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Abstract

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 predominantly due to radiation from impurities, this radiation producing a cold, highly resistive plasma. The observation in some discharges of m=2 magnetic fluctuations during start-up, which q-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. A comparison of the period of the oscillations with theoretical predictions has been made. 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.

Additional information

Authors: THOMAS P R, JET JOINT UNDERTAKING, ABINGDON (UK);WESSON J A JET JOINT UNDERTAKING, ABINGDON (UK), JET JOINT UNDERTAKING, ABINGDON (UK)
Bibliographic Reference: 10TH INTERNATIONAL CONFERENCE ON PLASMA PHYSICS AND CONTROLLED NUCLEAR FUSION RESEARCH, LONDON (UK), SEPT. 12-19, 1984 WRITE TO THE PUBLICATIONS OFFICER, JET JOINT UNDERTAKING, ABINGDON, OXON, OX14 3EA (UK), MENTIONING REPORT JET-P(84)05, 1984
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