Bringing up beams

Last month, commissioning began on CERN’s newest linear accelerator: Linac4. As the replacement machine for Linac2, Linac4 will take a negative hydrogen ion beam to a staggering 160 MeV. We check in to see how the Linac4 team is preparing its machine for its new role as the first link in the accelerator chain.


The Linac4 3 MeV beam line, with the ion source in the back, the RFQ in the middle and the chopping line in the front.

On 14 November, members of the Linac4 collaboration and the CERN Operations Group were brought together for their first “real day” in the Linac4 Control Room. Together, they successfully accelerated their first hydrogen ion beam to 3 MeV. It was an exciting moment for everyone involved and marked the start of one of the most critical commissioning phases for the new accelerator.

At the start of the Linac4 beam line sits the CERN-made Radio Frequency Quadrupole (RFQ). This vital piece of machinery takes the beam from 45 keV to 3 MeV in just 3 metres and also plays a key role for CERN’s entire accelerator chain: “Beam quality is decided in these first metres of the machine,” says Alessandra Lombardi, who is heading up Linac4’s commissioning. “The RFQ is not only responsible for the first stage of acceleration, it’s also where we prepare the beam for further acceleration down the line. It’s where we define the characteristics of the beam, where we form its emittance and where we define its current. After this point, these characteristics can only be degraded.”

So although this is just the beginning of a three-year-long commissioning phase, it is an important time for ensuring the overall quality of the beam. “At this energy, we also chop the beam – removing micro bunches – in order to prepare it for injection into the PS Booster,” says Carlo Rossi, RFQ project coordinator. “By chopping the beam at this energy, we take away at the earliest possible moment the part of the beam that we know we will lose at injection. It is a tricky process, as we need to remove selected micro-bunches without disturbing the rest of the beam. The settings of this chopper are essential for the overall quality of the beam.”

After commissioning to 3 MeV is completed in February 2014, subsequent radiofrequency (RF) accelerating structures will be installed in stages. The Linac4 team is using a temporary measurement line to check the beam at each stage. Once satisfied, the measurement line can be removed and the next RF element can be installed and tested. “Although this is complex and time consuming for those carrying out the installation,” says Alessandra Lombardi, “this step-by-step approach allows us to check the accelerator components as they are being installed.”

Although only a few months into installation, more than half of the Linac4 infra structure is already in place. Not only are the RFQ and Medium-Energy Beam Transport (home to the beam chopper) in their final locations, the majority of the RF klystrons on the surface have been installed. “After years of working with several teams (including EN-EL/CV and GS-ASE/SE) to complete and prepare the Linac4 hall, our focus now lies solely on the installation and commissioning of the machine itself,” explains Julie Coupard, who is in charge of the Linac4 installation. “By the end of next year we will be commissioning the machine at 100 MeV – at the same time it can become a back-up for Linac2 – and by 2016, we will be commissioning the 160 MeV design energy.”  

Made (and moved) by CERN

Linac4’s RFQ was the first of its kind to be completely constructed and brazed at CERN. This was an important advantage for the Linac4 team: the central production location dramatically reduced the risk of damage from transport, and allowed them to achieve greater precision in the machining and alignment.

However, the RFQ still had to be moved around the CERN site! “Getting the RFQ from the test stand to the Linac4 hall was a major feat,” says Carlo Rossi. “It is a 1.5 tonne machine set to a 100-micron precision, so we were understandably worried about damaging the quality of the accelerator.” CERN’s transport team used special equipment, including a truck with pneumatic suspension, to ensure the RFQ’s safe arrival.

Read more about the challenging construction of the Linac4 RFQ (“Sixteen silver wires to assemble 350 kg of copper”) and the testing of the RFQ module (“Testing begins on Linac4”) in the CERN Bulletin.


Different energies, different cavities

As Linac4 accelerates particles in the non-relativistic regime, the velocity of the beam changes as it becomes more energetic. While this is a (relatively) simple physics principle, it requires some complex adjustments from the Linac4 team: “As the beam becomes more energetic and faster, we have to change the type of RF cavities along the accelerator line,” says Alessandra Lombardi. “Different cavities are better adapted to handle different beam energies.”

In addition to the RFQ, the Linac4 accelerator is home to 3 types of RF structures: Drift Tube Linacs (DTL) will take the beam to 50 MeV; Cell-Coupled DTLs (CCDTL) will take it to 100 MeV; and, finally, PI-Mode Structures (PIMS) will take it up to 160 MeV.


by Katarina Anthony