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The paradigm of shear suppression of turbulence as the mechanism for the low to high confinement mode (L to H) transition is examined by quantitative comparison of the predictions of the paradigm with experimental results from the DIII-D tokamak [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159]. The L to H transition trigger is V×B rotation, not the main ion pressure gradient. The radial electric field Er shear increases before the fluctuation suppression, consistent with increasing Er shear as the cause of the turbulence suppression. The spatial dependence of the turbulence reduction is consistent with shear suppression for negative Er shear. For positive Er shear, the turbulence suppression is consistent with the effect of Er curvature for modes for which an Er well is destabilizing. Finally, the transport barrier depends on the phase angle between the density and potential fluctuations inside the Er well, an effect not included in existing L to H transition models.

Poloidal and toroidal rotation of the main ions (He2+) and the impurity ions (C6+ and Bs+) in Hmode helium plasmas have been measured via charge exchange recombination spectroscopy in the DIII-D tokamak. It was discovered that the main ion poloidal rotation is in the ion diamagnetic drift direction while the impurity ion rotation is in the electron diamagnetic drift direction, in qualitative agreement with the neoclassical theory. The deduced radial electric field in the edge is of the same negativewell shape regardless of which ion species is used, validating the fundamental nature of the electric field in L-H transition phenomenology.PACS numbers: 52.55.Fa, 52.55.Pi, 52.70.Kz Since its original discovery in ASDEX [1],the H mode has proven to be one of the most robust and ubiquitous modes of improved confinement in toroidal magnetic fusion devices. The physics of the L-mode to H-mode (L-H) transition has attracted a great deal of interest and effort from both the experimental and theoretical communities. Critical reviews of this subject have recently been made by Burrell et al. [2] and Groebner [3]. Although a complete, quantitative theory of the confinement improvement at the L-H transition does not yet exist, combined theoretical and experimental work has merged into a paradigm that a highly sheared EXB flow in the plasma edge can lead to better confinement through decorrelation of the fluctuations, decreased radial correlation lengths, and reduced turbulent transport [4,5]. The experimental observations that the edge impurity ion poloidal rotation [6-9] and the edge radial electric field [6-11] change dramatically and abruptly at the L-H transition have led to several theories which consider how the radial electric field or the (main) ion poloidal rotation changes across the L-H transition. Among these are theories based on bifurcation of the radial electric field [5,12], Stringer spin-up [13,14], turbulent Reynolds stress [15], temperature gradient-induced poloidal rotation [16], and particle and energy confinement bifurcation [17].Only the impurity ion rotation measurements have been available previously and some theoretical models were developed to explain the sudden (~I msec) increase of poloidal rotation (in the electron diamagnetic drift direction) at the L-H transition; these theories implicitly assumed that the main ion rotation and the impurity ion rotation are identical. However, recent neoclassical derivations of rotation velocities predict that the main ion poloidal rotation and the impurity ion poloidal rotation speed could be quite different [18,19]. Although not in H mode, a comparison of the main and the impurity ion poloidal rotation in Ohmic plasmas in TEXT was reported previously [20], which showed little diff'erence between them. Experimental clari6cation of the rotation behavior of the main and impurity iona in H-mode plasmas is important not only as a check of the neoclassical rotation theories, but also as a test of the existing L-H transition theories. However, charge exchange recom...

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