High-Precision Nonsingular Integrator of Guiding-Center Orbit Close to Magnetic Axis in Tokamak Equilibrium Configuration

Equations of guiding-center motion without the coordinate singularity at the mag- netic axis have been derived from the guiding-center Lagrangian. The poloidal magnetic flux is suggested to be included as one of the nonsingular coordinates for the computation of the guiding- center orbit. The numerical results based on different nonsingular coordinates are verified using the GCM code which is based on canonical variables. A comparison of numerical performance among these nonsingular coordinates and canonical coordinates has been carried out by check- ing the conservation of energy and toroidal canonical momentum. It is found that by using the poloidal magnetic flux in the nonsingular coordinate system, the numerical performance of non- singular coordinates can be greatly improved and is comparable to that of canonical variables in long-time simulations.