Physics of Turbulence Control and Transport Barrier Formation in Diii-d*

This paper describes the physical mechanisms responsible for turbulence control and transport barrier formation on DIII–D as determined from a synthesis of results from different enhanced confinement regimes, including quantitative and qualitative comparisons to theory. A wide range of DIII–D data support the hypothesis that a single underlying physical mechanism, turbulence suppression via ExB shear flow [1–5] is playing an essential, though not necessarily unique, role in reducing turbulence and transport in all of the following improved confinement regimes: H–mode, VH–mode, high-li modes, improved performance counter-injection L–mode discharges and high performance negative central shear (NCS) discharges. DIII–D data also indicate that synergistic effects are important in some cases, as in NCS discharges where negative magnetic shear also plays a role in transport barrier formation. This work indicates that in order to control turbulence and transport it is important to focus on understanding physical mechanisms, such as ExB shear, which can regulate and control entire classes of turbulent modes, and thus control transport. A more detailed description of recent results follows, grouped by type of operating regime: Edge L-H Transition Data. In fast L-H transitions detailed edge Langmuir probe measurements indicate quantitative agreement with ExB shear flow theories. The spatial dependence of the edge turbulence reduction is consistent with shear suppression for negative Er shear, while for positive Er shear the turbulence suppression is consistent with the effect of Er curvature for modes for which an Er well is destabilizing [6]. Phenomenologically distinct “fast,” “dithering,” and “slow” types of L-H transitions previously reported on DIII–D can all be explained by ExB shear suppression of turbulence and transport. An additional type of “very slow” transition has been observed on DIII–D in which the edge Dα emission can take more than 50 ms to evolve from L– to H–mode levels. Within the edge negative Er well density turbulence levels do not reduce until near the end of the Dα drop. Further out, in the vicinity of the separatrix, density and potential fluctuations are not reduced in H–mode, and may even increase. In this region the phase angle is responsible for a reduction in broadband turbulent driven flux. That the phase angle can play such a role in reducing transport has been observed previously [6], and has also been suggested by theoretical modeling [2,4]. Unlike fast transitions, SOL profiles remain unchanged from L– to H–mode. High-li and VH–mode Discharges. Magnetic braking of toroidal rotation in high-li enhanced confinement discharges reduces the ExB shearing rate, leading to an increase in turbulence and transport which demonstrates that ExB shear plays a causal role in the reduced turbulence and transport normally observed in these plasmas. Similar results indicating causality have been obtained with the magnetic brake in VH–mode discharges [7]. Controlling the current profile can directly affect the ExB shear mechanism as the magnitude of the shearing rate is proportional to the shear in Er/RBθ [8], and in high-li discharges the modified current profile increases the shear in Er/RBθ, resulting in enhanced turbulence suppression. Counter-injection L–mode Data. Reduced fluctuation levels have been observed in counter-injection L–mode plasmas, correlating with increasing ExB shear. These measure-