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(EX5/5) Behavior of Electron and Ion Transport in Discharges with an Internal Transport Barrier in the DIII-D Tokamak

C. M. Greenfield, G. M. Staebler, K. H. Burrell, J. C. DeBoo, J. S. deGrassie, P. Gohil, C. C. Petty, R. I. Pinsker, R. E. Waltz

General Atomics, P.O. Box 5608, San Diego, California 92186-5608, U.S.A.

C. Rettig, E. J. Doyle, W. A. Peebles, T. L. Rhodes, L. Zeng

University of California, Los Angeles, California 90024, U.S.A.

B. W. Stallard, B. W. Rice

Lawrence Livermore National Laboratory, Livermore, California 94551-9900, U.S.A.

M. E. Austin

The University of Texas at Austin, Austin, Texas 78712, U.S.A.

G. McKee

University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.

E. J. Synakowski
Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, U.S.A.

Abstract.  We report results of experiments to further determine the underlying physics behind the formation and development of internal transport barriers (ITB) in the DIII-D tokamak. The initial ITB formation occurs when the neutral beam heating power exceeds a threshold value during the early stages of the current ramp in low-density discharges. This region of reduced transport, made accessible by suppression of long-wavelength turbulence by sheared flows, is most evident in the ion temperature and impurity rotation profiles. In some cases, reduced transport is also observed in the electron temperature and density profiles. If the power is near the threshold, the barrier remains stationary and encloses only a small fraction of the plasma volume. If, however, the power is increased, the transport barrier expands to encompass a larger fraction of the plasma volume. The dynamic behavior of the transport barrier during the growth phase exhibits rapid transport events that are associated with both broadening of the profiles and reductions in turbulence and associated transport. In some, but not all, cases, these events are correlated with the safety factor q passing through integer values. The final state following this evolution is a plasma exhibiting ion thermal transport at or below neoclassical levels. Typically, the electron thermal transport remains anomalously high. Recent experimental results are reported in which rf electron heating was applied to plasmas with an ion ITB, thereby increasing both the electron and ion transport. Although the results are partially in agreement with the usual $\vec{E}\,$×$\vec{B}\,$ shear suppression hypothesis, the results still leave questions that must be addressed in future experiments.

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