Fast focal zooming is an indispensable key technology in direct-drive laser fusion, playing an irreplaceable role in maintaining irradiation uniformity, suppressing cross beam energy transfer (CBET), and enhancing implosion stability. Currently, no ideal fast focal zooming technical solution is available. This paper innovatively proposes a 3ω fast focal zooming scheme based on photochemical reactions, which utilizes the refractive index difference of the working medium before and after photochemical reactions, converted into focal spot movement through optical path design. The working medium must be efficiently photodissociated by 2ω laser light, while both reactants and products remain transparent to 3ω laser light. Taking iodine vapor as an example, this work establishes kinetic equations incorporating absorption, photodissociation and recombination processes. By solving these equations, the variation patterns of the medium's refractive index during the reaction are obtained. Based on these findings, a novel fast focal zooming scheme suitable for direct-drive laser fusion is designed through finite element simulations and parameter optimization. This new scheme performs zooming on a single laser beam and operates at the 3ω stage, thus leaving the laser output capability unaffected. The scheme is capable of repetitive operation, making it suitable for future fusion energy applications. It requires no additional energy injection, instead utilizing the inherent 2ω laser light to induce photochemical reactions. The scheme possesses self-triggering characteristics, eliminating the need for external trigger signals. As an additional benefit, the proposed scheme not only addresses the fast focal zooming problem but also adaptively improves near-field beam uniformity, achieving two goals with one approach. In summary, this paper provides a comprehensive solution for fast focal zooming in direct-drive laser fusion.

focal zooming,direct-drive,photochemical reactions