EFFECT OF THE CURRENT GRADIENT ON THE ELLIPTICITY AND TRIANGULARITY OF THE MAGNETIC-SURFACES IN TOKAMAKS AND ON THE STABILITY PROBLEM

The effect of the longitudinal-current gradient on the ellipticity and triangularity of the magnetic surfaces in tokamaks is studied theoret ically by solving the Linear Grad-Shafranov equation with allowance fo r the second and third harmonics of the Fourier expansion of the magne tic flux. Expressions for the ellipticity and triangularity in the cen tral region of the plasma column are obtained in terms of their values at the conducting wall and the plasma pressure. It is shown that the current gradient leads to the appearance of a part of the near-axis tr iangularity caused by the wall ellipticity and plasma pressure. The co nclusion is drawn that this part of the triangularity substantially re duces the plasma pressure threshold for the instability induced by the wall ellipticity.