The new linear accelerator, which from 2020 will be the first link in the accelerator chain, has entered a new stage of its commissioning

We couldn’t have imagined a more appropriate date: on 1 July (1.07), Linac4 reached an energy of 107 MeV. Having crossed the 100 MeV barrier, the linear accelerator is now on the home straight of its commissioning. “This stage was very quick – it took less than two weeks,” says Alessandra Lombardi, deputy project leader of Linac4, in charge of the commissioning.

In 2020, Linac4 will replace the existing Linac2 as the first link in the accelerator chain. It will accelerate beams of H- ions (protons surrounded by two electrons) to 160 MeV, compared to 50 MeV with Linac2.

The new machine is particularly sophisticated as it comprises four types of accelerating structure: the particles are accelerated in several stages, first to 3 MeV by a radio-frequency quadrupole (RFQ), then to 50 MeV by drift tube linacs (DTLs), then o 100 MeV by coupled-cavity drift tube linacs (CCDTLs), and finally to 160 MeV by Pi-mode structures (PIMS).

Linac4 during its installation in 2015. This photo was taken as part of the 2015 Photowalk competition. (Image: Federica Piccinni)

At the end of 2015, Linac4 accelerated beams to 50 MeV, the same energy as Linac2, for the first time. For the current stage, the Linac4 team put the last two types of accelerating structure, the CCDTLs and PIMS, into operation. All seven CCDTL cavities and one of the twelve PIMS cavities have been tested. “We were therefore able to verify that the entire acceleration chain was working,” explains Jean-Baptiste Lallement, from the Linac4 commissioning team.

The team is especially happy to have been the first in the world to use the innovative CCDTL cavities. They work on the same principle as normal DTLs: the particles travel through a series of tubes with spaces between them and are accelerated between the tubes by electric fields, entering the next tube when the oscillating field changes direction. In the shelter of the tube, they drift along to the next space, where the field accelerates them once again.

The difference between DTLs and CCDTLs is the way in which they are focused. DTL cavities contain permanent magnets, inside the tubes, that keep the bunches of particles together. “But this solution is quite expensive and, as the permanent magnets are inside the vacuum chamber, it’s difficult to work on them,” Maurizio Vretenar, Linac4 project leader, explains.

At a higher energy, a new solution was possible: placing quadrupole magnets between two series of tubes, outside the vacuum chamber. “This way, we can use electromagnets and can regulate the magnetic field to improve the focusing,” Vretenar continues. Maintenance is much easier and the manufacturing cost is lower.

The initial design of the CCDTL cavities, involving very specific coupling cells, was done at CERN. The development is the fruit of a collaboration between CERN and the Russian institutes VNIITF (Russian Institute for Technical Physics) and BINP (Budker Institute of Nuclear Physics). The Russian institutes then manufactured the components.

Fresh from this success, the commissioning of Linac4 will be stopped in a few days. The last PIMS cavities will be installed during the summer, along with the equipment that will inject the beam into the PS Booster – the second link in the accelerator chain. Commissioning will resume in September with the goal of reaching 160 MeV before the end of the year.