Long-Time Impurity Confinement as a Precursor to Disruptions in Ohmically Heated Tokamaks

The confinement times of impurities in Ohmically heated tokamaks are shown to lengthen considerably as the density-limit boundary is approached and exceeded either by the increase of $n_e$ itself or by the lowering of $B_T$ or $I_p$. Observations made on the Texas tokamak indicate that a change in the balance of diffusion and inward convection mechanisms governing impurity transport precedes the onset of partial or major disruptions. These studies also explain the reason for the wide differences of impurity behavior previously observed among various tokamaks.