Dimits transition in three-dimensional ion-temperature-gradient turbulence
Dimits transition in three-dimensional ion-temperature-gradient turbulence
复制标题
限制三维离子温度梯度湍流中的转变
DOI：
10.1017/s002237782200071x
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发表时间：
2022
影响因子：
2.5
通讯作者：
W. Dorland
中科院分区：
文献类型：
--
作者：
P. Ivanov;A. Schekochihin;W. Dorland
我们扩展了之前关于离子级湍流中二维 (2-D) Dimits 转变的工作（Ivanov 等人，J. Plasma Phys.，第 86 卷，2020，855860502），以包括沿磁场的变化。我们考虑一个三场流体模型，用于在没有磁剪切的恒定磁曲率几何结构中静电势、离子温度和离子平行流的扰动。它是在冷离子、长波长渐近极限的回旋运动理论中导出的。正如在二维模型中一样，存在低输运（Dimits）状态，并且发现该状态由强纬向流和纬向温度的准静态阶梯状排列主导。该纬向阶梯是由纬向流的负湍流粘度形成和维持的。与 2-D 模型不同，三维 (3-D) 模型不会遭受超出 Dimits 阈值的非物理爆炸（即楼梯变得非线性不稳定）。相反，建立了明确定义的有限振幅饱和状态。 2-D 和 3-D 模型之间的这种质量差异是由于小规模“寄生”模式的出现造成的，只有当我们允许扰动沿着磁场线变化时，这种模式才存在。这些模式从大尺度扰动中提取能量，并有效增强大尺度热扩散，从而有助于能量从大注入尺度转移到小耗散尺度。我们表明，在我们的模型中，寄生模式总是有利于区域流主导状态。事实上，只要系统沿磁场充分延伸并提供足够的并行分辨率，无论线性驱动的强度如何，都可以实现具有区域阶梯的迪米茨状态。
We extend our previous work on the two-dimensional (2-D) Dimits transition in ion-scale turbulence (Ivanov et al., J. Plasma Phys., vol. 86, 2020, 855860502) to include variations along the magnetic field. We consider a three-field fluid model for the perturbations of electrostatic potential, ion temperature, and ion parallel flow in a constant-magnetic-curvature geometry without magnetic shear. It is derived in the cold-ion, long-wavelength asymptotic limit of the gyrokinetic theory. Just as in the 2-D model, a low-transport (Dimits) regime exists and is found to be dominated by a quasistatic staircase-like arrangement of strong zonal flows and zonal temperature. This zonal staircase is formed and maintained by a negative turbulent viscosity for the zonal flows. Unlike the 2-D model, the three-dimensional (3-D) one does not suffer from an unphysical blow up beyond the Dimits threshold where the staircase becomes nonlinearly unstable. Instead, a well-defined finite-amplitude saturated state is established. This qualitative difference between the 2-D and 3-D models is due to the appearance of small-scale ‘parasitic’ modes that exist only if we allow perturbations to vary along the magnetic field lines. These modes extract energy from the large-scale perturbations and provide an effective enhancement of large-scale thermal diffusion, thus aiding the energy transfer from large injection scales to small dissipative ones. We show that in our model, the parasitic modes always favour a zonal-flow-dominated state. In fact, a Dimits state with a zonal staircase is achieved regardless of the strength of the linear drive, provided the system is sufficiently extended along the magnetic field and sufficient parallel resolution is provided.
影响因子：
8.6
作者：
Barnes, M.;Parra, F. I.;Schekochihin, A. A.
通讯作者：
Schekochihin, A. A.