LHC running

Taking a closer look at LHC

The proton source is where it all starts at CERN.

(A new ion source will be in operation at Linac4 in 2017-18)

At the present, "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.


The particles are accelerated up to 100 kV and then sent to a Radio Frequency Quadrupole (QRF) -- an accelerating component that both speeds up and focuses the particle beam. 4 vanes (electrodes) provide a quadrupole RF field that provides a transverse focusing of the beam.  Spacing of the vanes bunches and  accelerates up to 750 keV the beam.
From the quadrupole, the particles are sent to the linear accelerator (LINAC2). The linac tank is a multi-chamber resonant cavity tuned to a specific frequency which creates potential differences in the cavities that accelerate the particle up to 50 MeV. Protons cross the linac and reache the 157 m circumference circular accelerator  Proton Synchrotron Booster (PSB) in a few microseconds. Actually, PSB is a circular four rings accelerator.
Linac2 will be replaced for Linac4 in 2017-18.


Proton source


RF quadrupole - 90 KeV


LINAC2 - 50 MeV


LINAC4 - 160 MeV

The beam line to the PSB from the Linac is 80m long. 20 quadrupole magnets focus the beam along the line 2 bending and 8 steering magnets direct the beam. The PS Booster accelerates them to 1.4 GeV (factor of 28) in 530 ms, then after less than a microsecond they are injected in the 628 m circumference circular accelerator Proton Synchrotron (PS) .

In the PS protons can either: - be accelerated/manipulated/extracted in 1025 ms - or wait for 1.2 more seconds before being accelerated if they are part of the first PSB batch to the PS. They are accelerated to 25 GeV. The PS is responsible for providing 81 bunch packets with 25 ns spacing for the LHC.

Triplets of 81 bunches formed in the PS and injected into the 7 km circumference circular accelerator Super Proton Synchrotron (SPS), taking up ~27% of the SPS beamline. They wait for 10.8, 7.2, 3.6, or zero seconds whether they are part of the first, second, third, or fourth PS batch to the SPS. The SPS accelerates them to 450 GeV in 4.3 seconds, and sends it to the LHC.


PS Booster    1.4  GeV


Proton Synchrotron 25  GeV


Super PS   450 GeV

So the time it takes from the source to the exit of the SPS is between

0.53 + 1.025 + 4.3 = 5.86 seconds


0.53 + 1.2 + 1.025 + 10.8 + 4.3 = 17.86 seconds

Protons are finally transferred to the LHC (both in a clockwise and anticlockwise direction, the filling time is 4’20’’ per LHC ring). The total LHC beam consists of 12 “supercycles” of the 234 bunches from SPS. They have to wait up to 20 minutes on the LHC 450 GeV injection plateau before the 25 minutes ramp to high energy, and these 45 minutes dominates the transit time.

LHC   7 TeV (proton)

(2,76 TeV/nucleon Pb ions)

LHC en Live

The beams are stored at high energy for  10 hoursthe so called "beam lifetime", and particles make four hundred million revolutions around the machine. 

The more is the density of the stored particles the more decreases the beam lifetime.Coulomb scattering of charged particles traveling together causes an exchange of momentum between the transverse and longitudinal directions. Due to relativistic effects, the momentum transferred from the transverse to the longitudinal direction is enhanced by the relativistic factor γ. For stored beam, particles are lost if their longitudinal momentum deviation exceeds the RF bucket or the momentum aperture determined by the lattice. This is called the Touschek effect(after the austrian phyisicist Bruno Touschek) and is generally the limiting factor in beam lifetime.

After 10 h of beam collisions, the beam itself is exhausted and is dumped. The dipole magnets are then ramped down to 0.54 T and they stay at flat bottomfor some 20–40 min. Meanwhile beam injection is repeated before the magnets are ramped up again to 8.3 T for another cycle of high energy collisions. The machine is designed to withstand some 20 000 such cycles in 20 years’ lifetime, as well as 20–30 full thermal cycles.


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 (Spanish Postdoctoral Junior Grants Programme).

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). He has a Degree in Physics and in Chemistry, and is PhD for Santiago University (USC).



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