The New Generation of Spin Polarized Electron Sources

1)70 keV极化电子枪的研制高能加速器用极化电子枪已建成并投入使用。最严重的问题是由于有毒气体（H20、CO2等）引起的NEA（负电子亲和性）表面的降解。提出了一种制备清洁镜面不锈钢电极的工艺，在100 kV电压下，暗电流被抑制在10 nA以下。2）超晶格光电阴极克服“表面电荷极限”现象NBA阴极的最大电流受所谓的表面电荷极限（SCL）的限制，而不是受空间电荷极限的限制，因为传导电子被捕获在NBA表面的带弯曲区域中。首次证明了我们研制的超晶格结构光电阴极克服了SCL现象。事实上，超晶格光电阴极可以产生多束团结构的高密度束流，这一结果为将来的直线对撞机提供了很好的应用前景。3）超晶格结构光电阴极新材料的开发除了GaAs-AlGaAs、InGaAs-GaAs和InGaAs-AlGaAs超晶格结构外，还开发了新的材料组合--GaAs-GaAsP超晶格。由于导带的带偏小，而价带的带偏大，我们期望这种组合能得到更高的量子效率和更高的极化率。如预期的那样，第一个样品在803 nm的激光波长下给出了高于80%的偏振度和大于0.3%的量子效率的有希望的结果。进一步的结构参数优化正在进行中，以实现更高的性能。

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.