Realization of an iron-free superconducting quadrupole lens

It is possible to realize, by means of coils, field configurations with quadrupolar symmetry in which the transverse field gradient remains constant to some 10^-3in the useful central zone, thus permitting the realization of quadrupole lenses with good optical qualities. Optimum values better than one percent have been calculated for geometrical parameters leading to linearity of the field gradient. Two cases were studied: trapezoidal cross-section coils, with saddle-shaped extremities, and rectangular cross-section plane coils. In spite of greater construction difficulties, a quadrupole magnet of the first type was realized using Nb-Zr multicored cable. It has a useful diameter of 5.2 cm. mechanical length of 16 cm, and an equivalent magnetic length of 19.2 cm. The maximum field gradient obtained is 7000 G . cm^-1, the current being A. But the value of the stable working intensity is 225 A, giving a field gradient of 6800 G . cm^-1. The radial variations of the field are quite perfectly linear and in good agreement with theoretical results.