Magnetohydrodynamic equilibrium of tokamak plasmas with reversed current layers

An MHD code is used to find a sequence of equilibria where negative skin current penetrates progressively toward the magnetic axis, on which the toroidal current density (positive) and kinetic pressure are maintained constant. The initial equilibrium with no skin current has a central safety factor close to unity and an edge factor about three. Equilibria have been found for currents down to 5 per cent of the initial value (total negative current about half total positive current). For total currents less than a third of the initial current, the plasma becomes smaller because of the creation of a poloidal separatrix on the small major radius side. The vertical field necessary for equilibrium decreases more slowly than the total plasma current. This is explained by a large increase in the poloidal beta accompanied by a smaller increase in the internal inductance. The use of such a fast rampdown sequence to provide a controlled termination for an energetic tokamak plasma is discussed. (author)