Medical beam steering magnet on line

The Opera electromagnetic simulator has played an important role in the design of the particle beam steering magnet currently being commissioned at Italy's hadron therapy centre in Milan - the Centro Nazionale di Adroterapia Oncologica (CNAO).

Weighing 70 tonnes, 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 the high-performance magnet supplier Sigmaphi. The company specialises in custom magnetic systems and beam transport lines for particle accelerators.

As with almost every magnet design that Sigmaphi creates, the CNAO specification was unique, as well as extremely challenging. The specification called for a very large magnetic field region of 20 x 20 cm, combined with exceptional field homogeneity. Sigmaphi has already produced a similar bending magnet for the Heidelberg Ion Therapy Centre.

However, CNAO's stringent specifications called for even higher performance - with field homogeneity improved by a factor of two.

The sheer 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, the company uses simulation with 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.

Six months were spent designing and optimising this magnet before it was put into production.

The Opera simulator used in this application - known as Tosca - employs a discrete finite element model 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.