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Large first Fourier harmonic of the main magnetic field near the injection and extraction regions of the K500 superconducting cyclotron at VECC, Kolkata adversely affects the beam, guiding it off-center and inducing large amplitude coherent oscillations, thus, making efficient extraction very difficult. Copper-liners over the iron-poles make the poles inaccessible for local iron-shimming. In this paper we discuss an optimal correction of the field imperfection. We consider the magnetic field measurements data, analyze the contributions from different sources of error harmonics and find that it is mostly due to the misalignment of major components of the machine. An algorithm to provide effective correction of the first harmonic of the main magnetic field is proposed and successfully adopted in the K500 beam operation.

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