Abstract
Steady-state discharges with improved core confinement and H-mode edge with edge localized modes (ELMs) are investigated. In plasmas with an upper triangularity top close to zero an H-factor of HITER89-P = 2.7 and N = 2.2 could be maintained for 1 s and HITER89-P = 2.4 and N = 2.0 for 6 s, the latter corresponding to 40 confinement times or 2 1/2 resistive time scales for current redistribution, only limited by the duration of the possible discharge length. At a line averaged density of 4 × 1019 m-3 the central temperatures reach values of Ti = 10 keV and Te = 6.5 keV. The stationarity of the current profile is explained by magnetic reconnection driven by strong (m = 1, n = 1) fishbones, which, in the absence of sawteeth, also expel energy and impurities. Further increasing the pressure, is limited by neoclassical tearing modes. Raising the density by edge gas fuelling and the simultaneous increase of the neutral beam power, HITER89-P remained unchanged up to ne = 5.5 × 1019 m-3, accompanied by a substantial reduction of Zeff. Increasing top to 0.2, both confinement and -limit improved reaching values of HITER89-P = 3.0 and N = 2.4 at densities above ne = 5 × 1019 m-3. This resulted in the highest fusion product of nD,0Ti,0E = 0.9 × 1020 keV s m-3 so far observed in ASDEX Upgrade.
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