The nature of disruptions

Detailed investigations of the MHD activity during minor and major disruptions show clearly that the destabilization of several modes of different helicities causes a flattening of the electron temperature profile at the edge of the discharge. The transition from minor to major disruption is accompanied by an enhancement of the magnetic turbulence, while the energy confinement time decreases drastically. In both types of disruptions the presence of the ergodic zone seems to trigger an helical displacement of the hot region of the plasma which could be related to the temperature quenching in the discharge center. During the current quenching after a major disruption a part of the stored magnetic energy is dissipated in the center of a post-disruptive plasma. It is shown that the energy loss in the bulk of the plasma can be due to the presence of a significant magnetic turbulence ergodizing the magnetic field lines, while radiation and charge-exchange are dominant at the edge. The tokamak discharge then behaves as a natural ergodic divertor during the current quenching, provided the plasma discharge is clean enough so that the power balance at the plasma center is not dominated by impurity radiation