Not Much Helicity is Needed to Drive Large Scale Dynamos
  Understanding the in situ amplification of large scale magnetic fields in
turbulent astrophysical rotators has been a core subject of dynamo theory. When
turbulent velocities are helical, large scale dynamos that substantially
amplify fields on scales that exceed the turbulent forcing scale arise, but the
minimum sufficient fractional kinetic helicity f_h,C has not been previously
well quantified. Using direct numerical simulations for a simple helical
dynamo, we show that f_h,C decreases as the ratio of forcing to large scale
wave numbers k_F/k_min increases. From the condition that a large scale helical
dynamo must overcome the backreaction from any non-helical field on the large
scales, we develop a theory that can explain the simulations. For k_F/k_min>8
we find f_h,C< 3%, implying that very small helicity fractions strongly
influence magnetic spectra for even moderate scale separation.
