Electromagnetic simulator supports design of particle beam steering magnet

Weighing in at 70tonnes, the 1.81 Tesla dipole magnet is believed to be the largest ever produced for medical applications, and is positioned at the end of a particle acceleration line. It turns the particle beam through 90 degrees to direct it down onto a patient treatment table. The magnet has been produced by magnet supplier, Sigmaphi which specialises in custom magnetic systems and beam transport lines for particle accelerators. The CNAO specification was unique, calling for a very large magnetic field region of 20 x 20cm, combined with exceptional field homogeneity. The scale of the CNAO magnet meant that the design had to be right first time, as any post design modifications would have had a dramatic effect on project construction time and costs. To help ensure such outcomes, Sigmaphi makes extensive use of simulation using the Opera finite element analysis tool from Cobham Technical Services. For CNAO, the company created a detailed three dimensional model of the magnet concept and performed dozens of simulations of design variations before settling on the final optimised parameters for manufacture. The speed of simulation was aided by Opera's ability to use Biot-Savart calculations for computing coil fields, reducing the need for complex finite element meshing of the model - with its impact on computation time. Sigmaphi's chosen electromagnetic simulator for this design task is Opera, commonly used by both designers and end user organisations, which helps to simplify the specification, design and test cycle of projects. Frédérick Forest, engineering director at Sigmaphi, said: "We spent around six months designing and optimising this magnet before we put it into production. Even though the design model was complex, the 3D Opera simulations took only seven or eight hours on a standard PC, a speed that helped us to investigate a large number of design variations. I know from my own investigations into other simulators that Opera provides more accurate results in this magnetics application, and it's a very important tool in our work." The Opera simulator used in this application - known as Tosca - employs a discrete finite element model in order to solve the partial differential equations governing the behaviour of static electromagnetic fields. It computes the total magnetic scalar potential in the magnetic material and the reduced magnetic scalar potential in the regions where source currents in coils are specified. The reduced potential represents only that portion of the field produced by magnetisation, the remainder of the field being computed directly from source currents.