Marginal confinement in tokamaks by inductive electric field

Here diffusion and Ware pinch are analyzed as opposed effects for plasma confinement, when instabilities are not considered. In this work it is studied the equilibrium inductive electric field where both effects annul each other in the sense that the average normal velocity is zero, that is, marginal velocity confinement is reached. The critical electric field defined in that way is studied for different values of elliptic elongation, Shafranov shift and triangularity. A short complementary analysis is also performed of the variation of the poloidal magnetic field along a magnetic line. Magnetohydrodynamic transport theory in the collisional regime is used as in recent publications. Axisymmetric and up-down symmetry are assumed. (authors)