The fundamental nature of turbulent density fluctuations in standard Wendelstein 7-X (W7-X) stellarator discharges is investigated experimentally via phase contrast imaging (PCI) in combination with gyrokinetic simulations with the code GENE. We find that density fluctuations are ion-temperature-gradient-driven and radially localised in the outer half of the plasma. It is shown that the line-integrated PCI measurements cover the right range of wavenumbers and a favourable toroidal and poloidal location to capture some of the strongest density fluctuations in W7-X. Due to the radial localisation of fluctuations, measured wavenumber–frequency spectra exhibit a dominant phase velocity, which can be related to the $\boldsymbol {E\times B}$ rotation velocity at the radial position of a well in the neoclassical radial electric field. The match is robust against variations of heating power and line-integrated density, which is partly due to the localisation of fluctuations and partly due to effects of the radial gradient in the $\boldsymbol {E\times B}$ velocity profile on the wavenumber–frequency spectrum. The latter effect is studied with a newly built synthetic PCI diagnostic and global gyrokinetic simulations with GENE-3D.

标准 Wendelstein 7-X (W7-X) 仿星器放电中湍流密度波动的基本性质通过相衬成像 (PCI) 结合带有代码 GENE 的陀螺动力学模拟进行实验研究。我们发现密度波动是离子温度梯度驱动的，并且径向定位在等离子体的外半部分。结果表明，线集成 PCI 测量覆盖了正确的波数范围和有利的环形和极向位置，以捕获 W7-X 中一些最强的密度波动。由于波动的径向定位，测量的波数 - 频谱表现出主导相速度，这可能与$\boldsymbol {E\times B}$新古典径向电场中井径向位置的旋转速度。该匹配对于加热功率和线积分密度的变化是稳健的，这部分是由于波动的局部化，部分是由于$\boldsymbol {E\times B}$速度分布对波数的影响——频谱。后一种效应是通过新建的合成 PCI 诊断和 GENE-3D 的全局陀螺动力学模拟来研究的。