Magnetic Field Corrections in the Cyclotron

Abstract

It is pointed out that in order to obtain magnetic fields which will allow the ions in the cyclotron to attain high energies it is first necessary to render the field homogeneous within an accuracy of about one part in a thousand. The correction (insertion of "shims") for the most important source of inhomogeneity, namely the decrease in the field near the edge of the pole pieces, is considered. A general method is given for determining the dimensions for any shape of these shims. Results are given for the dimensions of shims in the form of a ring of rectangular cross section placed at the edge of the cyclotron chamber. The magnetic field resulting from the use of such shims is determined and compared with the field of plane parallel pole faces. For the special case of a ring whose thickness is $\frac{1}{16}$ the magnetic gap, the field is homogeneous within the required accuracy up to a distance from the edge of the poles of 0.4 of the magnetic gap. The uncorrected field is homogeneous only up to a distance from the edge equal to 0.9 of the magnetic gap.