Sixteen silver wires to assemble 350 kg of copper

Assembly of the first radiofrequency quadrupole (RFQ) module for the future LINAC4 linear accelerator has just been completed by CERN's Assembly and Forming section (EN) using a technique called vacuum brazing which involves furnace-baking at 800°C and micron precision, leaving absolutely nothing to chance.

The future LINAC4 will use four types of accelerating structures, each playing a different role in increasing the energy of the beam. The first of these are the radiofrequency quadrupole modules (RFQ, see box) which accelerate and focus the beam from the start. "The modules are complex items. Each had to be produced in 4 parts, corresponding to the 4 electrodes," explains physicist Serge Mathot, a vacuum brazing specialist in the Engineering (EN) Department. "To work properly, these modules must be aligned to a precision of a few microns. It is therefore essential for them to be machined and assembled with meticulous care."

To assemble the four parts of the RFQ module, Serge and his colleagues employe a technique known as vacuum brazing (see box). Sixteen silver-alloy wires are deposited on the surfaces to be assembled (see photo) and the whole assembly is placed in a vacuum furnace and heated to around 800°C. As the temperature rises, the 16 wires melt and interconnect the various components to form a single 350-kg copper block. "The problem is that each metre-long part expands by nearly 16mm when hot and must return to its original length with micron precision," Serge explains. "What's more, we can only keep the temperature above 800°C for a few minutes, otherwise the brazing material will run away into unwanted areas like the inner surfaces where the particles will be circulating." An error of just a few degrees Celsius or a slight over-exposure to high temperature and the silver will overflow, thus jeopardising the whole cavity. Although the special silver-alloy used at CERN allows optimal brazing it is also a material that emits many electrons when subjected to an electrical field such as the radiofrequency that flows between the electrodes.

The assembly of the first RFQ module was a success. "We had never before brazed such large components with such high precision. It's an excellent result for all the teams who worked on this project!" Serge enthuses. Although CERN has more than 50 years' experience of using furnaces to braze many components for its accelerators and experiments, the brazing of the RFQ for the LINAC has been greatly helped by use of the newly-acquired vacuum furnace. Several months of preliminary studies and tests were needed to determine the precise behaviour of the various materials during the high-temperature process.

The next step will be the installation of flanges on the module for assembly with the other modules that will soon enter production. LINAC 4 construction is proceeding according to the schedule, which provides for commissioning in 2014 and connection to the accelerator complex in early 2015.

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What's an RFQ?

Located at the very start of the linear accelerator, the RFQ (Radiofrequency Quadrupole) is used to accelerate and focus the particle beam. It comprises four electrodes and differs from conventional quadrupoles – such as the ones focussing the LHC beam close to the interaction points – in that the focussing operation is done by a radiofrequency electrical field that alternates the polarity on the electrodes (see photo). As the beam crosses the 3-metre long RFQ, the polarity of the field changes 300 times, as if it had passed through 300 conventional quadrupoles. In addition, the modulation on each of the electrodes generates a linear electrical field that accelerates the particles. These various features together enable the RFQ to perform three basic functions: to focus a low-energy beam, to bunch the particles into bundles from a continuous beam and to accelerate these bundles under the combined effect of the modulation and the supply of radiofrequency to the electrodes.
The electrodes are made of copper, a good conductor that facilitates circulation of the radiofrequency electric waves and dissipation of the heat.

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What is vacuum brazing?

Brazing is a technique for assembling metals and ceramics, closely related to welding, whereby a material is melted between the two parts to be assembled without their melting themselves. For this, you have to use an alloy with a lower melting point, i.e. which passes into a liquid state at a lower temperature than the materials to be assembled. A silver-based alloy is used for the RFQ, melting at 810°C, unlike the copper electrodes which melt at 1083°C. The alloy is placed in grooves (see photo above) and as it melts it flows into the space between the copper parts thereby brazing them together.
One of the special features of vacuum brazing is the use of a vacuum furnace, which heats the parts uniformly and in an oxygen-free atmosphere. Since there is no oxidisation, the liquid flows quite uniformly onto the surface of the parts to be assembled. The alloy would still have melted in a normal furnace but would not have been able to flow uniformly due to the oxidisation of the copper surface. Lumps would have formed and the two parts would not be properly fused together.
The great advantage of vacuum brazing is that no flux is needed to prevent surface oxidisation, as with flame-brazing for instance. Indeed fluxes should never be used for components that need to operate in a vacuum.

by Alizée Dauvergne