Laser system

The intense laser pulses for accelerating the electron beam and measuring the electron pulse width and timing jitter were obtained by splitting a laser pulse generated by a Ti:sapphire CPA laser system^47,48. This system comprises three multi-pass amplification stages. The pulses from the oscillator (Vitara-T HP, Coherent) were amplified with an eight-pass pre-amplifier and two five-pass power amplifiers. After four passes in the pre-amplifier, a nanosecond Pockels cell system (Model 5046SC, FastPulse Technology) was used to chop an 80-MHz pulse train from the oscillator. After the pre-amplifier, a sub-nanosecond Pockels cell system (UPC-088, Leysop) was used to reduce the nanosecond ASE. The laser intensity contrast ratio was approximately 10⁻⁶ at 2 ps before the peak of the main laser pulse and approximately 10⁻⁷ for the nanosecond ASE, whereupon the input power into the pre-amplifier was 340 mW. To adjust the pulse contrast ratio, the pulse energy of the chirped pulses after the pulse stretcher was attenuated using neutral density filters. The output energy from the power amplifiers was kept constant, and only the gain for the ASE was changed. In the present experiments, the input power into the pre-amplifier with the neutral density filters was 30 or 110 mW. The input energies of 30 and 110 mW correspond to changing the contrast ratio by 11-fold and 3.1-fold compared with the contrast ratio at an input energy of 340 mW. A plasma mirror with a reflectivity of 10⁻² was used to reduce the ASE level. The plasma mirror reduced the ASE level from 10⁻⁶ to 10⁻⁸ at 2 ps before the peak and from 10⁻⁷ to 10⁻⁹ for the nanosecond ASE.

The laser pulse from the compressor with a diameter of 50 mm was split in half by a gold mirror inside the vacuum chamber. Each laser pulse fired at the target foil and at the compression point was focused using an off-axis parabolic mirror with a focal length of 165 mm. The pointing stability of the laser pulse, which is critical for the timing jitter owing to its influence on the path length of the laser pulse and electron pulse, was sufficiently high. The RMS pointing stability was within 1.5 μrad in 1 h, corresponding to 0.25 μm in the focal plane.