Numerical study on fast particle confinement in Chinese first quasi-axisymmetric stellarator

Fast particle confinement in the Chinese first quasi-axisymmetric stellarator (CFQS) is investigated using the MEGA code, comparing the standard quasi-axisymmetric (QA) configuration with a finite-beta (〈β〉 = 0.74%) equilibrium featuring magnetic islands. In the standard QA configuration, the drift associated with the vertical magnetic curvature term, (∇ × b)_z, is identified as the dominant loss mechanism, especially for co-passing particles. In the finite-beta configuration, magnetic islands trap low-energy particles. The (∇ × b)_z drift modulates this trapping, promoting escape for co-passing particles while reinforcing trapping for counter-passing particles, and remains a significant contributor to overall losses. These findings underscore the critical role of the (∇ × b)_z drift and the added complexities of magnetic islands for energetic particle confinement in finite-beta stellarator plasmas.