Polarized electrons in the AmPS storage ring

AmPS is a 300-900 MeV electron ring. The machine was designed as a pulse stretcher to provide continuous external beams. After years of successful operation in this mode the nuclear physics program now concentrates on experiments with stored beams interacting with internal gas targets. These experiments require currents between 50 and 200 mA through 60 cm long 15 mm wide cylindrical target cells. Gas pressures in the open-ended cells correspond to target thickness of 10¹⁴ to 10¹⁶ atoms/cm². Strong stray fields from a spectrometer and from target holding fields disturb the closed orbit. Under operational conditions the linac provides a maximum energy of 700 MeV, so ramping is required to achieve 900 MeV. To further increase the luminosity at the target a reduced emittance lattice based on the existing magnet configuration has been designed. Late 1996 a polarized electron injector as well as a “Siberian Snake” to control the spin on the internal target became operational. The polarized injector delivers a few mA to the linac so stacked injection in the ring is required. With strained InGaAsP cathodes a polarization of 80% was obtained. Test results, performance and prospects are presented