Numerical analysis on the characteristics of particle orbits in quasi-axisymmetric stellarator

Based on the magnetic field configuration of the Chinese First Quasi-axisymmetric Stellarator (CFQS) device, three types of orbits such as the passing orbit, blocked trapped orbit, and localized trapped orbit are simulated using the Boris algorithm. Also, the orbital topology, orbit transition, and loss characteristics of these particles under different initial conditions are studied in this paper. It is found that there exists a transition from blocked trapped orbits to localized trapped orbits due to small continuous helical ripples. This phenomenon is analyzed as follows: we define the angle between the particle drift velocity (v→D) and the radial direction (ρ^) as β. If the blocked-localized transition appears in the region where β>90○, the localized particles will return to the blocked particles and be constrained. However, if the blocked-localized transition happens in the region where β<90○, localized particles will drift out of the last closed flux surface and be lost. The simulation results show that the loss of localized particles in the CFQS is mainly located in some specific regions within the vicinity of the toroidal angle φ=0○,180○ and the poloidal angle θ=90○.