Performance of the upgraded laser system for the Fermilab-NIU photoinjector

The laser system for the Fermilab-NIU photoinjector has been recently upgraded in order to improve reliability and to reduce amplitude fluctuations. Major modifications included the replacement of the oscillator by a diode-pumped passively mode locked Nd:YLF commercial laser. The oscillator delivers 5 ps long pulses at 81.25 MHz at an average power of 450 mW. The number of round trips in the multi-pass amplifier was reduced by half and image relaying was introduced throughout the optical system. The frequency of the IR (λ=1054 nm) pulse was quadrupled to UV (λ=263.5 nm) by two BBO crystals. The overall efficiency for frequency quadrupling was of order 20%. The shot to shot fluctuations in the UV are ∼5%. The UV energy on the cathode is 5 μJ/pulse (10 μJ/pulse without the pulse stacker), yielding charge of 10 nC/pulse.