The effect of plasma shape on H-mode pedestalcharacteristics on DIII-D

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The characteristics of the H-mode are studied in discharges with varyingtriangularity and squareness. The pressure at the top of the H-mode pedestalincreases strongly with triangularity, primarily due to an increase in themargin by which the edge pressure gradient exceeds the ideal ballooning modefirst stability limit. Two models are considered for how the edge may exceedthe ballooning mode limit. In one model, access to the ballooning mode secondstable regime allows the edge pressure gradient and associated bootstrapcurrent to continue to increase until an edge localized, low toroidal modenumber, ideal kink mode is destabilized. In the second model, the finite widthof the H-mode transport barrier and diamagnetic effects raise the pressuregradient limit above the ballooning mode limit. We observe a weak inversedependence of the width of the H-mode transport barrier, Δ, ontriangularity relative to the previously obtained scalingΔ∝(βPEP)1/2. The energy loss forType I ELMs increases with triangularity in proportion to the pedestal energyincrease. At low density, the energy confinement of high-triangularitydischarges is larger than discharges with low triangularity, as a result of anincrease in the energy in the H-mode pedestal. At high density, both thechange in pedestal pressure and the response of the density profile are foundto play a role in setting the energy confinement. The highest energyconfinement at high density was obtained in low-triangularity discharges.