We are one month apart from the beginning of a new era in High Energy Physics! Commissioning of the LHC is progressing and the new start-up is scheduled for November. The LHC consists of eight so called “sectors” which can be tested separately, and six out of eight are already at the operating temperature of few kelvin.
Why is such a low temperature needed? Let’s follow the path of the particles within the accelerator chain. All begins with hydrogen atoms, stripped of electrons and injected into the LINAC2, a linear accelerator where the electric field accelerates the positive particles, the protons, to roughly 1/3 of the speed of light. The following stage consists in splitting the particle packets and fed them into the Booster, a circular accelerator where the packets are squeezed and their energy increased to 92% the speed of light. A magnetic field bends the trajectory while the pulsating electric field raises the energy. Once the packets are recombined and sent to the PS (proton synchrotron), a circular accelerator of 600 m circumference, the protons make the transition. Their speed reaches 99% of the speed of light; since it cannot increase further, the additional energy acquired in the PS converts into mass. The proton mass is now 25 times larger than the mass at rest! The energy, measured in electron volt corresponds to 25 GeV (GeV =10E9 eV=1000000000 ev). The protons are ready to be channeled into the 7 km circumference SPS (super proton synchrotron) pushing the energy up to 450 GeV. The next transfer leads the protons into the gigantic LHC, nestled between the Jura and the Alps, 100 deep under ground with a circumference of 27 km. Additional energy is added such that each proton beam stores 7 TeV (TeV = 10E12 eV=1000000000000 ev) leading to a total energy per collision of 14 TeV. (during 2009-2010 the machine will operate at a maximum of 7 TeV in total.) The bending is possible because 1200 dipole magnets are powered along the path. The only possibility to have magnets strong enough to bend the high energy beam is to have them cold, which means superconducting magnets.
One of the superconducting magnets is lowered into the LHC tunnel via a specially constructed pit
In total, 1600 superconducting magnets are installed in the LHC with most weighing over 27 tonnes. “96 tonnes of liquid helium is needed to keep the magnets at their operating temperature of 1.9 K, making the LHC the largest cryogenic facility in the world at liquid helium temperature.” After the beam accident of last year, accelerator experts fixed the magnet inter-connections which, in normal superconducting state, should exhibit negligible electrical resistance.
15 m-long dipole magnets are seen lined up. To the right of the magnets the test hall can be seen.
Over 10000 high-current superconducting electrical connections were examined and conditions are now safe to start operating the machine!
A screenshot showing lead ions in the transfer line from the Super Proton Synchrotron (SPS) to the LHC
