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Abstract

Oscillating MHD modes in JET are often observed to slow down as they grow and generally lock when the amplitude exceeds a critical value, then continue to grow to large amplitudes (b-"rm r" /B-theta sim 1%). The mode can grow early in the current rise or after perturbations such as a pellet injection or a large sawtooth collapse and maintain a large amplitude throughout the remainder of the discharge. Such large amplitude quasi-stationary MHD modes can apparently have profound effects on the plasma including stopping central ion plasma rotation, reducing the amplitude and changing the shape of sawteeth, flattening the temperature profile around resonant q surfaces, and reducing the stored energy. Perhaps most importantly, large amplitude locked modes are precursors to most disruptions. Some large amplitude modes can be avoided by proper programming of the q evolution. The apparent reasons for the mode locking in a particular location will be given along with a comparison with theory.

Additional information

Authors: SNIPES J A, JET JOINT UNDERTAKING, ABINGDON (UK);CAMPBELL D J, JET JOINT UNDERTAKING, ABINGDON (UK);HAYNES P S, JET JOINT UNDERTAKING, ABINGDON (UK);HENDER T C, JET JOINT UNDERTAKING, ABINGDON (UK);HUGON M, JET JOINT UNDERTAKING, ABINGDON (UK);LOMAS P J, JET JOINT UNDERTAKING, ABINGDON (UK);LOPES CARDOZO N, JET JOINT UNDERTAKING, ABINGDON (UK);NAVE M F F, JET JOINT UNDERTAKING, ABINGDON (UK);SCHULLER F C JET JOINT UNDERTAKING, ABINGDON (UK), JET JOINT UNDERTAKING, ABINGDON (UK)
Bibliographic Reference: AVAILABILITY: PUBLICATIONS OFFICER, JET JOINT UNDERTAKING, ABINGDON, OXON, OX14 3EA (UK), MENTIONING REPORT JET-P(88)01, 1988
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