Editing HERA (particle accelerator) (section)

== HERA accelerators ==
[[Lepton]]s (electrons or positrons) were pre-accelerated to 450&nbsp;[[MeV]] in the linear accelerator LINAC&nbsp;II. From there, they were injected into the storage ring DESY&nbsp;II and accelerated further to 7.5&nbsp;[[GeV]] before their transfer into the storage ring [[PETRA]], where they were accelerated to 14&nbsp;GeV. Finally, they were injected into their storage ring in the HERA tunnel and reached a final energy of 27.5&nbsp;GeV. This storage ring was equipped with warm (non-superconducting) magnets keeping the leptons on their circular track by a magnetic field of 0.17&nbsp;[[tesla (unit)|tesla]].

Protons were obtained from originally negatively charged [[hydrogen]] [[ion]]s and pre-accelerated to 50&nbsp;MeV in a linear accelerator. They were then injected into the proton synchrotron DESY&nbsp;III and accelerated further to 7&nbsp;GeV. Then they were transferred to PETRA, where they were accelerated to 40&nbsp;GeV. Finally, they were injected into their storage ring in the HERA tunnel and reached their final energy of 920&nbsp;GeV. The proton storage ring used superconducting magnets to keep the protons on track.

The lepton beam in HERA became naturally transversely polarised through the [[Sokolov–Ternov effect]]. The characteristic build-up time expected for the HERA accelerator was approximately 40&nbsp;minutes. Spin rotators on either side of the experiments changed the transverse polarisation of the beam into longitudinal polarisation. The positron beam polarisation was 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.

On 30 June 2007 at 11:23&nbsp;pm, HERA was shut down,<ref name="CERNcourier_BW"/> and dismantling of the four experiments started. HERA's main pre-accelerator PETRA was converted into a [[synchrotron radiation]] source, operated under the name [[PETRA III|PETRA&nbsp;III]] since 2009. Today, a section of the HERA tunnel and 24 former superconducting dipole magnets are being used for the new ALPS experiment, which looks for [[axion]]-like particles.<ref>{{cite web|url= https://www.aei.mpg.de/636595/alps-ii| title=Any Light Particle Search (ALPS) II |publisher= MPG Albert Einstein Institute|access-date=15 September 2022}}</ref><ref>{{Cite web |url=http://www-mpy.desy.de/cgi-bin/status2004?Accelerator=HERA&Time=24+hours&Format=100%25 |title=Last run of HERA (30 June 2007) |access-date=6 October 2007 |archive-date=4 August 2009 |archive-url=https://web.archive.org/web/20090804203945/http://www-mpy.desy.de/cgi-bin/status2004?Accelerator=HERA&Time=24+hours&Format=100%25 |url-status=dead }}</ref>