Study on the mechanism of shock wave signal attenuation in wide-angle VISAR diagnostics

The wide-angle Velocity Interferometer System for Any Reflector (VISAR) is an important shock-wave diagnostic technique in inertial confinement fusion (ICF), enabling measurements of shock velocity and wavefront evolution over a wide range during implosion. However, its diagnostic range and duration can be limited by signal attenuation caused by the shock front tilt, the ionization of window materials caused by X-rays and shock waves, and the decreased reflectivity of the ellipsoidal mirror due to preheating. This study investigates the attenuation mechanisms of the wide-angle VISAR diagnostic signal associated with the above factors and establishes a normalized intensity model for the diagnostic signal. Simulation results indicate that wide-angle VISAR is highly sensitive to shock front tilt, while the thicknesses of the ablator and shielding layers, as well as the radiation temperature, are key parameters affecting window material ionization and ellipsoidal mirror reflectivity. A combined analysis of indirect-drive experimental results from a 10 kJ laser facility and the normalized intensity model reveals that, under the current target design and drive conditions, X-ray ionization effects can cause approximately 20% signal loss, the shock front reflectivity can reach 0.4, and the ellipsoidal mirror reflectivity is about 0.83. The contributions of X-ray ionization and changes in ellipsoidal mirror reflectivity to signal attenuation are limited, whereas shock front tilt is identified as the primary cause for the disappearance and subsequent reappearance of wide-angle VISAR signals. This study helps clarify the physical boundaries for shock wave diagnosis using wide-angle VISAR, and provides a basis for optimizing future target designs and achieving long-duration, wide-range shock wave diagnostics.