Hoop Force

From QED

Basic problem of toroidal equilibrium.
(Freidberg p.75)

A loop of wire tends to expand itself.

A toroidal plasma carrying a toroidal current has to deal with the same issue. The poloidal flux on the outside of the plasma axis has to equal the poloidal flux on the inside of the plasma axis. Geometrically, the inside flux passes through a smaller area than the outside flux, so

Leading to a net outward pressure.

The hoop force can be handled with a vertical B field.