The Production of Annular Magnetic Fields of Great Uniformity

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Abstract
In the magnetic focusing of electron or ion beams, it is important to secure magnetic fields which possess homogeneity over the region in space traversed by the beam. Helmholtz coils, frequently used for this purpose, provide the desired uniformity of field only for a limited region coaxial with the coils, but for regions off the axis in the median plane, the magnetic field is not uniform. It has been shown theoretically and verified experimentally that in order to obtain the necessary uniformity off the axis in the median plane, the axial separation of the coils must be diminished; this separation depends upon the region in which the uniformity is desired. A pair of coils whose separation may be varied has been constructed of mean radius 30.712 cm, each having 900 turns and an approximate square cross section of windings of side 6.856 cm. The field due to the coils has been explored off the axis in the median plane using a standard coil and magnetometer. The measurements reveal that the uniformity produced with the new separation is vastly superior to that which results from the usual Helmholtz separation.

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