The generalized offset toroidal rotation for total neoclassical toroidal viscosity torque in a tokamak*

For each ion species, its contribution to neoclassical toroidal viscosity (NTV) torque vanishes at the conventional offset rotation for that species. In general, the amplitude of NTV torque of each species can become comparable, thus the total NTV torque can be quite different from those of single species alone. Thus a generalized offset toroidal rotation for total NTV torque may exist for which the total NTV torque vanishes, defining a natural relaxation level of toroidal rotation above/below which the total NTV torque tends to brake/accelerate the toroidal flow. In this work, we evaluate the generalized offset toroidal rotation for total NTV torque and its dependence on plasma parameters using a coupling scheme between NIMROD and NTVTOK codes developed in our previous work. We find that the generalized offset toroidal rotation for total NTV torque can deviate significantly from those determined from single species contributions, especially in low collisionality and slow rotation regimes where ion and electron contributions are often comparable and opposite.