VIOLATION OF THE MERCIER CRITERION IN REVERSED SHEAR CONFINEMENT CONFIGURATIONS IN TOKAMAKS

It is found that the Mercier criterion can be violated in a reversed s heer plasma of a tokamak. A reversed shear plasma has negative magneti c shear and negative pressure gradient in the inner region of a plasma . In the negative shear region, stabilizing terms due to the parallel current and the magnetic well produced by the poloidal current change to destabilizing ones. As (q(0)-q(min))/q(min) increases, the destabil izing effects increase and the Mercier criterion can be violated. Here , q(0) and q(min) are the safety factor on the plasma axis and the min imum value along the minor radius, respectively. In JT-60U reversed sh ear plasmas, the value of (q(0)-q(min))/q(min) becomes large. The viol ation of the Mercier criterion seems to be consistent with the observa tion of magnetohydrodynamic activity localized near the internal trans port barrier.