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arXiv 2203.02769physics.plasm-ph
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三维离子温度梯度turbulence中的缺陷转变

Dimits transition in three-dimensional ion-temperature-gradient turbulence

Plamen G. Ivanov, Alexander A. Schekochihin, William Dorland

摘要:

我们扩展了我们先前关于离子尺度湍流中2D Dimits跃迁的工作(Ivanov等人,2020),以包括沿磁场的变化。我们考虑一个三场流体模型的扰动的静电势,离子温度,和离子平行流在恒定的磁曲率的几何形状没有磁剪切。它是在冷离子,长波长渐近极限的gyrokinetic理论。正如在2D模型中,低运输(Dimits)制度的存在,并被发现是由强纬向流和纬向温度的准静态阶梯状排列为主。这种带状阶梯是由带状流的负湍流粘度形成和维持的。与2D模型不同,3D模型不会遭受超出Dimits阈值的非物理爆破,其中楼梯变得非线性不稳定。相反,一个定义明确的有限振幅饱和状态的建立。2D和3D之间的这种质的差异是由于小尺度“寄生”模式的出现,这种模式只有在我们允许扰动沿着磁场线变化时才存在。这些模式从大尺度扰动中提取能量,并提供大尺度热扩散的有效增强,从而有助于从大注入尺度到小耗散尺度的能量转移。我们表明,在我们的模型中,寄生模式总是有利于带状流为主的状态。事实上,只要系统沿着磁场充分延伸并且提供足够的并行分辨率,则无论线性驱动的强度如何,都可以实现具有带状阶梯的Dimits状态。

英文摘要:

We extend our previous work on the 2D Dimits transition in ion-scale turbulence (Ivanov et al. 2020) 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 2D model, a low-transport (Dimits) regime exists and is found to be dominated by a quasi-static 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 2D model, the 3D 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 2D and 3D 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.

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