The PETRA storage ring (short for Positron–Electron Tandem Ring Accelerator) has been in operation since 1978. It has a circumference of 2,304 m. Until 1986, electrons and positrons collided in PETRA for research in particle physics (experiments JADE, MARK-J, TASSO and PLUTO). From 1990 on, PETRA served as a pre-accelerator for the HERA storage ring, and from 1995 on also as a synchrotron radiation source with two test experimental stations. Since 2009, the facility has been delivering hard X-ray beams of very high brilliance to over 40 experimental stations under the name PETRA III.