  The effect of magnetic perturbations (MPs) on the helical self-organization
of shaped tokamak plasmas is discussed in the framework of the nonlinear 3D MHD
model. Numerical simulations performed in toroidal geometry with the
\textsc{pixie3d} code [L. Chac\'on, Phys. Plasmas {\bf 15}, 056103 (2008)] show
that $n=1$ MPs significantly affect the spontaneous quasi-periodic sawtoothing
activity of such plasmas. In particular, the mitigation of sawtooth
oscillations is induced by $m/n=1/1$ and $2/1$ MPs. These numerical findings
provide a confirmation of previous circular tokamak simulations, and are in
agreement with tokamak experiments in the RFX-mod and DIII-D devices. Sawtooth
mitigation via MPs has also been observed in reversed-field pinch simulations
and experiments. The effect of MPs on the stochastization of the edge magnetic
field is also discussed.

Plasma Physics