Dynamics of energetic ion orbits in magnetically confined plasmas

The problem of the collisional evolution of fast-ion guiding centre orbits in magnetically confined plasmas is analysed in great detail. Regimes where the radial excursion of a particle orbit from a given magnetic surface is large compared with its minimum distance from the magnetic axis are specifically addressed. These non-standard orbits give rise to new interesting effects. For instance, the time scale for precessional and bounce motions become comparable, which may have important consequences for plasma stability. Inward orbit drifts and diamagnetic current effects are amplified in the nonstandard orbit regime. The neo-classical bootstrap current does not vanish on the magnetic axis, opening the possibility of entirely non-inductive tokamak operation. Non-standard fast ion orbits could also significantly affect the power deposition profile of waves in the ion cyclotron range of frequencies launched into the plasma for heating purposes. The analysis in this paper is based on a combination of analytical and numerical methods for the solution of the relevant Hamiltonian dynamical system and associated Fokker-Planck equation.