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Overview of recent experimental results from the DIII-D advanced tokamak programme


The goals of DIII-D advanced tokamak (AT) experiments are investigation and optimization of the upper limits of energy confinement and MHD stability in a tokamak plasma, and simultaneous maximization of the fraction of non-inductive current drive. Significant overall progress has been made in the past two years, as the performance figure of merit βNH89P of 9 has been achieved in ELMing H mode for over 16τE without sawteeth. The tokamak was also operated at βNH ≈ 7 for over 35τE or 3τR, with the duration limited by the hardware. Real time feedback control of β (at 95% of the stability boundary), optimizing the plasma shape (e.g., δ, divertor strike and X points, double/single null balance) and particle control (ne/nGW ≈ 0.3, Zeff < 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 a βNH89P of 7. The QDB regime has been obtained to date only with counter NBI. 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 ne/nGW ≈ 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 with external coils and control of neoclassical tearing modes with ECCD.

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