Characterization of fast ion loss in the EHL-2 spherical torus

This study analyzes fast ion losses in the EHL-2 fusion device, focusing on both beam ions and alpha particles as p-¹¹B fusion reaction products. Using the Monte Carlo orbit-following code TGCO, we evaluate particle confinement under various operational scenarios, including co-injected tangential neutral beam injection at beam energies of 60 keV, 80 keV, and 200 keV. Our simulations estimate the heat load driven by lost beam ions and find it to be within acceptable material limits for a plasma current on the order of mega-amperes. Additionally, we simulate the distribution of fusion products and observe a higher particle loss fraction for alpha particles compared to beam ions. However, due to the relatively low fusion power, these lost alpha particles are unlikely to significantly impact the plasma-facing materials. To assess the impact of the magnetic ripple, we compute the ripple field distribution by modelling the toroidal field (TF) coils as current filaments. The results indicate that the ripple field effect on particle confinement is minimal, primarily due to the large distance of over 1 m between the TF coils and the plasma on the low-field side. The analysis based on the test particle model is a foundational step in ensuring the basic safety aspects of the new device, which is essential for developing a robust design, optimizing performance, and maintaining safe operation.