LERs

LERs, short for Low Energy Rings, are a class of storage rings used in circular collider facilities to store the beam at energies lower than the companion ring, which is typically called the High Energy Ring (HER). In many two-ring electron-positron colliders, the beams collide at interaction points while circulating in separate rings. The LER handles the lower-energy beam while the HER handles the higher-energy beam; the two rings have separate RF systems, magnet lattices, and diagnostics, linked by the interaction region.

Design and operation: The LER must accommodate the lower magnetic rigidity, often using weaker dipoles and different lattice optics. It must manage synchrotron radiation, beam stability, and beam-beam interactions at the crossing point. Because the energy is lower, the ring tends to have different damping times and emittance control requirements; injection and lifetime considerations rely on dedicated damping and injection schemes. The luminosity is achieved through focusing of the colliding beams at the interaction point and careful tuning of the crossing angle, tunes, and impedance.

Examples: Two-ring collider facilities have used LERs to enable high-luminosity collisions. KEKB in Japan used an LER at about 3.5 GeV (positrons) and an HER at about 8 GeV (electrons); its upgrade, SuperKEKB, increases both energies to about 4 GeV in the LER and 7 GeV in the HER to reach higher luminosities. PEP-II at SLAC in the United States used a similar concept with a lower-energy LER and a higher-energy HER, enabling substantial luminosity during its operational period.

See also: Circular colliders, luminosity, damping rings.