Design And Study Of High-performance Electron Guns

To meet the requirements of high-quality electron beam for free electron lasers (FELs), the study of high-performance electron guns has been paid extensive attentions. In this paper, three different types of electron guns for FELs have been studied.A15keV gridded high-perveance electron gun is described firstly in this thesis This gun plays a key role as the external injecting electron source of the independently-tunable-cells (ITC) RF gun, the performance of which determines the beam quality in the injector for far IR radiation source. We proposed a double-anode structure with an intermediate electrode to overcome the strong longitudinal space-charge force on the emission surface and to improve the transverse focusing, which makes the goal of high perveance and high compression ratio achieved. This double-anode structure can adjust the voltage on intermediate electrode to beam outputs with different beam current at15keV. Through the beam dynamics simulations and opitmizations of the injector, the minimum output current could be reduced from4.2A to2.45A.The double-anode scheme makes the electrodes structure compact and complex. In order to avoid the possible breakdown in vacuum and creepage on the ceramic surface, two pulsed power supplies are employed for cathode and intermediate electrode. And the grid-controlled signal is superposed on the high-voltage pulsed signals. This approach not only can reduce the risk of berakdown and creepage, but also ensures that the output beam with good rising and falling edges.Considering the output beam is low-energy and high-current, we designed a test stand to measure the quality of the beam injected to ITC RF gun at the exit of anode.The measurement system is connected to the gridded gun through a ceramic chamber with a FCT outside. The anode is fixed on the flange of the ceramic chamber. The YAG screen can be drived from the downstream vacuum chamber to the position close to the anode. Besides, three beam current monitors were used in the power circuits to measure the values of the current emitted from the cathode and beam current lost on the electrodes, which gives a reference for gun tuning.The500kV high-voltage pulsed electron gun system is introduced in the second part of the thesis. In order to obtain the3A beam with low intial emittance, we used monocrystal CeB6as the cathode material basing on its excellent emission ability and anti-poisoning characteristic.The diameter of the cathode is3mm.The electrodes of the gun were optimized on the basis of gun simulations.We also designed an emittance measurement line using double-slit method. In addition, we finished the physical design of a pre-injector for XFEL and testified the possibility of using the500kV gun as the electron source for XFEL.The Schottky-like effect in photocathode RF gun is studied in the third part of the thesis. Considering the discrepancies between measured extracted charge and simulated results with Schottky-like effect from ASTRA code, PARMELA code was applied as benchmark. Since PARMELA cannot be used to simulate Schottky-like effect with its own modules, MATLAB scripts have been developed to implement this feature. The simulation results fit the experimental data well.Finally, the finished work has been summarized. The suggestions of problems to be solved in the next step and corresponding solutions are provided.