We described the output performance and temporal quality enhancement of the J-KAREN-P petawatt laser facility. After wavefront correction using a deformable mirror, focusing with an f/1.3 off-axis parabolic mirror delivered a peak intensity of 10²² W/cm² at 0.3 PW power levels. Technologies to improve the temporal contrast were investigated and tested. The origins of pre-pulses generated by post-pulses were identified and the elimination of most pre-pulses by removal of the post-pulses with wedged optics was achieved. A cascaded femtosecond optical parametric amplifier based on the utilization of the idler pulse rather than the signal pulse was developed for the complete elimination of the remaining pre-pulses. The orders of magnitude enhancement of the pedestal before the main pulse were obtained by using a higher surface quality of the convex mirror in the Öffner stretcher. A single plasma mirror was installed in the J-KAREN-P laser beam line for further contrast improvement of three orders of magnitude. The above developments indicate, although it has not been directly measured, the contrast can be as high as approximately 10¹⁵ up to 40 ps before the main pulse. We also showed an overview of the digital transformation (DX) of the system, enabling remote and automated operation of the J-KAREN-P laser facility.

We report on a new approach to measure the accumulated B-integral in the regenerative and multipass amplifier stages of ultrashort-pulse high-power laser systems by B-integral-induced coupling between delayed test post-pulses and the main pulse. A numerical model for such non-linear pulse coupling is presented and compared to data taken at the high-power laser Draco with self-referenced spectral interferometry (SRSI). The dependence of the B-integral accumulated in the regenerative amplifier on its operation mode enables optimization strategies for extracted energy vs. collected B-integral. The technique presented here can, in principle, be applied to characterize any type of ultrashort pulse laser system and is essential for pre-pulse reduction.

Ultra-high intensity femtosecond lasers have now become excellent scientific tools for the study of extreme material states in small-scale laboratory settings. The invention of chirped-pulse amplification (CPA) combined with titanium-doped sapphire (Ti:sapphire) crystals have enabled realization of such lasers. The pursuit of ultra-high intensity science and applications is driving worldwide development of new capabilities. A petawatt (PW = 10¹⁵ W), femtosecond (fs = 10⁻¹⁵ s), repetitive (0.1 Hz), high beam quality J-KAREN-P (Japan Kansai Advanced Relativistic ENgineering Petawatt) Ti:sapphire CPA laser has been recently constructed and used for accelerating charged particles (ions and electrons) and generating coherent and incoherent ultra-short-pulse, high-energy photon (X-ray) radiation. Ultra-high intensities of 10²² W/cm² with high temporal contrast of 10⁻¹² and a minimal number of pre-pulses on target has been demonstrated with the J-KAREN-P laser. Here, worldwide ultra-high intensity laser development is summarized, the output performance and spatiotemporal quality improvement of the J-KAREN-P laser are described, and some experimental results are briefly introduced.

We report on the design and characterization of the plasma mirror system installed on the J-KAREN-P laser at the Kansai Photon Science Institute, National Institutes for Quantum Science and Technology. The reflectivity of the single plasma mirror system exceeded 80%. In addition, the temporal contrast was improved by two orders of magnitude at 1 ps before the main pulse. Furthermore, the laser near-field spatial distribution after the plasma mirror was kept constant at plasma mirror fluence of less than 100 kJ/cm². We also present the results of investigating the difference and the fluctuation in energy, pulse width and pointing stability with and without the plasma mirror system.

Tight focusing with very small f-numbers is necessary to achieve the highest at-focus irradiances. However, tight focusing imposes strong demands on precise target positioning in-focus to achieve the highest on-target irradiance. We describe several near-infrared, visible, ultraviolet and soft and hard X-ray diagnostics employed in a ∼10²² W/cm² laser–plasma experiment. We used nearly 10 J total energy femtosecond laser pulses focused into an approximately 1.3-μm focal spot on 5–20 μm thick stainless-steel targets. We discuss the applicability of these diagnostics to determine the best in-focus target position with approximately 5 μm accuracy (i.e., around half of the short Rayleigh length) and show that several diagnostics (in particular, 3 $\omega$ reflection and on-axis hard X-rays) can ensure this accuracy. We demonstrated target positioning within several micrometers from the focus, ensuring over 80% of the ideal peak laser intensity on-target. Our approach is relatively fast (it requires 10–20 laser shots) and does not rely on the coincidence of low-power and high-power focal planes.

In this study, a detailed investigation of the dynamics of the generation of pre-pulse by post-pulses is presented, using single-shot self-referenced spectral interferometry (SRSI). The capability of SRSI in terms of the single-shot measurement of the temporal contrast of high-power laser systems has been experimentally demonstrated. The results confirm that the energy levels of the pre-pulses increase proportional to the square of the B-integral parametrizing the nonlinearity of the amplifier chain.