Production of picosecond square-shaped ultraviolet pulses for low-emittance electron bunch generation from an RF photocathode

We demonstrated generation of square-shaped ultraviolet pulses and that they reduces the emittance of electron beams from laser-photocathode RF-gun. Some frequency components of femtosecond pulses from a Ti:sapphire oscillator are modulated with a pulse shaper that consists of a lens pair, gratings, and a liquid crystal spatial light modulator (LC-SLM) to generate square-shaped pulses. The shaped pulses were amplified up to 3 mJ through a regenerative amplifier, and were converted to ultraviolet region with two different nonlinear crystals. Energy of the shaped pulses was about 100 μJ which is sufficient to generate electron charge of 1 nC. In our experiment, emittance of electron beam was reduced to as low as almost the half of that with non-shaped pulses.