Ultra-thin liquid sheets in laser plasma accelerators: theory, technology, and application frontiers

The interaction of ultrashort, ultra-intense lasers with thin-film targets represents a frontier in modern physics, providing a pathway to high-brightness radiation spanning from terahertz to gamma-rays, alongside high-energy protons and heavy ions. These novel radiation and particle sources, distinguished by their compact footprint, femtosecond-scale pulse duration, and unprecedented transient flux, hold significant potential across diverse applications. In oncology, for instance, the unique synergy of the proton Bragg peak with the ultra-high dose-rate FLASH effect promises a paradigm shift in cancer therapy, enabling more effective tumor cell destruction while minimizing collateral damage to healthy tissue. However, the transition from single-shot, proof-of-concept experiments to real-world applications is constrained by the limited ion numbers and radiation flux per pulse. Therefore, increasing the repetition rate of laser-target interactions is the principal bottleneck hindering practical deployment.This bottleneck is largely attributable to the limitations of traditional solid targets. Their inefficient preparation, limited switching speed, and catastrophic debris generation render them fundamentally incompatible with high-repetition-rate, continuous operation. In response, free-flowing, self-supporting liquid sheet targets emerge as a transformative solution, offering a continuously replenished, debris-free, and vacuum-compatible medium for laser interaction. Consequently, the development of a robust liquid target technology platform is critically important for advancing the entire field of laser-driven applications.This paper confronts these challenges by establishing a comprehensive research framework integrating advanced characterization, theoretical modeling, and application development for ultra-thin liquid sheet targets. A primary contribution is the development of a complete in-situ, online characterization system, specifically engineered to overcome the formidable difficulty of measuring sub-micron film dynamics within a vacuum environment. This system enables real-time, high-precision monitoring of critical parameters—including thickness, flatness, and spatial stability—which are essential for stable laser-plasma interaction. This diagnostic capability underscored the critical link between precise thickness control and the shot-to-shot reproducibility of the accelerator output, thus providing an indispensable foundational tool for optimizing particle source reliability.Building upon this diagnostic foundation, the paper advances the theoretical understanding and fabrication of these targets. A novel ternary liquid phase methodology was conceptualized and implemented to systematically decouple the complex interplay between viscosity and surface tension during liquid sheet formation. Extensive analysis revealed that surface tension primarily governs the global sheet geometry, whereas viscosity dominantly controls the spatial thickness distribution. Leveraging fundamental insights from radial momentum transfer and angular mass transport dynamics, the first predictive viscous collision model was established. This theoretical framework, combined with the ternary method, enabled the deterministic fabrication of ultra-thin liquid sheets, achieving internationally competitive sub-micron thicknesses identified as optimal for current laser ion acceleration applications.Finally, the paper pivots to demonstrate the practical utility of these liquid sheet targets as next-generation, micro-focus X-ray sources. A dedicated, vacuum-compatible femtosecond laser platform was constructed, a non-trivial engineering feat incorporating advanced differential pumping and sophisticated thermal management systems to ensure liquid sheet survival and stability. Experimental campaigns demonstrated continuous 10 Hz operation for over one hour with mJ-level lasers, generating a stable flux of high-energy hard X-rays with high photon yields and sub-20 μm focal spot sizes. These findings establish a viable technological pathway for developing low-cost, high-brightness X-ray sources, critically bridging the significant performance gap between conventional X-ray tubes and large-scale synchrotron radiation facilities.