In particle accelerators Weak focusing occurs when particles are kept in reasonably strong, uniform magnetic fields that causes them to move in circles under the influence of the Lorentz force

Lorentz force

In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...

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Weak focusing is a name for the fact that the trajectories of non-mutually interacting particles moving on these circles intersect.

The effect is reminiscent of the way that similar orbits in space intersect twice per revolution due to gravity.

See also

• Strong focusing
  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...
  
   focusing due to asymmetric magnetic fields can achieve much narrower focusing
• Particle accelerator
  Particle accelerator
  A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...