A New Record for HERA

The week of 24 February-3 March brought a major step forward for the exploitation of HERA II: longitudinal positron spin polarization has been obtained simultaneously at all three positron (electron) interaction regions equipped with rotator magnets and at the routine beam energy of 27.5 GeV.

Wednesday 4 May 1994 had already marked a turning point in the art of the manipulation of spins in electron storage rings: longitudinal electron spin polarization, with the spins oriented along the electrons direction of motion, was established at the East interaction region in the electron ring of HERA, DESY's hadron-electron ring accelerator facility. That was the first time in the history of high energy electron storage ring physics that the naturally occurring vertical spin polarization was, with the aid of spin rotators, converted to longitudinal polarization. It opened up a new range of physics, in particular for the HERMES experiment, which exploited the polarized electron and positron beams until mid-2000 when the modifications for the HERA luminosity upgrade began.

However, the original proposal for HERA foresaw longitudinal polarization at three interaction regions. But installation of spin rotators for electron-proton collision experiments of H1 and ZEUS was delayed until the HERA upgrade. In fact, with the necessary (space saving) removal of the compensating solenoids for HERA II, the more complicated magnetic fields around the H1 and ZEUS interaction regions as well as the stronger quadrupole fields in the arcs, the conditions for polarization are potentially much less favorable.

Nevertheless, starting on Monday 24 February with all three pairs of rotators switched on, with a special spin matched optic and with special orbit corrections (harmonic bumps) but with the solenoid magnets of H1 and ZEUS switched off, longitudinal positron polarization was generated at all three interaction regions. Then on Sunday 2 March a polarization of 50 percent was attained with the H1 and ZEUS solenoids switched on and during collision with the proton beam. This is to be compared with the theoretical maximum of about 83 percent that is allowed in this configuration.

Measurement and control of polarization would not be possible without high quality reliable polarimeters. At HERA, the polarization is measured with two polarimeters at places 3 km apart: vertical polarization in the West straight section and longitudinal polarization in the East straight section near the HERMES detector. The HERA upgrade also provided the opportunity to upgrade the polarimeters. As required, they observe the same value of the polarization.

The provision of high energy longitudinally spin polarized electron and positron beams at the East interaction region had already made HERA a unique facility. The provision of polarized beams for three experiments is even better and opens the way for a very exciting experimental program with HERA II.