Features of a Phase-only Shaper Relative to a Long Rectangular Ultraviolet Pulse

The generation of a very low-emittance high peak charge electron beam in a 3-GHz radiofrequency electron gun requires a rectangular shaped electron beam of 10 ps with a rise time less than 1 ps at the cathode surface. This is obtained illuminating a photocathode with a powerful ultraviolet laser pulse of that form. A programmable pulse shaper is added into the chain of the components of a femtosecond solid-state laser system for producing such a kind of pulse. The shaping system operates as a spectral phase-only filter. A genetic algorithm, in conjunction with a proper cost-function, is used to determine the phase function to be set in the shaper for generating the square pulse matching the specifications. The simulations show that the system is very sensitive to the signal parameters and alignment. The simulations show that the output signals at the fundamental and third harmonic have differences relevant for the setting of the system.