光阴极直流高压电子枪束流偏轴发射研究

离子反轰会造成直流高压电子枪内光阴极的量子效率衰减,降低阴极的工作寿命和稳定性,是限制光阴极直流高压电子枪性能的重要因素之一。基于中国工程物理研究院太赫兹自由电子激光(THzFEL)的直流高压电子枪,通过束流动力学模拟和阴极量子效率测量,研究了束流偏轴发射对离子反轰的抑制作用及其应用限制。模拟结果表明:束流偏轴发射时,离子主要轰击阴极的电中心附近,光电发射区域的离子通量较轴上发射时降低约95%;但束流发射度增大约4πmm·mrad,横向运动幅度增加约1倍。通过电子枪运行前后阴极量子效率测量,发现束流偏轴发射时离子反轰引起电中心附近量子效率大面积的衰减,而光电发射区域量子效率损失较小,验证了束流偏轴发射抑制离子反轰的可行性。

Beam off-axis emission in direct current high voltage photocathode gun

Ion back bombardment is believed to be a major cause of degradation of the quantum efficiency (QE) of the photocathode in direct current(DC) high voltage guns and will reduce the operational lifetime of the cathode. With particle tracking and QE mapping, the effects and limitations of the beam off-axis emission to suppress the ions striking the photo-emission region are studied. The simulation results suggest that the ions mainly bombard the electrostatic center (EC) and the flux of ions near the location of beam emission is reduced by 95% with the beam off-axis emission. The QE maps on the cathode before and after the DC gun operation with CW beam demonstrate that the QE decays seriously within a large region near the EC and the QE degradation in the photo-emission region is suppressed by the beam off-axis emission. However, the off-axis emission will also degrade the beam quality. With an off-axis emission of 6 mm from the EC, the simulation results show that the transverse emittance is increased by about 4mmmrad.