A high average-current electron source for the Jefferson Laboratory free electron laser

摘要

The spectral output power from the Jefferson Laboratory infra-red free electron laser is primarily limited by the performance of the electron injector. Free electron laser power is directly proportional to the electron beam current and at present the electron injector is limited to 10mA average current. To date the highest laser power achieved has been 14.2kW and the next goal is to reach 100kW. For this to occur a new electron injector has been designed that is capable of producing over 100mA average current. This thesis describes an investigation into the behaviour of this injector through simulation. Given that the layout of the injector is fixed, this thesis aims to find suitable operating regimes for various electron bunch charge scenarios. By determining the important features the electron beam must have at the exit of the injector, and the limitations of each component, this information was used to form an optimisation problem that could be solved to find the best operation point. To improve the simulation of electron bunches being launched from a photocathode, measurements were performed on a similar injector to evaluate the thermal energy and response time of the cathode. These values are a function of the laser wavelength used with the photocathode and so were repeated over a range of wavelengths from infra-red to green. The injector at Cornell University was used to take measurements of the electron beam that could then be compared against simulation to benchmark the code. The brightness and quality of electron beams in linac-based light sources, such as at Jefferson Laboratory, are limited by the properties of the beam in the injector. It is therefore important to have knowledge of the phase space distribution of the electron beam in addition to the rms emittance, to provide an insight into high brightness formation mechanisms. A tomography technique has been successfully used to reconstruct the transverse phase space of the electron beam delivered from the Cornell University ERL DC gun. The gun is similar to that in the 100mA JLab injector, therefore a tomography diagnostic could in future be applied to that case.