(EX/8-4) Edge Pedestal Control in Quiescent H-Mode Discharges in DIII-D Using Co Plus Counter Neutral Beam Injection

K.H. Burrell1), T.H. Osborne1), P.B. Snyder1), W.P. West1), M.S. Chu1), M.E. Fenstermacher2), P. Gohil1), W.M. Solomon3)
 
1) General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
2) Lawrence Livermore National Laboratory, Livermore, California 94550, USA
3) Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA

Abstract.  Quiescent H-mode (QH-mode) plasmas in DIII-D with co plus counter neutral beam injection have demonstrated active control of the edge pedestal that can be used to optimize the edge conditions in future burning plasma devices such as ITER. Edge localized mode (ELM)-free QH-mode plasmas have demonstrated that the three reactor requirements of: (1) steady operation at maximum stable pedestal pressure, (2) ELM-free operation and (3) rapid particle transport for helium exhaust can be met simultaneously in discharges which operate with constant density and radiated power. Altering the torque input to QH-mode plasmas allows continuous adjustment of the pedestal density, pressure and particle transport over a range of about a factor of two while maintaining the ELM-free state. This active control capability allows operation near but below the ELM stability boundary. These plasmas exhibit edge particle transport more rapid than that produced by ELMs while operating at reactor relevant pedestal β∼1% and collisionality ∼0.1; pedestal densities up to 1/2 the Greenwald density have been achieved. The essential feature, which distinguishes QH-mode from standard ELMing H-mode is the presence of an edge-localized electromagnetic mode, the edge harmonic oscillation (EHO). The EHO provides extra particle transport which prevents ELMs by keeping the edge pressure below the peeling-ballooning mode boundary. The EHO is spontaneously generated by the plasma itself and requires no external coils to generate a perturbed magnetic field as is necessary, for example, for ELM suppression via resonant magnetic perturbations. Edge stability calculations using the ELITE code show that the QH-mode operating point is near the peeling boundary. Much of the physics of the EHO is consistent with a model in which the EHO is an edge kink-peeling mode that is destabilized by shear in the edge toroidal rotation at an edge current density slightly below that on the standard ELM boundary. Peeling-ballooning stability calculations have been used as a guide in developing the best plasma shape. Experiments confirm the prediction that a high-triangularity, double-null plasma has the best stability against ELMs. Plasmas with the best stability require the least reduction in edge transport to achieve ELM-free operation.

Work supported by the US Department of Energy under DE-FC02-04ER54698.

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