The New Generation of Spin Polarized Electron Sources

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1)ConstructIon of 70 keV polarized electron gun The polarized gun for high energy accelerators was completed and has become into operation. The most serious problem was the degradation of NEA (Negative Electron Affinity) surface due to the poison gases (H20, Co2 etc.) produced by the field emission dark currents between the HV electrodes.The technology to fabricate the clean and mirror-flat surface of the stainless-steel electrode was developed and the dark current was supressed below 10 nA level under 100 kV application. As a result, the stable beam with the polarization more than 80 % and the peak current of 1.6 A can be produced by this gun.2)Overcome of "Surface Charge Limit" phenomenon by superlattice photocathodes The maximum current, extracted from an NBA cathode, is limited by so-called Surface Charge Limit (SCL), rather than by the space charge limit, as the conduction electrons are trapped in the band-bending region of NBA surface. It was first demonstrated by us that the SCL phenomena is overcome by using the superlattice structure photocathodes developed by us. In fact, the high density beam with a multi-bunch structure could be produced from the superlattice photo cathode and this result gave the promising perspective for aplication to future linear colliders.3)Developments of new material photo cathodes with superlattice structure In addition of GaAs-A1GaAs, InGaAs-GaAs and InGaAs-AIGaAs superlattice structures, the new combination of materials, GaAs-GaAsP superlattice was developted. We expected that this combination gives higher quantum efficiency and higher plarization, since it has the smaller band-offset for conduction-band, while the larger one for valence-band. As expected, the first sample gave the promising result with the polarization higher than 80 % and the quantum efficiency larger than 0.3% at laser wave-length of 803nm. Further optimizations of structure parameters are in progress to achieve the higher performances.