Disruptive instabilities in the TEXT-U tokamak

Disruptive instabilities in TEXT-U circular plasmas have been investigated. Hugill diagrams have been constructed for some single and consecutive disruptive shots, and the results indicated that the density limit boundary did not depend solely on plasma impurity content. Since the average radiated power intensity was typically only 30% of the input power, it is unlikely that major disruptions resulted from strong edge cooling and subsequent contraction of the plasma column. However, strong indications are found that a coupling between the m=1 and m=2 MHD modes could be the main triggering mechanism for the disruptions analysed, independently of where they occur in the Hugill operating space. After a major disruption has been triggered, the plasma current was observed to decay on a time-scale that varied considerably from one plasma discharge to another. This discrepancy was not found to depend, at least directly, on average quantities such as Zeff, plasma current, stored energy or plasma density. Finally, the installation of a set of locked mode detecting coils, for measuring the time derivative of the radial magnetic field associated mainly with the growth of the m=2 magnetic island, made it possible to observe that both minor and major disruptions were almost always preceded by a growth in Br. In particular, for some plasma discharges with long duration precursors, the characteristic fluctuations in the Mirnov coil signals and the soft X ray sawtooth oscillations are observed to disappear, which could suggest that a complete mode lock occurs before the disruption takes place. However, detailed investigation showed that the MHD modes were still rotating in the toroidal direction but with a very low frequency of about 50 Hz