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(EX6/2) Progress Towards Increased Understanding and Control of Internal Transport Barriers (ITBs) on DIII-D

E. J. Doyle¹⁾, C. M. Greenfield²⁾, M. E. Austin³⁾, L. R. Baylor⁴⁾, K. H. Burrell²⁾, T. A. Casper⁵⁾, J. C. DeBoo²⁾, D. R. Ernst⁶⁾, C. Fenzi⁷⁾, P. Gohil²⁾, R. J. Groebner²⁾, W. W. Heidbrink⁸⁾, G. L. Jackson²⁾, T. C. Jernigan⁴⁾, J. E. Kinsey⁹⁾, L. L. Lao²⁾, M. A. Makowski⁵⁾, G. R. McKee¹⁰⁾, M. Murakami⁴⁾, W. A. Peebles¹⁾, R. Prater²⁾, C. L. Rettig¹⁾, T. L. Rhodes¹⁾, J. C. Rost¹⁰⁾, G. M. Staebler²⁾, B. W. Stallard⁵⁾, E. J. Strait²⁾, E. Synakowski⁶⁾, D. M. Thomas²⁾, M. R. Wade⁴⁾, R. E. Waltz²⁾, and L. Zeng^1)
 
1) Dept. of Electrical Engineering and IPFR, University of California, Los Angeles, California USA
²⁾ General Atomics, San Diego, California USA
³⁾ University of Texas at Austin, Austin, Texas USA
⁴⁾ Oak Ridge National Laboratory, Oak Ridge, Tennessee USA
⁵⁾ Lawrence Livermore National Laboratory, Livermore, California USA
⁶⁾ Princeton Plasma Physics Laboratory, Princeton, New Jersey USA
⁷⁾ University of Wisconsin-Madison, Madison, Wisconsin USA
⁸⁾ Univeristy of California, Irvine, California USA
⁹⁾ Lehigh University, Bethlehem, Pennsylvania USA
¹⁰⁾ Massachusetts Institute of Technology, Cambridge, Massachusetts USA

Abstract.  Substantial progress has been made towards both understanding and control of internal transport barriers (ITBs) on DIII-D, resulting in the discovery of a new sustained high performance operating mode termed the Quiescent Double-Barrier (QDB) regime. The QDB regime combines core transport barriers with a quiescent, ELM-free H-mode edge (termed QH-mode), giving rise to separate (double) core and edge transport barriers. The core and edge barriers are mutually compatible and do not merge, resulting in broad core profiles with an edge pedestal. The QH-mode edge is characterized by ELM-free behavior with continuous multiharmonic MHD activity in the pedestal region, and has provided density and impurity control for 3.5 s ( > 20$\tau_{\mathrm{E}}^{}$) with divertor pumping. QDB plasmas are long-pulse high-performance candidates, having maintained a $\beta_{\mathrm{N}}^{}$H₈₉ product of 7 for 5 energy confinement times ( T_i $\leq$ 16 keV, $\beta_{\mathrm{N}}^{}$ $\leq$ 2.9, H₈₉ $\leq$ 2.4, $\tau_{\mathrm{E}}^{}$ $\leq$ 150 ms, DD neutron rate S_n $\leq$ 4×10¹⁵s^{- 1}). The QDB regime has only been obtained in counter-NBI discharges (injection anti-parallel to plasma current) with divertor pumping. Other results include successful expansion of the ITB radius using (separately) both impurity injection and counter-NBI, and the formation of ITBs in the electron thermal channel using both ECH and strong negative central shear (NCS) at high power. These results are interpreted within a theoretical framework in which turbulence suppression is the key to ITB formation and control, and a decrease in core turbulence is observed in all cases of ITB formation.

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