Investigating Guiding-Center Versus Full Orbit Effects with ORBIT-GPU

ORBIT is a particle-pushing code that uses Hamiltonian mechanics to calculate the Lorentz force on particles in magnetic confinement devices. ORBIT acts on particle guiding centers and ignores gyromotion. Given magnetic equilibria and perturbations, ORBIT can analyze particle transport, characterize wave resonant interactions, and produce Poincaré maps, all of which are useful for investigating energetic ions. Recent leaps in graphical processing unit (GPU)-enabled parallel processing such as compute unified device architecture have motivated ORBIT's adoption into a GPU-optimized format. One important performance factor is the speed of simulating many particles when analyzing distributional effects. Since the magnetic field strength drops off inversely to the major radius in a tokamak, the resulting Larmor radius of a given particle will increase as it nears the outer walls. While the guiding center approximation has been assumed appropriate, differences in the guiding center vs. full orbit representation may have noticeable effects on fast ion position and losses. This may be more pronounced in low aspect ratio tokamaks where the magnetic field gradient is steeper. Such effects are examined for energetic particles within National Spherical Torus Experiment – Upgrade discharges.