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.

Probe measurements in the PISCES linear device indicate the presence of plasma radially far from where it is produced. We show that this is mainly caused by large-scale structures of plasma with high radial velocity. Data from the Tore Supra tokamak show striking similarities in the shape of these intermittent events as well as the fluctuation density probability distribution and frequency spectrum. The fact that intermittent, large-scale events are so similar in linear devices and tokamaks indicates the universality of convective transport in magnetically confined plasmas.

Large-scale steady-state convection is found to dominate the particle transport just inside the last closed flux surface-(LCFS) in Ohmic discharges in the CCT tokamak. Near the limiter radius and in the scrape-off layer, fluctuation-induced transport is prevalent. At the L-H transition, the convection pattern near the LCFS is disrupted and a more poloidally symmetric, near-sonic plasma flow develops. Convective and turbulent particle transport are reduced across the entire edge region, resulting in the formation of the //-mode edge transport barrier. PACS numbers: 52.25.Fi, 52.35.Ra, 52.55.Fa, 52.55.Pi Results from a number of tokamak experiments have clearly demonstrated a degradation of energy and particle confinement with increasing auxiliary heating power [lj. This behavior, referred to as "L-mode" scaling, makes the achievement of a burning fusion plasma more difficult. The experimental observation of a regime with improved energy and particle confinement (the "//-mode" regime), first in ASDEX [2] and subsequently in other tokamaks [3], is thus significant. Recent measurements in Doublet III-D indicate that a localized radial electric field develops just inside the last closed flux surface (LCFS) at the L-H transition [4] and that the density fluctuation amplitude is reduced [5].Electrostatic edge turbulence has been shown to be an important transport mechanism in Ohmic plasmas in the TEXT tokamak [6] and during auxiliary heating in ISX-# and DITE [7,8]. It has been suggested that sheared poloidal plasma flow may locally decorrelate the turbulent fluctuations and thereby reduce the associated particle transport [9]. A transport barrier would then be created in the region of strongly sheared rotational flow. Here, we present experimental evidence that the edge turbulence and the associated particle transport are modified across the entire edge plasma region in the H mode. Furthermore, it is shown that large-scale steady-state ExB convection (observed in several devices [10-12] but often thought to be small or neglected [6]) is also an important transport mechanism in CCT during the Ohmic L mode. The radial component of this convective flow across the LCFS is substantially reduced during the //-mode phase, providing a second mechanism for the formation of an edge plasma transport barrier.The results presented in this paper are obtained at a magnetic field of 0.25 T (q a~ 3) with a total plasma current of 38 kA, a line-averaged density of AI=3X10 12 cm -3 , a loop voltage of 1.4 V, and plasma major and minor radii of 148 and 36 cm, respectively. The inside midplane of the conducting vacuum chamber liner is used as the limiter, and the gap between the LCFS and the outside midplane wall is 4 cm. The plasma position is controlled to within 0.5 cm by means of a fast response feedback system.A poloidal Langmuir probe array with 15° poloidal spacing between probes is used to measure the equilibrium plasma profiles, the steady-state convective transport, and the turbulence-induced transport over the region 30° above...