In its 2005 run, DESY’s HERA collider achieved the largest integrated luminosity it has ever produced in one year. Colliding 27.5 GeV electrons with 920 GeV protons, HERA delivered a total of 213 pb-1 to the experiments H1 and ZEUS in 318 days of running. Compared with the positron-proton luminosity production of 2004, the integrated luminosity and the average luminosity were increased by factors of 2.2 and 1.5, respectively. The peak luminosity reached 5.1 ×1031 cm-2s-1 – the design luminosity for the upgraded HERA collider.
This success is particularly remarkable since, compared with running with positrons, additional complications were expected for electron-proton collisions, due to increased synchrotron radiation in the interaction regions and degradations in the lifetime of the electron beam. The synchrotron radiation problems were successfully avoided by improved beam control. Problems with the electron-beam lifetime proved rare and were not relevant for production of luminosity. Nevertheless the electron-beam current reached only 90% of the positron currents in 2004.
The proton intensity improved slightly in 2005 due to improved beam transfer from the injector, while the specific luminosity (luminosity/current) increased considerably beyond the design value owing to the additional focusing of the electron beam by the beam-beam force. The large beam-beam forces made longitudinal electron-spin polarization more difficult: the average peak polarization decreased from 50% in 2004 to 45% in 2005. All in all, however, the operating efficiency improved noticeably compared with the previous running.
Operation at HERA is scheduled to resume at the end of January 2006. Various improvements that have been added during the shutdown – such as refurbished normal conducting magnet coils, enhanced RF interlocks, active damper and feedback systems – will further improve the availability, peak luminosity and background conditions for the run during 2006.
