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H-mode experiments on Alcator C-Mod [I. H. Hutchinson, R. L. Boivin, F. Bombarda et al., Phys. Plasmas 1, 1511 (1994)], which exhibit an internal transport barrier (ITB), have been examined with gyrokinetic calculations, before barrier formation. Ion temperature gradient (ITG) and electron temperature gradient (ETG) modes are unstable outside the barrier region and not strongly growing in the core; in the barrier region ITG is only weakly unstable. Linear calculations support the picture that ITG and ETG modes drive high transport outside the ITB, and that weakly unstable ITG modes in the barrier region correlate with reduced particle transport and improved thermal confinement even before the ITB is established, without the need for E×B shear stabilization. Long-wavelength mode stability in the barrier region is analyzed in the context of a phase diagram for ion and electron drift waves by varying the temperature and density scale lengths. Results from the gyrokinetic code GS2 [M. Kotschenreuther, G. Rewoldt, W. M. Tang et al., Comput. Phys. Commun. 88, 128 (1995)] are compared to standard threshold models and benchmark successfully against the experiment in the plasma core. [ABSTRACT FROM AUTHOR]

The radially local magnetohydrodynamic (MHD) ballooning stability of a compact, quasiaxially symmetric stellarator (QAS), is examined just above the ballooning beta limit with a method that can lead to estimates of global stability. Here MHD stability is analyzed through the calculation and examination of the ballooning mode eigenvalue isosurfaces in the 3-space (s,α,θ[sub k]); s is the edge normalized toroidal flux, α is the field line variable, and θ[sub k] is the perpendicular wave vector or ballooning parameter. Broken symmetry, i.e., deviations from axisymmetry, in the stellarator magnetic field geometry causes localization of the ballooning mode eigenfunction, and gives rise to new types of nonsymmetric eigenvalue isosurfaces in both the stable and unstable spectrum. For eigenvalues far above the marginal point, isosurfaces are topologically spherical, indicative of strong “quantum chaos.” The complexity of QAS marginal isosurfaces suggests that finite Larmor radius stabilization estimates will be difficult and that fully three-dimensional, high-n MHD computations are required to predict the beta limit. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]