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Why can an electron and a positron travel in opposite directions around the same synchrotron ring?

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Diagram of a linear proton resonance accelerator, or linac, showing protons entering from a source, passing through a series of labeled drift tubes powered by a radio-frequency source, and exiting as a beam through a cavity. — particle accelerator The accelerator is a large-diameter tube within which an electric field oscillates at a high radio frequency. Within the accelerator tube are smaller diameter metallic drift tubes, which are carefully sized and spaced to shield the protons from decelerating oscillations of the electric field. In the spaces between the drift tubes, the electric field is oriented properly to accelerate the protons in their direction of travel.

An electron and a positron can travel in opposite directions around the same synchrotron ring because they are a particle-antiparticle pair with opposite electric charges.12

When a charged particle moves through a magnetic field, it experiences a force that bends its path. Because the positron has a positive charge and the electron has a negative charge, they respond differently to the same field when moving in the same direction. However, if the positron travels in the opposite direction to the electron, the combination of its opposite charge and reversed velocity causes it to be bent along the same curved path as the electron.341

Furthermore, the same electric fields used to accelerate the particles can be utilized for both beams. While an electric field that accelerates an electron would normally decelerate a positron moving in the same direction, a positron traveling in the opposite direction experiences an opposite force, allowing it to be accelerated by the same field. This efficiency allows a single ring to function as a colliding-beam storage ring.12