Neoclassical momentum transport in a collisional stellarator and a rippled tokamak

Short mean-free path two-fluid equations are employed to evaluate the lowest order nonambipolar radial current in plasma confined by a three-dimensional toroidal magnetic field. The result is used to obtain a necessary condition for intrinsic ambipolarity of plasma transport in such a field and to derive a criterion for the importance of the toroidal field ripple for collisional tokamak plasma rotation. The ripple effects on toroidal plasma rotation are found to be negligible if the characteristic perpendicular length scale is determined by the pedestal width of order the poloidal ion gyroradius (as may be the case in the H-mode regime), but may conceivably become important for more shallow plasma gradients. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3104715]