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Understanding the in situ amplification of large scale magnetic fields inturbulent astrophysical rotators has been a core subject of dynamo theory. Whenturbulent velocities are helical, large scale dynamos that substantiallyamplify fields on scales that exceed the turbulent forcing scale arise, but theminimum sufficient fractional kinetic helicity f_h,C has not been previouslywell quantified. Using direct numerical simulations for a simple helicaldynamo, we show that f_h,C decreases as the ratio of forcing to large scalewave numbers k_F/k_min increases. From the condition that a large scale helicaldynamo must overcome the backreaction from any non-helical field on the largescales, we develop a theory that can explain the simulations. For k_F/k_min>8we find f_h,C< 3%, implying that very small helicity fractions stronglyinfluence magnetic spectra for even moderate scale separation.

Not Much Helicity is Needed to Drive Large Scale Dynamos