that even small modifications of the original equilibrium can lead to significantly different values of the stability parameter and can therefore strongly affect the onset and the dynamics of the mode. For example, a current corrugation of amplitude around $5\%$ of the equilibrium and with scale length comparable with the resistive layer can triple the growth rate of the tearing instability. The study presented in this paper has two clear implications for tokamak experiments.

First, it implies that the calculation of the stability parameters from experimental data is an extremely delicate procedure. Indeed, the current density profiles used to evaluate $\Delta'$ cannot be directly measured. While the magnetic field can be deduced from Motional Stark Effect measurements, the best spatial resolution achievable is around 5cm, too coarse to rule out the possibility of current corrugations. Another option is to reconstruct the current profile from temperature and density data by using equilibrium codes. However, the error bars of the measurement and the approximations used in the current reconstruction make it virtually impossible to obtain a precise value of the stability parameter.

The second implication is related to the trigger of the NTMs in a relatively quiescent plasma. Experiments in several machines 