STRETCH-TWIST-FOLD AND ABC NONLINEAR DYNAMOS - RESTRICTED CHAOS

We present direct numerical simulations for nonlinear dynamos. based o n a Lagrangian approach that allows us to compute for relatively high effective magnetic Reynolds numbers, R-m similar to(1-3)X10(4). The pa rticular systems we study and contrast are the stretch-twist-fold (STF ) and the ABC flow dynamos. In the case of the STF dynamo, we show tha t whereas small-scale magnetic fluctuations are suppressed in the nonl inear regime, they still remain sufficiently large so that the STF dyn amo still cannot be considered (in this nonlinear regime) a paradigm f or a fast dynamo. Our numerical study of the ABC flow dynamo indicates , first, that during the period of kinematic behavior, there is no gro wth of a large-scale magnetic field, and that any large-scale field co mponents are subject to classical turbulent diffusion; second, we show that if back reactions (due to magnetic tension) are taken into accou nt this diffusion is highly restricted. We refer to this behavior as ' 'restricted chaos.''