Reducing laser plasma instability in inertial confinement fusion by polarization smoothing based on stress birefringence

Polarization smoothing is one of the effective means to reduce laser plasma instability (LPI) in inertial confinement fusion (ICF). However, with the increase in laser power, there is currently a lack of stable single-beam polarization smoothing methods suitable for engineering applications. In our previous work, we proposed a polarization smoothing idea based on stress birefringence of edge loading fused silica window, but two issues were left unresolved: determination of the optimal edge loads distribution and design of the loading structure within limited available space. In this article, a method for determining the optimal loads combining continuous load discretization processing and genetic algorithm is proposed. And guided by the optimized results, we propose a pre-tightening loading method based on the reaction force of deflection deformation of special-shaped beams. Enabling the loading structure to provide the optimal edge loads for the stressed optical window while being thin enough to allow the element to be installed into ICF final optics assembly. We conducted tests on the element in the ultraviolet (351 nm wavelength) section of a high-power laser system, with a laser energy of 2kJ/3 ns. The results showed that compared to the situation without polarization smoothing, the backscattered energy decreased by 50∼60%, proving the effectiveness of this polarization smoothing scheme in reducing LPI.