Magnetic Measurements and Commissioning of the Fast Ramped 90$^{\circ}$ Bending Magnet in the PROSCAN Gantry 2 Project at PSI

The Paul Scherrer Institute (PSI/Switzerland) is developing, within the PROSCAN project, a new method of proton radiotherapy for treating cancers using a gantry based spot scanning technique for irradiating deep-seated tumors. Among the innovative elements are the laminated magnets in the beam lines, including the newly conceived Gantry 2, that will enable rapid energy changes (<150 ms) to modulate the range of the scanning pencil beam in three dimensions over the tumor volume. The last and multifunctional 90° bending magnet in the Gantry 2 transport system is not only the largest, but also the most challenging element. The dynamic eddy current effects during ramping were reduced with a special arrangement of laminated parts in the central pole region of the magnet. The construction of the Gantry 2 is finished and the first proton beam reached the treatment area in May 2008. The start of the patient treatment program is planned for the near future. In this paper, the 90° bending magnet construction and the results of the magnetic measurements (both static and dynamic) are presented, together with the commissioning experiences of the Gantry 2 magnet system. The results of the magnetic measurements validated the design, showing a very successful reduction of the eddy current effects so that the fast switching between two treatment energy steps (1 to 2% of the total energy) can be realized in less than 80 ms.