On collisionality dependence of the neoclassical tearing modes

The problem of validity of the semi-phenomenological model of collisionalit y dependence of the polarization current contribution into the generalized Rutherford equation for neoclassical tearing modes proposed by Wilson H R e t al (1996 Plasma Phys. Control. Fusion 38 A149) and later discussed and an alysed by Maraschek M et al (1999 Plasma Phys. Control. Fusion 41 L1) is co nsidered. It is shown that the collisionality dependence of the polarizatio n current in this model contradicts that in the theory of linear MHD instab ilities; Because of this, the above model is, to some extent, ad hoc. Since this model cannot be substantiated by any theory developed up until now, i t should be used with care for evaluation of the plasma behaviour in ITER. The discussed disagreement between experiments, for which interpretation of this model was used, and a recent theory of magnetic islands may serve as a stimulus for further development of the theory.