Development of ultra-low-emittance electron source

为正负电子直线对撞机研制了200 keV自旋极化电子源系统。它采用了两种物理机制的组合：光学极化过程和极化传导电子的NEA（负电子亲和势）表面发射机制。这种电子源不仅能产生极化电子，而且能产生高电流密度和低发射度的电子束，用这种200 keV电子枪获得了下列性能：（1）极高真空度为2？在<-9>光电阴极（PC）周围获得了10^ -Pa的真空度，NEA表面的暗寿命延长了300小时以上。(2)通过使用Mo材料作为阴极和Ti材料作为阳极来制造低暗电流电极。在200 kV的偏压下，暗电流小于1 nA，并且实现了超过500小时的稳定工作。(3)作为极化电子源，实现了3.0 MV/m的世界最高场梯度。(4)采用GaAs-GaAsP应变超晶格光电阴极，获得了92%的自旋极化率和0.5%的量子效率。(5)高电流密度200千伏束; 8-nC/束（1.6-ns束宽），即5-A的峰值电流，可以产生。(6)50-？？A可以从相同的NEA表面提取超过120小时。(7)用胡椒罐法测量了体GaAs和超晶格光阴极NEA表面发射电子束的热发射度。(8)作为结果，相同的发射度值为0.1？？嗯？对于带隙激发，两种光阴极都观察到Emrad，但是由于较高的激光能量激发，超晶格的发射度增长小于体GaAs光阴极。

A 200-keV spin polarized electron source system for the electron-positron linear collider has been developed and completed. It employs a combination of two physical mechanism; optical polarization process and NEA (Negative Electron Affinity) surface emission mechanism for polarized conduction electrons. This type of electron source has the advantages to produce not only the polarized electrons, but also the high current density beam and the low emittance beam.Following performances are obtained by using this 200-keV electron gun(1) An extremely high vacuum of 2?~10^<-9>-Pa is obtained around the photocathode (PC), and the dark-lifetime of NEA surface is extended for more than 300-hours.(2) The low dark current electrode is fabricated by using the Mo material for a cathode and the Ti material for an anode. At a bias voltage of 200-kV, the dark current is less than 1-nA, and the stable operation of more than 500-hours is achieved.(3) As the polarized electron source, the world highest field gradient of 3.0-MV/m is achieved.(4) The spin polarization of 92-% and the quantum efficiency of 0.5-% are achieved by using the GaAs-GaAsP strained superlattice photocathode.(5) High current density 200-kV beam; 8-nC/bunch (1.6-ns bunch-width) that is 5-A peak-current, can be produced.(6) The constant average current of 50-??A can be extracted for more than 120-hours form the same NEA surface.(7) The thermal emittance for the electron beam emitted from the NEA surface of both of bulk-GaAs and superlattice photocathode are measured by using a pepper-pot method.(8) As the results, the same emittance value of 0.1 ??mm?Emrad is observed for the both photocathode for the band-gap excitation, but the emittance-growth due to the higher laser energy excitation is smaller for the superlattice than that of the bulk-GaAs photocathode.