On the Amplitude of Sawtooth Pre-Cursors at the Onset of Neo-Classical Tearing Modes

Most observations of Neo-classical Tearing Modes (NTM) in JET experiments, occur in regimes where the sawteeth are present. Recent JET experiments showed that it is possible to control the time of onset of the NTM by controlling the frequency of occurrence and the amplitude of sawteeth crashes using ICRH. A clear correlation between the time of sawtooth crashes and the onset of NTMs was obtained from a database of pre 2000 JET experiments [1]. In this paper we study the correlation between sawteeth and other central n=1 MHD modes such as fishbones and the onset of the m=3 n=2 NTMs. In addition we study the amplitude of the n=1 sawtooth precursors around the time the NTM starts. A larger database including 2000-2001 NTM studies is considered. This includes experiments for the study of shape effects on the NTM trigger, NTM studies in ASDEX-Upgrade similarity discharges, ITER like discharges and experiments to control NTMs by sawtooth control. Only discharges, which are dominantly NBI heated, are considered. Studies show that large sawteeth in dominantly ICRH discharges can have a low b limit, b <1 and conclusions for these discharges may differ from these reached here.
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