Optimized design and analytical modeling of a differentially pumped windowless gas target for low-energy nuclear cross-section measurements in the 200 keV Cockcroft-Walton accelerator

• We believe this manuscript is an excellent fit for the scope of Results in Engineering, which welcomes concise and complete accounts of novel experimental and theoretical results across all areas of physics. Our contribution is significant for several reasons: • Interdisciplinary Methodology: It provides a clear, step-by-step framework that bridges vacuum science, computational physics, and nuclear instrumentation, offering a valuable reference for experimentalists • Validated Results: The close agreement (∼15%) between our analytical model and sophisticated COMSOL/TPMC simulations provides high confidence in the proposed design parameters. • Practical Utility: The design is directly applicable to upgrading existing Cockcroft-Walton facilities worldwide, enabling precise low-energy cross-section measurements without the drawbacks of windowed targets. • Generalizable Approach: The methodology is explicitly extensible to different gases, number of stages, and beam energies, making it a versatile tool for the broader community. An optimized, validated methodology for windowless differentially pumped gas targets is presented, combining analytical multi-regime conductance modeling, constrained Sequential least-square quadratic programming (SLSQP) optimization, Stopping and Range of Ions in Matter (SRIM) beam-transport and range simulations, COMSOL Multiphysics validation and Test Particle Monte Carlo (TPMC) simulations in Molflow. For deuterium, a three-stage Differential Pumping System (DPS) with pumps of 472, 400, and 350 L/s and ≥ 1 cm ID pipes (total length 175 cm) maintains a target pressure of 7.5 mbar while achieving 7[MULSGN] 10 -6 mbar vacuum pressure in the final stage. SRIM indicates a target chamber of 45 cm length and 10 cm diameter for full stopping of 200 keV ions. COMSOL results agree with the analytical model within ∼ 15% and TPMC simulation demonstrates that molecular beaming does not compromise downstream vacuum performance in the context of DPS parameters estimation. The approach is readily extensible to additional stages and alternative gases and is compatible with integration into a 200 keV Cockcroft–Walton linear accelerator.