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The long-pulse, slowly evolving single-null divertor (SND) discharges in DIII-D with H-mode, ELMs, and sawteeth are found to be limited significantly below (factor of 2) the predicted ideal limit beta N = 4l(i) by the onset of tearing modes. The tearing modes are metastable in that they are explained by the neoclassical bootstrap current (high beta theta) destabilization of a seed island which occurs even if Delta prime < 0, (ie otherwise stable). For sufficiently high beta theta, there is a region of the modified Rutherford equation such that dw/dt > 0 for omega larger than a threshold value; the plasma is metastable, awaiting the critical perturbation which is then amplified to the much larger saturated island. Experimental results from a large number of tokamaks indicate that the high beta operational envelope of the tokamak is well defined by ideal magnetohydrodynamic (MHD) theory.

Additional information

Authors: LA HAYE R J ET AL, General Atomics, San Diego (US);CALLEN J D ET AL, University of Winconsin, Madison (US);SAUTER O ET AL, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne (CH);WILSON H R, AEA Fusion, Culham Laboratory, Abingdon, Oxon (GB)
Bibliographic Reference: Article: Proceedings of 24th EPS Conference on Controlled Fusion and Plasma Physics, Berchtesgaden 9-13 June 1997
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