Optimization and application of shock wave measurement technology for shock-timing experiments on small-scale capsules

Abstract

In laser fusion research, the precision of shock-timing technology is pivotal for attaining optimal adiabatic tuning during the compression phase of fusion capsules, which is crucial for ensuring the high-performance implosion. The current main technological approach for shock-timing experiments is to use keyhole targets and VISAR (velocity interferometer system for any reflector) diagnostics to measure the shock velocity history. Nonetheless, this approach encounters limitations when scaling down to smaller capsules, primarily due to the reduced effective reflection area available for VISAR diagnostics. In this work, a novel high-precision shock-timing experimental methodology is used to realize a double-step radiation-driven implosion of a 0.375 mm radius capsule on a 100 kJ laser facility. By calculating the intensity of VISAR images with spherical reflective surfaces, a new experimental technical route is proposed, i.e. using the keyhole cone reflection effect to enhance the VISAR diagnostic spatial area, which can effectively increase the effective data collection region by nearly threefold for small-scale capsules. The technique has been adeptly used to measure shock waves in cryogenic liquid-deuterium-filled capsules under shaped implosion experimental conditions, thus obtaining high-precision shock-timing experimental data. The experimental data reveal that the application of this technology can markedly enhance both the image quality and the precision of data analysis for shock wave velocity measurements in small-scale capsules. Furthermore, it is discovered that under similar laser conditions, there exist considerable variations in the shock velocity profiles. Simulation analysis shows that the difference in chasing behavior of the “<i>N</i>+1” reflected shock wave caused by small changes in laser intensity is the main reason for the significant difference in merging speed. It is demonstrated that small changes in laser parameters can significantly affect the transmission behavior of the shock wave. This experiment highlights the complex sensitivity of shock wave transmission in high-performance forming implosion physics process on a current small capsule scale, making it essential to conduct shock-timing experiments to accurately adjust actual shock wave behavior. This research not only lays a robust technical foundation for promoting adiabatic tuning experiments ofour 100 kJ laser facility but also has profound significance for the ultra-high pressure physics research based on the spherical convergence effect.