  Equations describing plasma equilibria are derived from the total energy of
the system. The MHD equilibrium is shown to hold even for systems where the
magnetic field may locally vanish. Through conservation of helicity,
confinement is shown to be intimately related to electromagnetic tension. For
an axisymmetric device, the presence of a magnetic field or current circling
the axis is shown to be destabilising. This provides a simple explanation for
the poor performance of some devices for which MHD predicts stability.
