Hadron Elektron Ring Anlage

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A view in to the HERA accelerator tunnel at DESY, Hamburg.

HERA (Hadron-Elektron-Ringanlage, or Hadron-Electron Ring Accelerator) is a particle accelerator at DESY in Hamburg. Its operation started in 1992. At HERA, electrons or positrons are collided with protons at a center of mass energy of 318 GeV. It is the only lepton-proton collider in the world today. Also, it is still on the energy frontier in certain regions of the kinematic range.

HERA is located under the DESY site and the nearby Volkspark around 15 to 30 m underground and has a circumference of 6.3 km. There are four interaction regions, of which three are used by active experiments H1, ZEUS and HERMES. The fourth interaction region was used the now defunct HERA-B experiment. All these experiments are particle detectors.

Leptons and protons are stored in two independent storage rings on top of each other inside the HERA tunnel.

Leptons (electrons or positrons) are preaccelerated to 450 MeV in a linear accelerator LINAC-II. From there they are injected into the storage ring DESY-II and accelerated further to 7.5 GeV before their transfer into PETRA where they are accelerated to 14 GeV. Finally they are injected into their storage ring inside the HERA tunnel and reach a final energy of 27.5 GeV. This storage ring is equipped with warm magnets keeping the leptons on their circular track by a magnetic field of 0.17 tesla.

Protons are obtained from originally negatively charged hydrogen ions and preaccelerated to 50 MeV in a linear accelerator. They are then injected into the proton synchrotron DESY-III and accelerated further to 7 GeV. Then they are transferred to PETRA and where they are accelerated to 40 GeV. Finally, they are injected into their storage ring inside the HERA tunnel and reach their final energy of 920 GeV. The proton storage ring uses superconducting magnets to keep the protons on track.

The lepton beam in HERA becomes naturally transversely polarised through the Sokolov-Ternov effect. The characteristic build-up time expected for the HERA accelerator is approximately 40~minutes. Spin rotators on either side of the experiments change the transverse polarisation of the beam into longitudinal polarisation. The positron beam polarisation is measured using two independent polarimeters, the transverse polarimeter (TPOL) and the longitudinal polarimeter (LPOL). Both devices exploit the spin-dependent cross section for Compton scattering of circularly polarised photons off positrons to measure the beam polarisation. The transverse polarimeter was upgraded in 2001 to provide a fast measurement for every positron bunch, and position-sensitive silicon strip and scintillating-fibre detectors were added to investigate systematic effects.

The end of HERA is predicted to be in summer 2007, when its main pre-accelerator PETRA will be converted into a synchrotron radiation source.