Thermal transport and confinement improvements with high power and high energy neutral beam in JT-60U ELMy H-mode plasmas

Improvements in thermal transport and global energy confinement with the high power and high energy Neutral Beam (NB) have been discussed in JT-60U ELMy H-mode plasmas in terms of fast-ion effects on thermal transport. The fast-ion effects have been investigated in various tokamak devices and the mechanisms have been discussed with theoretical models. This paper presents the characteristics of thermal transport and global energy confinement with a power modulation of Positive-ion-based NB (PNB) of 85 keV and Negative-ionbased Neutral Beam (NNB) of 0.4 MeV in JT-60U positive shear ELMy H-mode plasmas. The NNB was one of the unique items in JT-60U but was hardly utilized for thermal transport studies. Both global thermal confinement and local thermal transport were improved by adding NNB to the PNB injected H-mode plasmas. Power degradation of the global energy confinement was not observed with the NNB power scan in the series of experiments. The H-mode confinement enhancement factor increased with increasing power or the fast-ion beta. Less profile stiffness in the ion temperature, in other words, an enhancement of an Internal Transport Barrier (ITB), has been identified with increasing heating power. The physics mechanism of the ITB formation are discussed based on the transport analyses; the ion thermal diffusivity remained constant or decreased with increasing heating power, the ion thermal diffusivity tended to decrease at higher fast-ion pressure produced by NBs and the ExB shearing rate. Linear gyrokinetic simulations support the experimental observations; the calculated linear growth rates of ion temperature gradient modes reduced with adding fast-ion components.