Exploratory Research On Realization Of Ultra-low Emittance In The Photocathode RF Gun

The exploration on elementary particles and the craving for knowledge of the structure of matter at ultra-fast and ultra-small scales drive the development of modern large-scale electron linear accelerator applications.In order to meet the requirements of electron source in these applications,the photocathode RF electron gun and photoinjector are usually employed to provide electron beam with low emittance and high peak current.After the electron beam is generated from the photocathode surface,the beam emittance will increase during subsequent acceleration,focusing and bunching.Further reduction of the intrinsic emittance and suppression of the emittance growth have been the hotspots of recent research.The main purpose of this dissertation is to exploratory study the possible methods to generate beams with ultra-low emittance in the photoinjector.We firstly study the influence of the cathode surface condition on the thermal emittance,then propose and verify a new method to suppress the emittance growth caused by the non-linear effects in the photoinjector.The real-life cathode surface is gently undulating,the undulations on the surface will not only cause the photoelectron emission direction dispersion(slope effect),but also produce transverse electric field component(field effect),which lead to the thermal emittance growth.In order to predict thermal emittance growth caused by the surface roughness,we introduce the point spread function to calculate the slope effect,and evaluate the field effect by applying the approximate electric field distribution formula for arbitrary gently undulating surface.The roughness emittance growth analytical formula for the real-life cathode surface has been derived,and the numerical simulation has been performed.The analytical and simulation results show that the growth of the roughness emittance is negligible for a finely machined photocathode.A nano-patterned metal photocathode can be excited by the incident laser and pro-duces the surface plasmon polaritons which could significantly enhance the photon ab-sorption of the cathode.The emission characteristics of the nano-patterned cathode have been investigated experimentally.Firstly we optimized the structure parameters of a sil-ver nano-patterned photocathode by numerical simulations,then performed the off-line reflectivity spectrum measurement and finally the high power test was carried out.The experimental results show that the silver nano-patterned cathode could work under the infrared laser and provide more than 400 times charge yield gain compared to the flat cathode case,however the normalized thermal emittance is almost doubled compared to the flat copper cathode as a trade-off.Therefore the nano-patterned cathode might be more suitable for applications which require high charge but not ultra-low emittance.In order to suppress the emittance growth caused by the nonlinear force,pancake beam and pencil beam emission regime are generally employed.We analyze the thermal emittance limit of both emission conditions and show that the thermal emittance limit of the pencil beam regime is slightly better.To overcome the RF emittance growth caused by the long initial length and the relatively low peak current of the pencil beam,the concept of the spatially separated two frequency RF gun was proposed and integrated into the photoinjector.The beamline parameters of the photoinjector are then optimized by a genetic algorithm optimizer.The optimization results show that by introducing a HOM cavity into the photoinjector,the emittance could be reduced by 25%with the bunch charge of 200 pC while keeping the peak current of 30 A at the exit of the photoinjector.If the normalized thermal emittance could be further reduced to 0.5 ?m/mm,a optimization point with peak current of 30 A and slice emittance of 0.1 mm · mrad at the photoinjector exit could be achieved.