  Using direct numerical simulations (DNS) and mean-field simulations (MFS),
the effects of non-uniformity of the magnetic field on the suppression of the
turbulent pressure is investigated. This suppression of turbulent pressure can
lead to an instability which, in turn, makes the mean magnetic field even more
non-uniform. This large-scale instability is caused by a resulting negative
contribution of small-scale turbulence to the effective (mean-field) magnetic
pressure. We show that enhanced mean current density increases the suppression
of the turbulent pressure. The instability leads to magnetic flux
concentrations in which the normalized effective mean-field pressure is reduced
to a certain value at all depths within a structure.
