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The destabilisation of neoclassical tearing modes (NTM) requires a finite size seed island which is often provided by sawtooth activity. It is shown that the beta value at the onset of NTMs can be significantly modified by altering the sawtooth period using ICRH in JET . Sawtooth stabilisation, leading to long sawtooth periods, enables NTMs to be triggered at low beta value, consistent with larger induced seed islands. By destabilising sawteeth, that is with a shorter sawtooth period, larger beta values are required to trigger NTMs. This is the first clear direct demonstration of the relation between sawteeth and NTMs and opens the possibility of increasing the beta threshold by sawtooth destabilisation. Adding a slow power ramp-down, after the mode onset, shows that NTMs are metastable as long as the discharge is in H -mode in JET, emphasising the role of seed island control to avoid NTMs.

Additional information

Authors: SAUTER ET AL, Centre de Recherches en Physique des Plasmas, EURATOM Association-Confederation Suisse, Ecole Polytechnique Federale de Lausanne (CH);WESTEROF E, FOM- Instituut voor Plasmafysica (NL);MAYORAL ET AL, EURATOM/UKAEA Fusion Association, Culham Science Centre, Abington (UK);NGUYEN ET AL, DRFC, CEA de Cadarashe (FR);MANTSINEN M.J, Helsinki University of Technology (FI);BELO ET AL, Centro de Fusao Nuclear, Instituto Superior Tecnico, Lisboa (PT);HELLSTEN ET AL, EFDA-JET, Culham Labs. Abington (UK);JOHNSON ET AL, Association EURATOM-NFR, Royal Institute of Technology, Stockholm (SE);LAMALLE P, LPP/ERM-KMS, TEC, Association-EURATOM (BE);PINCHES S, Max-Planck-IPP-EURATOM Association, Garching (DE);PODDA S, Frascati, Association-EURATOM-ENEA (IT);RAPP J,
Bibliographic Reference: An oral report given at: The 28th EPS Conference of Controlled Fusion and Plasma Physics. Held in: Madeira (PT) June 2001
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