MHD behaviour and discharge optimization in JET

The electron density in JET is limited by disruptions, the disruption boundary having rather precise dependence on the plasma current. Disruptions on the current fall are preceded by a reduction in the rate of fall of electron density, this reduction being correlated with the spectroscopic signals. During the current quench which follows the disruption, the energy loss is mainly due to radiation from impurities, this radiation producing a cold, highly resistive plasma. The observation in some discharges of m=2 magnetic fluctuations during startup, while qsub(a) is still above 10, indicates a rapid peaking of the current. Higher m-modes are found to be correlated with broad current profiles. The growing magnetic oscillations which appear before disruptions often become phase-locked but the amplitude of the non-oscillatory signal continues to grow. In some cases, the growth of the magnetic perturbation occurs entirely without oscillation. Soft X-ray sawtooth oscillations are observed on all high current discharges, the period being typically 30-40 ms. The onset is sometimes accompanied by oscillations at about 500 Hz. The period of the oscillations has been compared with theoretical predictions. JET plasmas are vertically unstable if b/a exceeds 1.2. The growth of the instability is limited by eddy currents induced in the vacuum vessel, allowing the plasma position to be feedback-stabilized. The loss of stabilization in one discharge caused a force of hundreds of tonnes to be transmitted to the vacuum vessel. (author)