Negligible Kinetic Helicity Can Drive Large Scale Dynamos

Turbulent helical velocities drive large scale magnetic field growth and steepen the small scale magnetic energy spectrum, but the minimum sufficient fractional kinetic helicity $f_{h,C}$ to do so has not been previously quantified. Using direct numerical simulations, we show that $f_{h,C}$ strongly decreases as the ratio of forcing to large scale wavenumbers $k_F/k_{min}$ increases. We also develop a simple theory that explains the simulation results. {For $k_F/k_{min}\ge6$ we find $f_{h,C}\la 5\%$, and our theory predicts that, in the asymptotic limit $k_F/k_{min}\rightarrow\infty$, $f_{h,C}\sim(k_F/k_{min})^{-5}$,} implying that very small helicity fractions strongly influence magnetic spectra for {even moderate} scale separation.