Strong focusing

In accelerator physics strong focusing or alternating-gradient focusing is the principle that the net effect on a particle beam of charged particles passing through alternating field gradients is to make the beam converge. By contrast, weak focusing is the principle that nearby circles, described by charged particles moving in a uniform magnetic field, only intersect once per revolution. In accelerator physics strong focusing or alternating-gradient focusing is the principle that the net effect on a particle beam of charged particles passing through alternating field gradients is to make the beam converge. By contrast, weak focusing is the principle that nearby circles, described by charged particles moving in a uniform magnetic field, only intersect once per revolution. Earnshaw's theorem shows that simultaneous focusing in two directions at once is impossible. However, ridged poles of a cyclotron or two or more spaced quadrupole magnets (arranged in quadrature) alternately focus horizontally and vertically. Strong focusing was first conceived by Nicholas Christofilos in 1949 but not published (Christofilos opted instead to patent his idea), In 1952, the strong focusing principle was independently developed by Ernest Courant, M. Stanley Livingston, Hartland Snyder and J. Blewett at Brookhaven National Laboratory, who later acknowledged the priority of Christofilos' idea. The advantages of strong focusing were then quickly realised, and deployed on the Alternating Gradient Synchrotron.