Design Optimization of the Fast Switched Chopper Dipole Magnet for the MedAustron Project

The MedAustron hadron therapy centre currently under construction in Wiener Neustadt, Austria, is a synchrotron based accelerator facility for cancer treatment with protons and carbon ions. The concept for such a machine first originated at the European Organization for Nuclear Research (CERN) in 1999 as the Proton-Ion Medical Machine Study (PIMMS). The first centre based on this concept was the National Centre for Oncological Treatment (CNAO) built in Italy, which treated its first patient in November 2012. The MedAustron accelerator complex consists of three particle sources, a linear accelerator, a synchrotron, an extraction line, and four irradiation rooms (1 experimental area with horizontal fixed beam, 2 fixed beam line rooms (one horizontal, and one horizontal and vertical) and a rotating gantry treatment room). It will be capable of accelerating 1H+ protons to energies of 60-250 MeV for clinical purposes, and up to 800 MeV for research purposes. It will also accelerate 12C6+ carbon ions to energies of 120-400 MeV/u. An energy of 400 MeV/u for the carbon ions corresponds to a beam rigidity Bρ of 6.35 Tm, which determines the maximum strength of the required magnetic elements. The treatment rooms will be able to switch the beam on and off rapidly (< 240 µs) during routine operation, or during emergencies. For the purpose, a beam chopper system will be installed in the extraction line, comprising four identical fast switched dipole magnets electrically connected in series. In the “ON” state, the magnets will deviate the beam around a dump block mounted inside the beam vacuum chamber. When the magnets are switched off the beam will be absorbed by the dump, thus making the chopper system a device in the safety chain to prevent a mismatched beam from being sent to a treatment room inadvertently. The initial design of the magnets has been based on experience from CNAO, and realised at CERN by the MedAustron Special Magnets work package. The production of the magnets has been subcontracted to the company Danfysik, based in Taastrup, Denmark. The author was involved in the 3D Finite Element electromagnetic design optimisation and was responsible for the validation of the mechanical models and drawings, and the contract follow-up, ensuring that the specified magnet parameters were respected in the final design. Being the link between the FEM models and the mechanical design, the author optimized the magnet coil heads, and ferrite pole shape in the end-regions, validating their impact on the desired ± 0.2 % field homogeneity in the magnet gap. He examined different insulation schemes and their impact on the field quality, as well as the impact of any possible manufacture and assembly defects and misalignments. The author also studied the influence on the effective magnetic length of the device of the brazing collars for the flanges on the ceramic vacuum chambers. The author was involved in the final magnetic measurements and factory acceptance tests, thus assuring that the magnets delivered to the project would perform according to the technical specification.