Alternating focusing and defocusing quadrupole magnets, separated by drifts, can give stable net focusing in both transverse planes. Stability depends on the full transfer matrix of the lattice cell, not on the sign of one magnet alone.
Strong focusing is a technique used in particle accelerators and certain types of beam optics, particularly in the context of magnetic fields. It refers to a method of focusing charged particle beams using specially designed magnetic fields that can maintain better control over the particle trajectories compared to traditional methods. In strong focusing, a sequence of alternating gradient (AG) magnetic fields is employed.
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