While on its long road to restart, yet another milestone was reached at the Large Hadron Collider (LHC) over the weekend.
Protons were generated by the LHC's source and blasted through a ‘daisy-chain' of smaller accelerators before being intentionally smashed into a metaphorical brick wall. The particle beam didn't reach the LHC's famous 17 mile (27 kilometer) accelerator ring, they were stopped just short, but the event was used to begin calibration efforts of the massive experiment's detectors before the whole system is powered back up again early next year.
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The world's most powerful particle collider, which is located at and managed by the European Organization for Nuclear Research (CERN) near Geneva, Switzerland, was powered down in February 2013 for repairs and upgrades. Now, with the improvements complete, CERN engineers are testing and calibrating each component of the complex system before experiments can restart in 2015.
"These initial tests are a milestone for the whole accelerator chain," said the LHC's chief engineer Reyes Alemany Fernandez. "Not only was this the first time the injection lines have seen beams in over a year, it was also our first opportunity to test the LHC's operation system. We successfully commissioned the LHC's injection and ejection magnets, all without beam in the machine itself."
Before protons are ‘fed' into the LHC's main ring to be accelerated to relativistic speeds by the circuit of superconducting electromagnets, a series of smaller accelerators are used, each one ramping-up the particles' energies before being passed to the next accelerator.
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Starting at the source, which is responsible for creating a reservoir of protons (hydrogen atoms are stripped of their electrons, leaving the positively-charged protons behind), the protons are fed into the Linac2 accelerator - the first boost on the proton beam's journey through the LHC. After Linac2, the particle beam is accelerated by the Proton Synchrotron Booster (a component that has seen some of the most radical upgrades since shutdown), then the Proton Synchrotron (PS) and then the Super Proton Synchrotron (SPS).
But during this test, the high-speed protons were fed through the LHC injection lines, after being accelerated by the SPS, only to end their short journey smashing into 21.6 tons of graphite, aluminum and copper known as "beam dumps." These dumps are used to absorb the energy of particle beams before they enter the LHC proper. This is useful during tests and can help prevent significant damage to the LHC's electromagnets and sensitive detectors should there be a fault or "quench" event. The purpose of these dumps is to diffuse the particle beam, thereby scattering its energy, and then absorbing the rest.
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However, as the beams are dumped, muons - subatomic particles that are basically high-mass electrons - are generated by the collisions and their signal can be detected by detectors along the LHC's main beam line. This provides LHC physicists with an opportunity to calibrate their detectors before the system is fully switched on.
"The experiments where given the precise timing of each beam dump, which allowed them to tune their detectors and trigger to the LHC clock," said Verena Kain, SPS supervisor.
So it seems the most complex and awe-inspiring experiment ever created by humankind is almost ready for restart. The next big milestone is February 2015 when particle beams will be unleashed into the LHC's main ring and accelerated to a new and exciting high-energy regime.