Generation and delivery of an ultraviolet laser beam for the RF-photoinjector of the AWAKE electron beam

In the AWAKE experiment, the electron beam is used to probe the proton-driven wakefield acceleration in plasma. Electron bunches are produced using an rf-gun equipped with a Cs$_2$Te photocathode illuminated by an ultraviolet (UV) laser pulse. To generate the UV laser beam a fraction of the infrared (IR) laser beam used for production of rubidium plasma is extracted from the laser system, time-compressed to a picosecond scale and frequency tripled using nonlinear crystals. The optical line for transporting the laser beam over the 24 m distance was built using rigid supports for mirrors and air-evacuated tube to minimize beam-pointing instabilities. Construction of the UV beam optical system enables appropriate beam shaping and control of its size and position on the cathode, as well as time delay with respect to the IR pulse seeding the plasma wakefield.