first accelerator in the accelerator chain ending at the LHC.

"To make the protons", physicists inject hydrogen gas into the metal cylinder -Duoplasmatron-  then surround it with an electrical field to break down the gas into its constituent protons and electrons. This process yields about 70 percent protons.

We can simplify the process as follows:

H2 + e-  →  H2+ + 2e-

H2+ + e-  →  H+ + H + e-

H + e-  →  H+ + 2e-

 H2   →   2 H+ + 2e-

For the LHC beam, we need:

2808 bunches x 1.15·1011 = 3·1014   protons per beam

or,  1014  protons for the two beams   (1)

(We are using as protons per bunch the value initially planned for the LHC. With the upgrades and improvements made during successive long shutdowns (LS), this value has been increasing.)

A single cubic centimetre of hydrogen gas at room temperature contains

with  P = 105 Pa   ,,   V = 10-6 m3   ,,   T=293 K

using   P·V = n·R·T  -   

n = 4·10-5 moles   ,,     N = 4·10-5 x 6·1023 = 2.4·1019 molecules  

So,  about 5·1019 atoms of hydrogen      (2)

Taking into account (1) and (2), the LHC can be refilled about 100000 times with just one cubic centimetre of gas – and it only needs refilling twice a day!

The particles are accelerated by a 90 kV supply and leave the Duoplasmatron with 1.4% speed of light, i.e. ~ 4000 km/s.
Then they are sent to a radio frequency quadrupole, QRF -an accelerating component that both speeds up and focuses the particle beam. From the quadrupole, the particles are sent to the linear accelerator (LINAC2).

A small commercial hydrogen cylinder contains about 5 kg of gas. So the amount of hydrogen molecules is:

n =  5000/2 = 2500 moles

2500  x 6·1023 = 1.5·1027 molecules

N = 2 x 1.5·1027 = 3·1027 atoms

Taking into account that the process yields about 70% protons we have:

  0,7 x 3·1027 = 2.1 ·1027atoms

With (1), this cylinder can be used:

2.1·1027 / 6  ·10143.5·1012  times

Since the LHC is filled every ten hours, this cylinder could be used for:

10 x 3.5·1012  = 3.5·1013 hours

So, about 4 ·10years

The hydrogen will diffuses out of the bottle faster.

A we said at the top, a new ion source (providing in this case H-) is already working in the new linear accelerator, Linac4.

1.34 x 1020 protons were accelerated in the accelerator complex in 2016. This might sound like a huge number, but in reality it corresponds to a minuscule quantity of matter, roughly equivalent to the number of protons in a grain of sand. In fact, protons are so small that this amount is enough to supply all the experiments. The LHC uses only a tiny portion of these protons, less than 0.1%, as shown in the diagram.


Taken from the Annual Report 2016 - CERN Document Server


Xabier Cid Vidal, PhD in experimental Particle Physics for Santiago University (USC). Research Fellow in experimental Particle Physics at CERN from January 2013 to Decembre 2015. Currently, he is in USC Particle Physics Department ("Ramon y Cajal", Spanish Postdoctoral Senior Grants).

Ramon Cid Manzano, secondary school Physics Teacher at IES de SAR (Santiago - Spain), and part-time Lecturer (Profesor Asociado) in Faculty of Education at the University of Santiago (Spain), until his retirement in 2020. He has a Degree in Physics and in Chemistry, and is PhD for Santiago University (USC).



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 For the bibliography used when writing this Section please go to the References Section

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