Enhanced heat confinement in the flexible heliac TJ-II

Recent experimental results show that the core electron temperature in the TJ-II stellarator almost doubles previously obtained values for the same heating power. These plasmas, heated with electron cyclotron waves, are characterized by their low density, and by having highly peaked electron temperature profiles and flat, or even hollow, density profiles. The conditions for obtaining these high electron temperature discharges regarding their density, injected power and dependence on plasma species are described. Neoclassical and experimental transport analyses are performed for these discharges, showing a reduction in the electron heat conductivity at the plasma core. The relations of this observed confinement enhancement to the CHS internal transport barrier and the W7-AS neoclassical electron root feature are discussed.

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