The finite element method for calculating saturation magnetic field of H-120 cyclotron main magnet using the vector potential over a wide region was described in this paper. The simultaneous non-linear equations which were finally deduced were solved by Newton-Raphson iterative method with a small increment (the increment of surrent density) . LDL

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analysis method was used in every step.

Comparing the values calculated with those measured

it was found that the errors of the field distribution calculated were less than 1%.