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(OV1/3) Overview of Recent Experimental Results from the DIII-D Advanced Tokamak Program

S. L. Allen¹⁾, and the DIII-D Team^2)
 
1) Lawrence Livermore National Laboratory, Livermore, California USA
²⁾ General Atomics, P.O. Box 85608, San Diego, California USA

Abstract.  The goals of DIII-D Advanced Tokamak (AT) experiments are to investigate and optimize the upper limits of energy confinement and MHD stability in a tokamak plasma, and to simultaneously maximize the fraction of non-inductive current drive. Significant overall progress has been made in the past 2 years, as the performance figure of merit $\beta_{\mathrm{N}}^{}$H_89P of 9 has been achieved in ELMing H-mode for over 16 $\tau_{\mathrm{E}}^{}$ without sawteeth. We also operated at $\beta_{\mathrm{N}}^{}$ $\sim$ 7 for over 35 $\tau_{\mathrm{E}}^{}$ or 3$\tau_{\mathrm{R}}^{}$, with the duration limited by hardware. Real-time feedback control of $\beta$ (at 95% of the stability boundary), optimizing the plasma shape (e.g., $\delta$, divertor strike- and X-point, double/single null balance), and particle control ( n_e/n_GW $\sim$ 0.3, Z_eff < 2.0) were necessary for the long-pulse results. A new quiescent double barrier (QDB) regime with simultaneous inner- and edge- transport barriers and no ELMs has been discovered with $\beta_{\mathrm{N}}^{}$H_89P of 7. The QDB regime has been obtained to date only with counter neutral beam injection. Further modification and control of internal transport barriers (ITBs) has also been demonstrated with impurity injection (broader barrier), pellets, and ECH (strong electron barrier). The new Divertor-2000, a key ingredient in all these discharges, provides effective density, impurity and heat flux control in the high-triangularity plasma shapes. Discharges at n_e/n_GW $\sim$ 1.4 have been obtained with gas puffing by maintaining the edge pedestal pressure; this operation is easier with Divertor-2000. We are developing several other tools required for AT operation, including real-time feedback control of resistive wall modes (RWMs) with external coils, and control of neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD).

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