UPGRADE OF THE 50 KEV GaAs SOURCE OF POLARIZED ELECTRONS AT ELSA

Since 2000, an inverted source of polarized electrons has been operated routinely at the electron stretcher accelerator ELSA, providing a pulsed beam with a current of 100 mA and a polarization of about 80 %. One micro-second long pulses with 100 nC charge are produced in space-charge limitation by irradiating a strained-layer superlattice photocathode (8 mm in diameter) with laser light from a flash lamp pumped Ti:Sapphire laser. Part of the future hadron physics programme requires significantly higher beam intensity, which can be supplied by enlarging the emission area or by improving the quantum efficiency (QE). Both will significantly influence the beam parameters and the optics of the transfer line. Numerical simulations of the space-charge dominated beam transport show that a quasi lossless transport to the linear accelerator is achievable using the existing setup of magnets. Dedicated beam diagnostics like wire scanners and fluorescence monitors will allow an in situ optimization of the optics and the transfer efficiency.