Inverse cascades and α-effect at low magnetic Prandtl number

Dynamo action in a fully helical Beltrami (ABC) flow is studied using both direct numerical simulations and subgrid modeling. Sufficient scale separation is given in order to allow for largescale magnetic energy build-up. Growth of magnetic energy obtains down to a magnetic Prandtl number PM = RM/RV close to 0.005, where RV and RM are the kinetic and magnetic Reynolds numbers. The critical magnetic Reynolds number for dynamo action R M seems to saturate at values close to 20. Detailed studies of the dependence of the amplitude of the saturated magnetic energy with PM are presented. In order to decrease PM , numerical experiments are conducted with either RV or RM kept constant. In the former case, the ratio of magnetic to kinetic energy saturates to a value slightly below unity as PM decreases. Examination of energy spectra and structures in real space both reveal that quenching of the velocity by the large-scale magnetic field takes place, with an inverse cascade of magnetic helicity and a force-free field at large scale in the saturated regime.