Manufacturing Magnet Components for CERN

Written by HV Wooding on 21st July 2015

This paper will take you through the scientific side of the business, in relation to manufacturing of components for the research industry and how we have supported their prestigious contracts over the years.

Specifically big magnets, and the largest and most complex set of instruments there are, this being CERN. To best describe the component parts we manufacture for CERN and similar establishments, it’s best to explain what CERN is and with the aid of illustrations and photographs working backwards from the finished assembly to identify our components.

CERN is the European Organisation for Nuclear Research which is financed by 21 member states. It is the largest and most complex set of instruments to study the basic constituents of matter, the fundamental particles. These particles are made to collide together at close to the speed of light. This process gives Physicists clues about how particles interact, and provide insights into the fundamental laws of nature. The Large Hadron Collider at 27kms straddles the Suisse/French border near Geneva and is tunnelled underground at 100 metres deep and as such is the world’s largest and most powerful particle accelerator. CERN houses 10 times more engineers and technicians than research physicists. Without superconducting magnets the LHC would require a circumference of 120kms.

Ten’s of millions of particle collisions per second occur and these are monitored by four main detector stations. The biggest of these experiments, ATLAS and CMS, use general-purpose detectors to investigate the largest range of physics possible. Having two independently designed detectors is vital for cross-confirmation of any new discoveries made.

ALICE and LHCb have detectors specialized for focusing on specific phenomena. These four detectors sit underground in huge caverns on the LHC ring. ALICE detects quark-gluon plasma, a state of matter thought to have formed just after the big bang whereas the Atlas uses a 7000-tonne machine using a detector probing for fundamental particles.

A chain of accelerators speed up and increase the energy of a beam of particles by generating electric fields that accelerate the particles, and magnetic fields that steer and focus them.

The LHC is the last accelerator in the chain. The LHC has 1,232 Dipole magnets at 15 mtr. lengths to bend the beams and 392 Quadrupole magnets at 5-7 mtr. lengths to focus the beams.

From this diagram of a split section through the Dipole magnets we can now start to see the components we make. Inside the casing, the yellow area being the steel yoke laminations which retain the stainless steel collars in green. These in turn secure the superconductors and the beam screen shown in brown.

Our involvement has been mainly with the yokes and collars for various accelerator prototypes and upgrades but we have also supplied someflexible busbars and are currently looking at pressing some coil tapes.

The yokes are made from a low carbon steel with a gauge of 5.8mm with an outside diameter of 580mm. Dimensional tolerances range from 0.1mm for the less important features through to sub 20 microns for collar retention and line up locations.

Prototype Dipoles may start at one metre long, and progress in length through their experimental stages. This means an order can start for just one metre’s worth of laminations, 172 in quantity. A 15 metre Dipole will have over 2,500 laminations. We start this yoke manufacturing process by laser cutting from strip material the basic profile. This material comes to us in 4 metre lengths and 600mm wide.

The flatness of this steel is very important. Any bow in the material will influence dimensions once parts are clamped together. Material must also be contamination and rust free. Laser cutting does not like rusty material. We need to be able to maintain tolerances near 0.1mm for laser cutting otherwise the follow on CNC machining and wire erosion processes will be too great for the tighter tolerances. For some mill machining and wire erosion, parts can be stacked but this has to be assessed depending on the tolerances required. CERN had not come across this combination of machining methods previously.

For the collars, material is sourced in coil form. We de-coil, straighten and crop stainless steel into 2 metre strips. This material for the collars has to be fully non- magnetic. It is of a particularly hard and tough grade. Flatness both across and down the strip is even more important than the yokes.

The photograph here shows the laser cut collars, cut all facing one direction. Collars are generally of a tight tolerance, so some are laser cut then wire eroded, others fully wire eroded.

If the upgrade trials are successful, then larger numbers of yokes and collars will be required. For commercial and production volume reasons, pressing of these parts is the preferred route. Fine Blanking is the preferred pressing method which gives a squarer cut edge and flatter part than conventional pressing. However, some of the very tight tolerances achieved by wire erosion and some profile detail will be very challenging for pressing.

Most of our work with CERN has been in relation to the upgrade work from the current 8 Tesla to 11 Tesla. Other research institutes such as Fermilab in Chicago and Brookhaven in New York work actively with CERN and this close co-operation has led to us winning similar scientific work with Brookhaven and more closely to home STFC, Rutherford, UKAEA and Diamond Light. Activity at CERN is particularly strong and we look forward to working with them and their associates for many years to come.