Intermittent plasma objects ͑IPOs͒, featuring higher pressure than the surrounding plasma, are responsible for ϳ50% of the EϫB T radial transport in the scrape off layer ͑SOL͒ of the Doublet III D ͑DIII-D͒ tokamak ͓J. L. Luxon, Nucl. Fusion 42, 614 ͑2002͔͒ in L-and H-mode discharges. Conditional averaging reveals that the IPOs are positively charged and feature internal poloidal electric fields of up to 4000 V/m. The IPOs move radially with EϫB T /B 2 velocities of ϳ2600 m/s near the last closed flux surface ͑LCFS͒, and ϳ330 m/s near the wall. The IPOs slow down as they shrink in size from 2 cm at the LCFS to 0.5 cm near the wall. The skewness ͑i.e., asymmetry of fluctuations from the average͒ of probe and beam emission spectroscopy data indicate IPO formation at or near the LCFS and the existence of positive and negative IPOs which move in opposite directions. The particle content of the IPOs at the LCFS is linearly dependent on the local density and decays over ϳ3 cm into the SOL while their temperature decays much faster ͑ϳ1 cm͒.

Cross-field fluctuation-driven transport is studied in edge and scrape-off layer (SOL) plasmas in the DIII-D tokamak using a fast reciprocating Langmuir probe array allowing local measurements of the fluctuation-driven particle and heat fluxes. Two different non-diffusive mechanisms that can contribute strongly to the cross-field transport in the SOL of high-density discharges are identified and compared. The first of these involves intermittent transport events that are observed at the plasma separatrix and in the SOL. Intermittence has qualitatively similar character in L-mode and ELM-free H-mode. Low-amplitude ELMs observed in high-density H-mode produce in the SOL periods with cross-field transport enhanced to L-mode levels and featuring intermittent events similar to those in L-mode. The intermittent transport events are compatible with the concept of plasma filaments propagating across the SOL due to E × B drifts. The intermittent character of the transport in the SOL is also in agreement with predictions of the non-linear numerical simulations performed with an imposed driving flux. Another type of non-diffusive transport is often seen in high-density H-modes with prolonged ELM-free periods, where the transport near the separatrix is dominated by quasi-coherent modes driving particle and/or heat fluxes exceeding L-mode levels. These modes may play an important role by providing particle and/or heat exhaust between ELMs.

IAEA-CN-116/EX/P5-29 This is a preprint of a paper intended for presentation at a scientific meeting. Because of the provisional nature of its content and since changes of substance or detail may have to be made before publication, the preprint is made available on the understanding that it will not be cited in the literature or in any way be reproduced in its present form. The views expressed and the statements made remain the responsibility of the named author(s); the views do not necessarily reflect those of the government of the designating Member State(s) or of the designating organization(s). In particular, neither the IAEA nor any other organization or body sponsoring this meeting can be held responsible for any material reproduced in this preprint.

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Based on semianalytic modeling of experimental H-mode density profiles, a pedestal density limit is computed for a prototypical next generation burning plasma. For a range of input assumptions, the projected limit lies near or above Greenwald density limit [M. Greenwald, J. L. Terry, S. M. Wolfe, S. Ejima, M. G. Bell, S. M. Kaye, and G. H. Neilson, Nucl. Fusion 28, 2199 (1988)]. Two separate models are developed and utilized: the first model determines the edge density profile from consideration of cross-field transport in the steep gradient region and the distribution of fueling sources. It is shown that the observed shape of the density profile in H-mode can be explained with constant particle diffusivity within the separatrix. The second model determines the separatrix density above which the conduction limited scrape-off layer (SOL) transport is no longer sustainable. These results imply that further increases in pedestal density (while maintaining separatrix density below the power balance limit) are achievable through control of the fueling profile, e.g., with shallow pellets from the high-field side and divertor pumping.