  The effect of the perturbed ion polarization current on the stability of
neoclassical tearing modes is calculated using an improved, neoclassical,
four-field, drift-MHD model. The calculation involves the self-consistent
determination of the pressure and scalar electric potential profiles in the
vicinity of the associated magnetic island chain, which allows the chain's
propagation velocity to be fixed. Two regimes are considered. First, a regime
in which neoclassical ion poloidal flow damping is not strong enough to enhance
the magnitude of the polarization current (relative to that found in slab
geometry). Second, a regime in which neoclassical ion poloidal flow damping is
strong enough to significantly enhance the magnitude of the polarization
current. In both regimes, two types of solution are considered. First, a freely
rotating solution (i.e., an island chain that is not interacting with a static,
resonant, magnetic perturbation). Second, a locked solution (i.e., an island
chain that has been brought to rest in the laboratory frame via interaction
with a static, resonant, magnetic perturbation). In all cases, the polarization
current is found to be either always stabilizing, or stabilizing provided that
eta_i = d ln T_i/d ln n_e does not exceed some threshold value. In certain
ranges of eta_i, the polarization current is found to have have a stabilizing
effect on a freely rotating island, but a destabilizing effect on a
corresponding locked island.
