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Crab cavities are a form of electromagnetic cavity used in particle accelerators to provide a transverse deflection to particle bunches. They can be used to provide rotation to a charged particle bunch by applying a time varying magnetic field. This rotation of the bunch can be used as a diagnostic tool to measure the length of a bunch (the longitudinal dimension is projected into the transverse plane, and imaged) or as a means of increasing the luminosity at an interaction point of a collider if the colliding beams cross each other at an angle (then called crab crossing). They can also be used in order to minimise beam-beam effects, which are important for circular colliders. The KEKB accelerator introduced this technology in its last upgrade.