Linac 4 is the latest accelerator to join the CERN complex. Credit: Andrew Hara / CERN
De CERN Control Center (CCC) is abuzz again. The second long shutdown (LS2) has come to an end for CERN’s latest accelerator – Linac 4 – and the slow-moving accelerator complex of a two-year hibernation of repair and repair has begun. The three-week machine development ran until mid-August saw trees with low energy of negative hydrogen ions (H–) fly through the first part of the accelerator for the first time since it was connected to the PS Booster. On August 20, the first beams at the rated energy of 160 MeV were accelerated by the whole machine and placed in a dedicated beam at the end of the linac. In the coming months, the new accelerator will be ready to order and will be ready to deliver several December to the PS Booster in December.
CERN is famous for its circular accelerators, in particular the 27-kilometer-wide Large Hadron Collider. But the protons circulating in these larger machines first undergo acceleration in a humble and relatively small linear accelerator, like linac. In 2018, Linac 2, which has been feeding protons to CERN’s acceleration complex since 1978, eventually retired, with the 86-meter-long Linac 4 ready to take its place. But a new machine comes with new challenges for the team that operates it.
The end-of-July development phase was handled by the team of Accelerators and Beam Physics (ABP) responsible for the proton sources, which previously also performed the Linac 2 operations. “ABP ensured that we could send beam through the first structure in Linac 4, the so-called low frequency radio frequency quadrupole or RFQ,” said Bettina Mikulec, who leads the Operations Group (OP) team. t are not only responsible for Linac 4 but also for the PS Booster. During the three weeks, ABP also worked on optimizing the proton source and adjusting it to get a better angle for the particles entering the RFQ. ABP then handed over the accelerator for assignment to the OP team.
Linac 4 differs significantly in behavior from its predecessor, in terms of forming the profile of the proton beams that flow downstream. “With Linac 4, we can adjust additional tree parameters so we can feed the Booster in a lossless process,” Mikulec adds. “We can also adjust the energy amplification of the beams to accommodate the Booster, while with Linac 2 we can only adjust the length of the beam for injection.” The newer accelerator will inject particles into the PS Booster at an energy of 160 MeV, significantly higher than the 50-MeV operation of Linac 2. This in turn allows the Booster to inject beams at an energy of 2 GeV in the Proton Synchrotron (PS), higher than the previous value of 1.4 GeV.
The commissioning phase is crucial for long-term operation of Linac 4. The equipment qualifies, optimizes tree instrumentation and much more can only be done with tree in the accelerator. This week, Linac 4 was phased in for operation at its maximum energy. “Among other things, we are working with ABP to check the optics of the machine to provide the optimal conditions for the injection point of the PS Booster,” Mikulec points out.
The beams are now being sent to the dedicated Linac 4 tree dump, and from September onwards the injection line will be sent down to the PS Booster before sitting in the tree dump just above the Booster. The Linac 4 team is back in full operation and looks forward to delivering December 7th boom in the PS Booster.
