The magnetic system that focuses the beam of particles arising from the graphite target of the CERN Neutrinos to Gran Sasso project (CNGS) has been installed in its final position in the tunnel. This represents the final milestone of the project prior to testing all systems in preparation for the first commissioning with beam, at the end of May.
The CNGS secondary beam magnetic system consists of two elements: the horn and the reflector, both acting as focusing lenses for the positively-charged pions and kaons produced by proton interactions in the target. Most of these pions and kaons will decay in a 1 km-long vacuum pipe. At the end of this, a barrier, comprising 3 m of graphite and 15 m of iron, will absorb the remaining hadrons, leaving behind a beam of muons and neutrinos. Muons are quickly absorbed downstream in the rock, leaving only muon-neutrinos to traverse the Earth’s crust towards the Gran Sasso laboratory 732 km away in Italy.
Both the horn and the reflector came originally from LAL/IN2P3 before major modification at CERN. They are 7 m long and weigh more than a tonne each. They work with high pulsed currents, 150 kA for the horn and 180 kA for the reflector. These currents flow for a few milliseconds at the instant when the proton-beam pulse hits the target.
The heat that the current produces and the energy deposited by stray particles require a complex water cooling system. To avoid modifications in the mechanical properties of the aluminium alloy that makes up the whole system, the temperature must not exceed 80 °C. Cooling power is extracted from the chilled-water network by means of a heat exchanger. Demineralized water is sprayed onto the inner conductor, then collected at the bottom of the horn and the reflector, and finally pumped back to the system in a closed circuit.
The neutrino beam will be remotely monitored from the newly built central control room at CERN’s Prévessin site. The completion of the CNGS project, that is the hand- over to the teams in charge of regular operation of the beam, is planned for mid July.
