Recent advances in gyrokinetic simulation have allowed for quantitative predictions of core turbulence and associated transport. However, numerical codes must be tested against experimental results in both turbulence and transport. In this paper, we present recent results from ohmic plasmas in the Alcator C-Mod tokamak using phase contrast imaging (PCI) diagnostic, which is capable of measuring density fluctuations with wave numbers up to 55 cm−1. The experiments were carried out over the range of densities covering the 'neo-Alcator' (linear confinement time scaling with density, electron transport dominates) to the 'saturated ohmic' regime. We have also simulated these plasmas with the gyrokinetic code GYRO and compared numerical predictions with experimentally measured turbulence through a synthetic PCI diagnostic method. The key role played by the ion temperature gradient (ITG) turbulence has been verified, including measurements of turbulent wave propagation in the ion diamagnetic direction. It is found that the intensity of density fluctuations increases with density, in agreement between simulation and experiments. The absolute fluctuation intensity agrees with the simulation within experimental error (±60%). In the saturated ohmic regime, the simulated ion and electron thermal diffusivities also agree with experiments after varying the ion temperature gradient within experimental uncertainty. However, in the linear ohmic regime, GYRO does not agree well with experiments, showing significantly larger ion thermal transport and smaller electron thermal transport. Our study shows that although the short wavelength turbulence in the electron temperature gradient (ETG) range is unstable in the linear ohmic regime, the nonlinear simulation with kθρs up to 4 does not raise the electron thermal diffusivity to the experimental level, where kθ is the poloidal wavenumber and ρs is the ion-sound Larmor radius. At the present time, it is not known whether even shorter wavelength turbulence would account for the measured electron transport.
近年来的回旋动理学模拟进展已能够对芯部湍流及相关输运进行定量预测。然而，数值程序必须在湍流和输运两方面与实验结果进行检验。本文介绍了在Alcator C-Mod托卡马克装置上利用相位衬度成像（PCI）诊断获得的欧姆等离子体最新结果，该诊断能够测量波数高达55 cm⁻¹的密度涨落。实验覆盖了从“新欧姆”（neo-Alcator）定标（约束时间随密度线性增加，电子输运占主导）到“饱和欧姆”（saturated ohmic）状态的密度范围。我们还利用回旋动理学程序GYRO对这些等离子体进行了模拟，并通过合成PCI诊断方法将数值预测与实验测量的湍流进行了比较。离子温度梯度（ITG）湍流所起的关键作用已得到验证，包括对离子逆磁方向湍流波传播的测量。研究发现，密度涨落强度随密度增加而增大，模拟与实验在误差范围内（±60%）一致。在饱和欧姆状态下，模拟得到的离子和电子热扩散系数在离子温度梯度于实验不确定度内调整后也与实验一致。然而，在线性欧姆状态下，GYRO与实验吻合不佳，模拟显示离子热输运显著偏大而电子热输运偏小。我们的研究表明，尽管在电子温度梯度（ETG）范围内短波长湍流在线性欧姆状态下是不稳定的，但kθρs高达4的非线性模拟并未将电子热扩散系数提升至实验水平，其中kθ为极向波数，ρs为离子声速拉莫尔半径。目前尚不清楚更短波长的湍流是否能解释所测得的电子输运。