After an almost two-year shutdown for repairs and upgrades, CERN’s Large Hadron Collider (LHC) is beginning to fire back up for its next phase of probing the mysteries of physics. Its newest particle accelerator, Linac 4, completed its first test run over the past few weeks, with the potential to provide much more energetic beams than ever before.
The LHC paused operations in December 2018, beginning a massive overhaul called the High-Luminosity Large Hadron Collider (HL-LHC). When it’s fully finished and finally fired up in 2026, the upgraded facility will be seven times more powerful and will collect around 10 times more data in the following decade than it did during the previous run.
And now, the first incremental stage of this upgrade is coming online. The new linear accelerator, called Linac 4, has been installed and tested over the last few weeks. This device is the starting point for accelerating protons, which are then injected into the Proton Synchrotron (PS) Booster and onto the rest of the accelerator complex.
Linac 4 replaces Linac 2, which was in operation at CERN for 40 years. As you might expect the new model is significantly more powerful, injecting particles into the PS Booster at energies up to 160 MeV – much higher than Linac 2’s 50 MeV. By the time these beams are boosted, they’ll reach energies of 2 GeV, compared to the 1.4 GeV that Linac 2 was capable of.
This extra energy is thanks to the fact that scientists can tweak Linac 4’s beams in much more detail than its predecessor.
“With Linac 4, we can adjust additional parameters of the beam so we can feed the Booster in a loss-free process,” says Bettina Mikulec, team leader at the operations group for Linac 4. “We can also adapt the energy spread of the beams to match the Booster’s acceptance, whereas with Linac 2 one practically only adjusted the length of the beam before injection.”
In the three weeks up to mid-August, Linac 4 was tested with low-energy beams of negative hydrogen ions, running only through the first part of the accelerator. On August 20, it was finally cranked right up to maximum energy, with beams accelerated through the whole machine. These were then sent into a “beam dump” at the end, a device that catches and absorbs the particles.
Further testing will take place over the next few weeks and months. In September, the beams will be sent down the injection line towards the PS Booster, but will be caught in a beam dump before they arrive.
Currently, the first beam to be delivered into the PS Booster is scheduled for December 7. After that, the first test beams will be sent into the LHC at the end of September 2021 – representing a four-month delay thanks to the COVID-19 pandemic.
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