@article{oai:nifs-repository.repo.nii.ac.jp:00011648,
 author = {BEURSKENS, Marc and BOZHENKOV, Sergey and FORD, O. and XANTHOPUOLOS, Pavlos and ZOCCO, Alessandro and TURKIN,  Yuri and ALONSO, Juan Arturo and BEIDLER, Craig and CALVO, I. and CARRALERO, Daniel and ESTRADA, Teresa and FUCHERT, Golo and GRULKE, O. and HIRSCH, M. and IDA,  Katsumi and JAKUBOWSKI,  Marcin and KILLER, Carsten and KRYCHOWIAK, Maciej and KWAK, Sehyun and LAZERSON, Samuel and LANGENBERG, Andreas and LUNSFORD, Robert and PABLANT, Novimir and PASCH, E. and PAVONE, Andrea and REIMOLD, F. and ROMBA, Thilo and STECHOW, A. von and SMITH, H. M. and WINDISCH, T. and YOSHINUMA,  Mikirou and ZHANG, Daihong and WOLF, Robert and the W7-X Team},
 issue = {11},
 journal = {Nuclear Fusion},
 month = {Oct},
 note = {0000-0002-3354-0279, The neoclassical transport optimization of the Wendelstein 7-X stellarator has not resulted in the predicted high energy confinement of gas fueled electron-cyclotron-resonance-heated (ECRH) plasmas as modelled in (Turkin et al 2011 Phys. Plasmas 18 022505) due to high levels of turbulent heat transport observed in the experiments. The electron-turbulent-heat transport appears non-stiff and is of the electron temperature gradient (ETG)/ion temperature gradient (ITG) type (Weir et al 2021 Nucl. Fusion 61 056001). As a result, the electron temperature Te can be varied freely from 1 keV–10 keV within the range of PECRH = 1–7 MW, with electron density ne values from 0.1–1.5 × 1020 m−3. By contrast, in combination with the broad electron-to-ion energy-exchange heating profile in ECRH plasmas, ion-turbulent-heat transport leads to clamping of the central ion temperature at Ti ∼ 1.5 keV ± 0.2 keV. In a dedicated ECRH power scan at a constant density of 〈ne〉 = 7 × 1019 m−3, an apparent 'negative ion temperature profile stiffness' was found in the central plasma for (r/a < 0.5), in which the normalized gradient ∇Ti/Ti decreases with increasing ion heat flux. The experiment was conducted in helium, which has a higher radiative density limit compared to hydrogen, allowing a broader power scan. This 'negative stiffness' is due to a strong exacerbation of turbulent transport with an increasing ratio of Te/Ti in this electron-heated plasma. This finding is consistent with electrostatic microinstabilities, such as ITG-driven turbulence. Theoretical calculations made by both linear and nonlinear gyro-kinetic simulations performed by the GENE code in the W7-X three-dimensional geometry show a strong enhancement of turbulence with an increasing ratio of Te/Ti. The exacerbation of turbulence with increasing Te/Ti is also found in tokamaks and inherently enhances ion heat transport in electron-heated plasmas. This finding strongly affects the prospects of future high-performance gas-fueled ECRH scenarios in W7-X and imposes a requirement for turbulence-suppression techniques.},
 title = {Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas},
 volume = {61},
 year = {2021}
}