a lower threshold for both the critical $\zeta$ and $\delta\Delta'$. An interesting discussion of the stochastic behavior of Rutherford equation was given by Itoh *et al.* in Ref.[@Itoh2004]. Furthermore, the calculation presented here is conservative and the critical values obtained are very stringent, since we have assumed an initial condition for Eq.\[13\] with no magnetic island. In reality, the presence of a small, but finite seed island island \[$w(0)<w_{seed}$\] would move the stability boundaries in Fig.\[fig8\], broadening the unstable region 3.

Conclusions
===========

We have investigated the effect of current corrugations on the stability of the tearing mode. We have obtained a theoretical scaling which relates the change of $\Delta'$ to the characteristics of the corrugation, such as its amplitude, typical scale length and phase with respect to the resonant surface. The scaling has been tested on a simple slab linear problem, giving excellent agreement between theoretical predictions and numerical data. Then, by using a shooting code, we have investigated the effect of the corrugations in cylindrical geometry, and found qualitative and quantitative agreement with the analytic scaling. Finally, we have addressed the problem of the seedless trigger of a NTM through a current corrugation.

From our results we conclude 