Field formation in cyclotron cc18/9


Completed orders:

Efremov Inst. (St.-Petersburg, Russia) – cyclotron CC18/9


Desired isochronous field was obtained by iterative solving a self-consistent problem on the basis of precise 3D field simulations with the use of the specially developed software package. A detailed calculation of an original shimming system was performed. The system was optimised using magnetic measurements. A comparison between the calculated and measured data is presented

The numerical procedure of the CC18/9 field formation included the following main stages.
1. At the initial stage basic parameters of the magnet system were determined from the magnetooptic calculations.

2. The second stage involved the development of a realistic 3D model for the magnet system. The model used detailed descriptions of the magnet geometry, media interfaces, and non-linear properties of steel. For initial calculations a standard near-realistic B-H curve was used as reference. Then steel properties were corrected using the results of magnetic measurements on samples of steel used in the fabrication of the magnet. The curves В(Н), m(Н), m(В), m / H ) obtained for the real magnet were used to simulate the expected field distribution and to choose the shimming method. Figures 1,2 present a part of the finite-element model of CC12 magnet system (magnetic circuit and coils only). The model covers a 1/16 of the magnet system and includes boundary conditions with respect to the magnet symmetry. The external boundary for the calculated region was taken so that to avoid the influence of the boundary conditions on the field behaviour inside the working zone and type of field decay with distance from the magnet. The finite-element mesh has about 180000 nodes.

3. The finite-element model was applied to preliminary analyse a spatial field distribution. From the magnetooptic analysis required isochronous curves was found to provide proton and deuterion acceleration. The next step was to vary geometrical parameters in order to calculate the influence functions [2] for different magnet components and select the shimming method. From the results of the calculation, the azimuthal shimming by shaping the sector sides was chosen to form a desired field distribution.

4. The influence functions obtained were used to form a required isochronous field to provide both proton (shim moved in) and deuterion (shim moved out) acceleration regimes. The proposed numerical method allowed effective field formation in CC18/9 taking into consideration realistic properties of steels used in the fabrication. The influence function obtained made it possible to estimate manufacture/assembly tolerances, which will be used for magnetic measurements and magnet adjustment.



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