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The maximum normalized beta achieved in long pulse tokamak discharges falls significantly below both that observed in short pulse discharges and that predicted by the ideal magnetohydrodynamics (MHD) theory. Studies using the reconstructed equilibria show that soft and hard beta limits of 20% up to as much as 50% below the ideal MHD limit are observed. Recent theories on neoclassical destabilization of tearing modes, including the effects of a perturbed helical bootstrap current and polarization current, are successful in explaining the behaviour of the tearing mode beta limits in long-pulse discharges and recent Tokamak fusion test reactor (TFTR) results support the size of the saturated islands. Also, a strong correlation is observed between the onset of these low m/n modes with sawteeth, edge localized modes (ELM), or fishbone events, consistent with the seed island required by the theory.

Additional information

Authors: SAUTER O, Centre de Recherches en Physique des Plasmas, Ecole Polytechnic Fédérale de Lausanne, Association Euratom Confédération-Suisse (CH)
Bibliographic Reference: Article: Bulletin of the American Physical Society, Vol. 41 (1996) No. 7, p. 1528
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