The HERA storage ring (short for Hadron–Electron Ring Accelerator) operated from 1992 to 2007. It has a circumference of 6,336 m. It was DESY's largest ring accelerator and Germany's largest research instrument to date. Until 2007, HERA was the only storage ring facility in the world to enable collisions of electrons or positrons with protons for particle physics (experiments H1, ZEUS, HERMES and HERA-B) for studies of the inner structure of the proton.