Interpretation of Precursors of Internal Disruptions

A collisional instability driven by electron temperature gradients is described which appears to explain the major characteristics of the precursors of the internal disruptions. The threshold is reached when a certain parameter vertical-bar..mu..vertical-bar>..sqrt..3. For the ST and TFR tokamaks, we estimate 1approximately-less-thanvertical-bar..mu..vertical-barapproximately-less-than5 as the radius where q=1. For vertical-bar..mu..vertical-bar=3, the theoretical growth rate ..gamma../sub th/approx. (2--3) x 10/sup 3/ sec/sup -1/ compares well with the experimental value ..gamma../sub exp/approx.3 x 10/sup 3/ sec/sup -1/. The predicted shape of the wave profile is also in qualitative agreement with experiment. (AIP)