  Numerical simulations of the magnetorotational instability (MRI) with zero
initial net flux in a non-stratified isothermal cubic domain are used to
demonstrate the importance of magnetic boundary conditions.In fully periodic
systems the level of turbulence generated by the MRI strongly decreases as the
magnetic Prandtl number (Pm), which is the ratio of kinematic viscosity and
magnetic diffusion, is decreased. No MRI or dynamo action below Pm=1 is found,
agreeing with earlier investigations. Using vertical field conditions, which
allow the generation of a net toroidal flux and magnetic helicity fluxes out of
the system, the MRI is found to be excited in the range 0.1 < Pm < 10, and that
the saturation level is independent of Pm. In the vertical field runs strong
mean-field dynamo develops and helps to sustain the MRI.
