NIFS-842

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Author(s): N. Ohyabu et al. Title: Super Dense Core Plasma due to Internal Diffusion Barrier in LHD Date of publication: Oct. 2006 Key words: 21 IAEA Fusion Energy Conference , EX/8-1 Abstract: A Super Dense Core (SDC) plasma operational regime has been discovered in the LID (Local Island divertor) discharges in LHD. The observed new confinement improvement regime is very attractive to future helical device. The SDC plasma has been established naturally by multiple pellet injection. A core region with density as high as 4.5 × 10^20 m^-3 and temperature of 0.85 keV is maintained by an Internal Diffusion Barrier (IDB). The density gradient in the IDB is very high and the time constant for the core density decay is about 1 s. The temperature gradient in the outer region beyond the IDB is similar to that in the outer region of the non-SDC discharge with similar plasma conditions. The size of the SDC increases with increasing beta and increasing R_ax (vacuum magnetic axis). The maximum achieved stored energy is 1.1 MJ (at 10 MW of NBI power), achieving n_o tau_E T_o (fusion triple product) of ~ 4.6×10^19 keVm^-3 s. The density in the outer region is kept low by pumping of the recycled particles by the LID, helping to raise the temperature gradient there and hence the core temperature.

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