Session 6IB - Tokamak Optimization.
INVITED session, Wednesday afternoon, November 13
Grand Ballroom, Adam's Mark
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 MHD theory. Recent long-pulse experiments, in particular those aimed at simulating ITER-like discharges with low collisionality (\nu*), are limited by low (m,n) non-ideal MHD modes. The effect of the saturated modes is a reduction of the confinement time by 10%-20% (soft beta limit), depending on the island size and location.(Z. Chang et al., Nucl. Fus. 34, 1309 (1994).) If \beta_N is further increased, more modes are excited leading to a disruption (hard beta limit).
\par Detailed 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. Linear and non-linear resistive MHD cannot clearly explain the dependence of stability on \nu* and \beta. However more recent theories on neoclassical destabilization of tearing modes,(H. Wilson et al, Phys. Plasmas 3, 248 (1996) and references herein.) including the effects of a perturbed helical bootstrap current and polarization current, are succesful in explaining the behavior of the tearing mode beta limits in long-pulse discharges and recent TFTR results support the size of the saturated islands(Z. Chang et al, Phys. Rev. Lett. 74, 4663 (1995).). Also, a strong correlation is observed between the onset of these low m/n modes with sawteeth, ELMs, or fishbone events, consistent with the seed island required by the theory.
\par We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories and the experimental results of ASDEX-U, COMPASS, DIIID, JT60U and TFTR, which have all observed these low (m,n) non-ideal modes. This will enable us to project the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods which would increase the soft \beta limit, decrease the seed perturbations and/or diminish the effects on confinement.
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