Observation of spontaneous neoclassical tearing modes

In this paper data are presented from the Tokamak Fusion Test Reactor (TFTR) [D. Meade and the TFTR Group, in Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, pp. 9-14] which challenge the commonly held belief that extrinsic magnetohydrodynamic (MHD) events such as sawteeth or edge localized modes are required to provide the seed islands that trigger neoclassical tearing modes (NTMs). While sawteeth are reported to provide the trigger for most of the NTMs on DIII-D [J. Luxon and DIII-D Group, in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. I, p 159] and ASDEX-U [O. Gruber, H.-S. Bosch, S. Gunter , Nucl. Fusion 39, 1321 (1999)], the majority of NTMs seen in TFTR occur in plasmas without sawteeth, that is, which are above the beta threshold for sawtooth stabilization. Examples of NTMs appearing in the absence of any detectable extrinsic MHD activity will be shown. Conversely, large n=1 modes in plasmas above the NTM beta threshold generally do not trigger NTMs. An alternative mechanism for generating seed islands will be discussed. (C) 2002 American Institute of Physics.