Helical dynamo growth at modest versus extreme magnetic Reynolds numbers

Understanding large-scale magnetic field growth in astrophysical objects is a persistent challenge. We tackle the long-standing question of how much helical large-scale dynamo growth occurs independent of the magnetic Reynolds number (Rm) in a closed volume. From modest-Rm numerical simulations, we identify a pre-saturation regime when the large-scale field grows independently of Rm, but to an Rm-dependent magnitude. For plausible magnetic spectra however, the analysis predicts the magnitude to be Rm-independent and substantial as Rm$\to \mathrm{\infty }$. This gives renewed optimism for the relevance of closed dynamos and pinpoints how modest Rm and hyper-diffusive simulations can cause misapprehension of the Rm$\to \mathrm{\infty }$ behavior.