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(CD2/EX9/3) Progress Towards Sustainment of Advanced Tokamak Modes in DIII-D

B. W. Rice, B. W. Stallard

Lawrence Livermore National Laboratory, Livermore, California, U.S.A.

K. H. Burrell, J. R. Ferron, C. M. Greenfield, G. L. Jackson, L. L. Lao, R. J. La Haye, T. C. Luce, E. J. Strait, T. S. Taylor, A. D. Turnbull

General Atomics, P.O. Box 85608, San Diego, California 92186-9784, U.S.A.

E. J. Synakowski

Princeton Plasma Physics Laboratory, Princeton, New Jersey, U.S.A.

M. R. Wade
Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.

Abstract.  Improving confinement and beta limits simultaneously in long-pulse ELMy H-mode discharges is investigated. The product $\beta_{\mathrm{N}}^{}$H_98y serves as a useful figure-of-merit for performance, where $\beta_{\mathrm{N}}^{}$ $\equiv$ $\beta$/(I/aB) and H_98y is the ratio of the thermal confinement time relative to the most recent ELMy H-mode confinement scaling established by the ITER confinement database working group. In discharges with q_min > 1.5 and negative central magnetic shear, $\beta_{\mathrm{N}}^{}$H_98y $\sim$ 4 is sustained for $\sim$ 1 s. Although peaked profiles are observed, strong internal transport barriers are not present. Further increases in $\beta_{\mathrm{N}}^{}$ in these discharges is limited by neoclassical tearing modes (NTM) in the positive shear region. In another recently developed regime, $\beta_{\mathrm{N}}^{}$H_98y > 6 has been sustained during large infrequent ELMs in non-sawtoothing discharges with q₀ $\sim$ 1. This level of performance is similar to that obtained in ELM-free regimes such as VH-mode. The limitation on $\beta_{\mathrm{N}}^{}$ and pulse length in these discharges is also the onset of NTMs.

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