Application and Development of a Method to Shim the Isochronous Field in Small Cyclotrons

Several small cyclotrons have been constructed at CIAE to extract 14-MeV proton beam from 100 to 400 μA for various medical applications, e.g., isotope production. The magnet mapping and shimming for the isochronous field is commonly an iterative process, which is critical for a commercial cyclotron to save the manufacturing time and reduce the cost. A numerical method is established to get the isochronous field by cutting the shimming bars at both sides of the sector pole. The method has been employed to shim the magnet for two small cyclotrons in CIAE. During the shimming process for the second cyclotron, this method has been developed to reduce the number of iterations and overall time for field mapping with the experience from the first cyclotron and accurate results of three-dimensional finite-element simulation. The shimming result shows that it satisfies the requirement of isochronism and focusing characteristics. In this paper, the development of the method is presented in detail, and the improvement for the shimming procedure is illustrated with the shimming data for the two small cyclotrons.