From the drawing board to the test bench

After seven years of design, prototyping and manufacturing, the Linac4 Drift Tube Linac (DTL) is being assembled in CERN's Building 181. In fact, the first DTL tank is just a few tests away from installation into the Linac4 tunnel. Let's check in on how this essential element of the accelerator complex has - at last - come into being.


A peek inside a Linac4 DTL tank.

The Linac4 DTL is designed to take the new linear accelerator's H- ion beams from 3 to 50 MeV. Divided into three tanks, holding a total of 108 drift tubes in vacuum, each made from a dozen parts, and assembled with ancillary elements, cooling circuits and hundreds of bolts and screws, it’s no surprise it has been nicknamed a 'puzzle with thousands of pieces'.

A trying tale
Although the first tank will soon be ready for installation, constructing the DTLs has been a trying tale for the Linac4 team. “Not only were we tasked to design CERN's first drift tube linac for over a quarter of a century, we also encountered numerous manufacturing issues,” says Suitbert Ramberger, project engineer for the Linac4 DTL. “A precision of ±0.1 mm is required when positioning drift tubes in the 7.3 m long vacuum vessels and companies rarely have such expertise. So we turned to the Los Alamos team responsible for the Spallation Neutron Source (SNS) DTL, who provided extremely helpful advice on how to proceed.”

Innovative design
“We wanted to reduce the complexity of the design and make it reliable for 30 years of operation.” says Maurizio Vretenar, project leader of the Linac4 project. “While traditionally DTLs are equipped with screws in order to adjust the drift tube position after the assembly, we concluded that manufacturing techniques had improved to the point where DTLs could be built with fewer means of adjustment. This way we could make do without bellows or double sealing.” In fact, this new design was the subject of its own patent (for more information, read "The invention that is shaping Linac4").

Extreme precision
Some of the most challenging pieces to manufacture were the girders that hold the drift tubes in place. With geometry of parts measured in terms of a few tens of microns, they tested the limits of traditional manufacturing. “Several companies tendered for the contract but were just not able to guarantee the tolerances," says Ramberger. "Not only does the external manufacturing company need the right machinery, they also need significant expertise. Interestingly, the latter can be traded for the former. Although two girders were final machined at CERN, the main workshops did not have the equipment or environment usually considered appropriate for this type of job! It is amazing what you can do with the right people."

Assembling the Linac4 DTL in Building 181.

Imitation is the best form of flattery
Now, with seven years of experience under their belts, the Linac4 DTL team are experts in their own right and a resource to other teams looking to build drift tube linacs. "The DTL in Bilbao's planned 50 MeV light ion facility of the ESS-Bilbao (ESSB) is an exact replica of the Linac4 DTL," says Vretenar, “as is the mechanical design of the DTL for the European Spallation Source (ESS) currently under development at the INFN, Legnaro (LNL). Both institutes contributed to Linac4's DTL development, and now their teams are following the work of our team very closely." No matter the discipline, imitation is always the best form of flattery.

Happily ever after
"The beauty of the Linac4 DTL design is its puzzle-like simplicity," concludes Ramberger. "Each complex piece was designed to slot into place for straight-forward assembly... with no extra welding required. It's been a speedy assembly process and soon all the tanks will be ready to go!"

by Katarina Anthony