Linac 4 became the first accelerator in the LHC injection chain

the CERN Control Center (CCC) is buzzing again. The second long shutdown (LS2) has come to an end for CERN’s newest accelerator – Linac 4 – and the slow recovery of the accelerator complex from a two-year repair and recovery hibernation has begun. The three-week machine development through mid-August saw low-energy beams of negative hydrogen ions (H^–) fly over the first part of the throttle for the first time since it was connected to the PS Booster. On August 20, the first beams at nominal energy of 160 MeV were accelerated through the entire machine and into a dedicated beam dump located at the end of the linac. Over the next few months, the all-new accelerator will finish commissioning and will be ready to deliver various beams to the PS Booster in December.

CERN is famous for its circular accelerators, in particular the Large Hadron Collider with a circumference of 27 kilometers. But the protons circulating in these larger machines first undergo acceleration in a humble and relatively small linear accelerator, or linac. In 2018, Linac 2, which had supplied protons to CERN’s accelerator complex since 1978, was finally retired, with the 86-meter-long Linac 4 ready to take its place. But a new machine presents new challenges for the team operating it.

The development phase of the machine from the end of July was handled by the Accelerators and Beam Physics (ABP) team responsible for the proton sources, which also led Linac 2 operations. “ABP ensured that we can send a beam through the first structure of Linac 4, the so-called Radio Frequency Quadrupole or RFQ, with low beam losses, ”notes Bettina Mikulec, who heads the Operations Group (OP) team. which are responsible not only for Linac 4 but also for the PS Booster. Over the three weeks, ABP also worked on optimizing the proton source and realigning it to get a better angle for particles entering the RFQ. ABP then handed over the accelerator to the OP team for commissioning.

The behavior of Linac 4 differs considerably from its predecessor, in terms of shaping the profile of the proton beams that are shot downstream. “With Linac 4, we can adjust additional beam parameters so that we can feed the Booster in a lossless process,” adds Mikulec. “We can also adapt the energy distribution of the beams to match the acceptance of the Booster, whereas with Linac 2, we practically adjusted the length of the beam only before injection.” The new 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 allows the Booster in turn to inject beams at an energy of 2 GeV into the proton synchrotron (PS), greater than the previous value of 1.4 GeV.

The commissioning phase is crucial for the long-term operation of the Linac 4. Equipment qualification, optimization of beam instrumentation and much more can only be done with the beam in the line. ‘accelerator. This week, Linac 4 was set up to operate at its maximum energy. “We are working with ABP among others to check the optics of the machine in order to provide the optimal conditions for the injection point of the PS Booster,” Mikulec emphasizes.

The beams are now sent to Linac 4’s dedicated beam dump, and from September will be sent along the injection line to the PS Booster before snapping into the beam dump located just upstream of the Booster. The Linac 4 team is back in full swing and can’t wait to deliver the beam to the PS Booster on December 7th.