Analysis of the role of the ion polarization current on the onset of pre-disruptive magnetic islands in JET

Abstract

The problem of the trigger of the neoclassical tearing mode is addressed in this work by evaluating the non-linear terms of the generalized Rutherford equation (GRE) for a set of JET disruptions. The linear stability index Δ′0, even for positive values, is not enough to describe the trigger of the mode as the stabilizing non-linear effects tend to prevent the growth of a mode below a certain width. First, an analysis on the contribution of the stabilizing effect of the curvature and the destabilizing effect of the bootstrap is done. Second, the work focuses on the role of the ion polarization current, a return current that flows parallel to the magnetic field due to the difference in the drift motion of the electrons and ions. This contribution is thought to play an important role in the onset of an island of width W, because it scales as 1/W3, making it a dominant term in the GRE when W is small. The assessment is carried out over a subset of pulses, producing interesting observations that were then generalized across the entire database, obtaining consistent results.