Study of secondary instability of precursor magnetic island in COMPASS density limit disruptions

A systematic study of COMPASS density limit disruptions (DLDs) was carried out to analyze the relation between the precursor magnetic island and its secondary instability (SI). In all of these plasmas, the SI was always found present at the onset of energy confinement erosion. The duration of this erosion was shorter for precursor quasi-locked island than for rotating island, and the larger the island amplitude, the shorter the duration. It was also found, at the onset of the erosion of energy confinement, an inverse relation between the magnetic island amplitude and its rotation frequency, showing that erosion can start at different values of precursor island amplitude, depending on what is its rotation frequency. DLDs occur either with smaller precursor island rotating faster or larger island rotating slower (quasi-locked).A systematic study of COMPASS density limit disruptions (DLDs) was carried out to analyze the relation between the precursor magnetic island and its secondary instability (SI). In all of these plasmas, the SI was always found present at the onset of energy confinement erosion. The duration of this erosion was shorter for precursor quasi-locked island than for rotating island, and the larger the island amplitude, the shorter the duration. It was also found, at the onset of the erosion of energy confinement, an inverse relation between the magnetic island amplitude and its rotation frequency, showing that erosion can start at different values of precursor island amplitude, depending on what is its rotation frequency. DLDs occur either with smaller precursor island rotating faster or larger island rotating slower (quasi-locked).