Consider a cylindrical system, {r, phi, z}.  Suppose there is a magnetic field 
which is purely B_phi.  Show that a plasma in the shape of an axisymmetric 
donut, ie p = p(r,z), cannot be in equilibrium in this system.  Use the ideal 
MHD equilibrium equations for this.  Scale the MHD momentum equation and 
deduce the time scale over which such an initial condition will convect out 
assuming that the minor radius and the major radius of the donut are a and R, 
respectively.   Compare this problem with P5.