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

Improvements in thermal transport and global energy confinement with a 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. Fast-ion effects have been investigated in various tokamak devices and their 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-ion-based NB (NNB) of 0.4 MeV in JT-60U positive shear ELMy H-mode plasmas. NNB is one of the unique items in JT-60U but is hardly utilized for thermal transport studies. Both global thermal confinement and local thermal transport are improved by adding an NNB to the PNB injected H-mode plasmas. Power degradation of the global energy confinement was not observed with the NNB power scan in this series of experiments. The H-mode confinement enhancement factor increased with increasing power of the fast-ion beta. Less profile stiffness in the ion temperature, in other words an enhancement of internal transport barrier (ITB), has been identified with increasing heating power. The physical mechanisms of ITB formation are discussed based on the transport analyses; the ion thermal diffusivity () remained constant or decreased with increasing heating power, and tended to decrease at higher fast-ion pressures produced by NBs and the E×B shearing rate. Linear gyrokinetic simulations support the experimental observations, and the calculated linear growth rates of ion temperature gradient modes are reduced by adding fast-ion components.