Study On Ion Anti - Hammer In GaAs Cathode DC High Voltage Electron Gun

High brightness electron source has became a research hotspot with the driving of the future high average power free electron laser and energy recovery linac which require the high repetition, low emittance and high average current electron beams. Negative electric affinity (NEA) gallium arsenic (GaAs) DC high-voltage photocathode injector is one of the most important electron sources to drive the future accelerator facility. But the maintainment of the high operation lifetime and stability of the GaAs electron source is still a great challenge. This dissertation is based on the DC high-voltage photocathode injector of CAEP THz-FEL, and devoted to investigating the ion back bombardment in a DC gun and the measurement of quantum efficiency (QE) of photocathode both theoretically and experimentally.Firstly, we systematically studied the ion back bombardment in a photo-gun which was believed to be a dominant mechanism limiting the operating lifetime of the DC high-voltage photocathode guns. We established a preliminary model of the distribu-tion of the residual gases pressure within the cathode/anode gap and downstream of the anode. With this model, we deeply studied the effects of gas pressure, voltage and electric field distribution, beam emitted off-axis on the dynamic of ion and the distri-bution of ion striking the photocathode by simulation. We also preliminarily estimated the damage of ion implantation in the photocathode wafer. To suppress the ion back bombardment in a DC high-voltage photocathode gun, we gave some simple and effec-tive solutions such as reducing the gas pressure inside DC gun, improving the voltage of gun, beam emitted off-axis and positive potential barrier. We measured the QE dis-tribution of the GaAs wafer surface before and after the beam emitted off-axis which was proved to be a feasible method to suppress the ion back bombardment of the beam emitted region. The quality of electron beam would be degraded with the beam emitted off-axis which was also systematically studied. To suppress the QE decay due to ion bombardment, an electrically isolated anode was designed to repel the ion produced downstream of the anode.Then, we devoted much efforts to developing a simple and cost-effective photo-cathode quantum efficiency scanner. We generated a QE map by scanning the laser spot on the GaAs wafer surface and measuring the current flow across the photocathode. A program was wrote to control the scanning lens and data acquisition. To the best of our knowledge, this is the Chinese first photocathode QE scanner. The non-uniformity of QE distribution and the delay of QE were observed by measuring the QE distribution of GaAs photocathode in darkroom.