Understanding of neoclassical offset rotation based on DIII-D experiments

Abstract

Neoclassical offset rotation induced by non-axisymmetric magnetic perturbations in tokamaks is investigated using NTVTOK model based on plasma profiles in one DIII-D discharge. The calculated counter-Ip (Ip indicates plasma current) ion root of neoclassical offset rotation is found to be consistent with DIII-D experimental observations. The modeling results predict that this DIII-D plasma regime is close to the marginal condition for the co-Ip electron root to exist. The importance of bounce–drift resonance is highlighted in the calculation, which affects the neoclassical offset rotation, especially the electron root. The ion root usually exists for various parameter regimes, while the electron root is only possible in low collisionality (e.g., high temperature and/or low density) regimes. The magnetic perturbation spectrum is found to influence the existence of electron roots when electrons are closer to resonant superbanana plateau regime than ions. By adjusting the plasma collisionality and tuning the spectrum of magnetic perturbations, it is possible to control the plasma rotation and hence to optimize the plasma confinement.