Precise Laser Energy Control for Laser-Shock Experiments at the Dynamic Compression Sector*

Very high shot-to-shot reproducibility, as well as precise control of the stress amplitude in shock-compressed materials is essential for understanding material phenomena such as structural transformations (including meting) and inelastic deformation under extreme conditions. The ~100 Joule, 351 nm (UV) laser at the Dynamic Compression Sector (DCS) located at the Advanced Photon Source at Argonne National Laboratory can produce shock stresses up to 500 GPa with high shot-to-shot reproducibility. To maintain that reproducibility while also precisely controlling the sample stress, we have developed a convenient two-stage energy tuning method that does not affect the laser pulse shape (time varying intensity) or the beam smoothness (spatial profile). The energy is adjusted by operating the laser at full energy and inserting one of nine available beam splitters (10-90% transmission in ~10% increments) in the laser output. Since the splitter is located after the final frequency tripling optics, the UV laser pulse shape is unchanged. For finer adjustment in 3% increments, the infrared pulse is attenuated inside the laser before the final amplifier. The demonstrated variation in shot-to-shot reproducibility is less than +/- 2.5 Joules (5% of the full energy) irrespective of the laser output energy. We will present a detailed description of this capability and describe recently published results – showing precise determination of phase transition and melting stress thresholds – that were only possible because of the developments described here.