Highly-efficient 20 TW Ti:sapphire laser system using optimized diverging beams for laser wakefield acceleration experiments

• Developed a compact and highly efficient 20 TW/40 fs laser system.
• Investigated the thermal lensing effect with high-peak pumping power.
• Achieved optimization of the beam divergence to compensate the thermal lensing effect.
• High conversion efficiency from the pump energy to laser output was achieved.
• Generated stable quasi-monoenergetic e-beams with energies up to 150 MeV.

We developed a highly efficient and compact Ti:sapphire multipass amplifier that uses divergent seed beams to generate 20 TW/40 fs laser pulses with a repetition rate of 10 Hz. The laser system consists of a mode-locked oscillator, a regenerative amplifier, and a multipass power amplifier. The thermal lensing effect is very important in this system, especially in the multipass amplifier, as it limits the conversion efficiency. In order to compensate the thermal lensing effect, we calculated the optimum divergence of the seed beam and used the result for the multipass amplifier, where the thermal focusing is taken into account. In this way, we achieved a very high conversion efficiency of 41%, which is close to the theoretical limit. The laser system was then used with a capillary gas cell to generate stable high-energy electron beams with electron energies of about 150 MeV and a beam divergence of 4 ± 1 mrad. In this paper, details of the laser system development and experimental results for electron generation are presented.