A superconducting quadrupole magnetic lens with 30 mm aperture

It is not possible to make a sufficiently correct calculation of such a system without a knowledge of the focusing properties of the quadrupole lenses, which depend to a great extent on the characteristics of the superconducting and magnetic materials used. We therefore constructed a model of an element of the focusing system. A variant of lens design was found which is technologically simple and makes it possible, by direct summing of a number of elements, to obtain a focusing system of any length. The pole tips of the lens are of cylindrical shape. It is shown theoretically 3 and experimentally 4 that the optimum value of the radius of curvature of a pole tip must be R = 1.10 to 1.15 a (where a is the radius of the aperture). The change in magnetic field gradient will then be not more than + 0.2% at distances r ~< 0.8a from the lens axis.