where | ∇ ∇ p | ∼ ∼ p L p is the pressure gradient g e f f = c s 2 | ∇ ∇ B B | ∼ ∼ 1 / R 0 is the effective gravity produced by a non-homogeneous magnetic field, R 0 is the major radius of the device, L p is a characteristic length of the pressure gradient, and c s is the plasma sound speed.