On‐Target Contrast Diagnostic via Specular Reflectivity Measurement

High‐power laser contrast is challenging to measure, especially in the real target irradiation conditions. We present a convenient and relatively simple contrast diagnostic technique based on the measurement of target specular reflectivity at full laser power. The reflectivity remains high even at intensities above 1019 W/cm2 in the case of a high‐contrast prepulse‐free laser. On the contrary, the specular reflectivity drops in the case of lower contrast, due to the beam break‐up and increased absorption caused by the preformed plasma. The technique was demonstrated using three different laser systems with several contrast conditions: Astra (CLF, RAL), TiS laser at APRI, GIST, and J‐KAREN (APRC, JAEA).