A dipole magnet, in particle accelerators, is a magnet constructed to create a homogeneous magnetic field over some distance. Particle motion in that field will be circular in a plane perpendicular to the field and collinear to the direction of particle motion and free in the direction orthogonal to it.
Thus, a particle injected into a dipole magnet will travel on a circular or helical trajectory.
In particle accelerators, dipole magnets are used to realize bends in the design trajectory (or 'orbit') of the particles, as in circular accelerators. Other uses include injection/ejection of particles and the correction of orbit errors.
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