Analytic model for the propagation of a collisionless neutral beam

This paper introduces an analytical model for the propagation of collisionless neutral particles in neutral beam injection (NBI) systems. The model incorporates a novel approach using composite Gaussian basis functions to represent non-Gaussian source distributions and extends to two-dimensional source configurations under orthogonal separability assumptions. The method efficiently computes particle velocity and spatial distributions along beam trajectories, accounting for truncation effects due to transmission losses. The model has been implemented as a computational module in the Minerva framework and interfaced with the ITER Integrated Modelling & Analysis Suite (IMAS). A case study of the MAST Upgrade NBI system demonstrates the model's ability to predict particle distributions from the source grid to the plasma cavity while accommodating detailed baffle geometries and calculating transmission factors. Comparisons reveal that reduced Gaussian basis representations can achieve an order-of-magnitude reduction in computational time with negligible impact on accuracy. The proposed model provides a fast and rigorous alternative to Monte Carlo simulations, enabling enhanced diagnostic modelling and efficient integration with Bayesian inference frameworks.