Investigation of Turbulence Dynamics in the Presence of Flow Shear, Electrode Biasing, Magnetic Shear and X-Points

在聚变、空间和实验室等离子体中，热、动量和粒子在空间变化(剪切)流动中的湍流和湍流传输是普遍存在的。已经观察到，当流动中的剪切量足够大时--无论是平衡的ExB流，还是湍流的雷诺应力自驱动(分区)流--都可以减少或抑制湍流和/或湍流的传输。在环形聚变装置中广泛观察到的高限制模(H模)就是这种流动剪切抑制的一个例子。在理解充分发展的湍流(及其相关输运)和流动剪切之间的相互作用方面，已经做了大量的工作，包括实验、计算和理论。然而，由于观测到的到被抑制或减弱的湍流态的转变通常发生在非常快的时间尺度上，因此很少有实验工作来阐明这种转变的详细的非线性动力学。由于观察动力学的困难，对这种转变的理论理解也很缺乏，因为几乎没有可能对模型进行详细的测试。本研究的目的是在受控的实验室环境中提高对不稳定漂移涨落(包括宽带湍流)和涨落抑制态之间转变的详细动力学的理解。在新墨西哥大学(UNM)的双源HelCat(螺旋形阴极)装置上进行的实验，正在研究受控磁切变和磁X点对这些动力学的影响，以及电极偏置的详细影响。这些实验得到了与完全非线性的全球布拉金斯基代码以及其他计算工具的直接比较的补充。主要的数值代码是全3D全球(全等离子体平衡和涨落)，用于模拟和理解大尺度的平衡流动和涨落。该代码还补充了线性稳定性代码和一维3V胞内粒子(PIC)代码，前者将用于解释波动的性质，后者用于了解电极偏置到任意电压以影响流动时等离子体电位分布的细节。此外，线性HelCat设备的测量和模拟正在与瑞士洛桑洛桑联邦理工学院等离子体物理中心(CRPP-EPFL)的环形TORPEX基本等离子体设备进行比较。国际研究小组包括来自UNM、阿拉斯加大学费尔班克斯分校和瑞士洛桑CRPP-EPFL的研究人员(他们免费参加NSF)。该团队由全职教员、研究生和本科生研究人员组成，包括女性和少数民族学生，因此具有很强的STEM教育组成部分。这项工作将增加等离子体湍流/流动剪切动力学的基本知识，并有望对天体物理、空间、实验室和聚变等离子体以及中性流体系统产生影响。

Turbulence and turbulent transport of heat, momentum, and particles in the presence of spatially varying (sheared) flows are ubiquitous in fusion, space, and laboratory plasmas. It has been observed that when the shear in the flow -- either "equilibrium" ExB flows, or turbulent Reynolds stress self-driven (zonal) flows -- is sufficiently large, turbulence and/or turbulent transport can be reduced or suppressed. The high confinement mode (H-mode) widely observed in toroidal fusion devices is one example of such flow shear suppression. There has been a significant amount of work, including experimental, computational, and theoretical, on understanding the interactions between fully developed turbulence (and its associated transport) and flow shear. However, since the observed transitions to suppressed or reduced turbulence states typically occur on very fast time scales, little has been done experimentally to elucidate the detailed nonlinear dynamics of this transition. Because of the difficulty in observing the dynamics, a theoretical understanding of the transition is also lacking, since few detailed tests of models have been possible.The goal of this research is to improve the understanding of the detailed dynamics of transitions between states of unstable drift fluctuations (including broadband turbulence) and fluctuation-suppressed states in a controlled laboratory environment. Experiments, which are being conducted in the dual-source HelCat (Helicon-Cathode) device at the University of New Mexico (UNM), are investigating the effects of controlled magnetic shear and magnetic X-points on these dynamics, as well as the detailed effects of electrode biasing. These experiments are being complemented by direct comparisons with a fully nonlinear global Braginskii code, as well as other computational tools. The primary numerical code, a fully 3D global (full plasma equilibrium and fluctuations), is being used to model and understand both large-scale equilibrium flows and fluctuations. This code is complemented by a linear stability code, which will be used to interpret the nature of the fluctuations, and a 1D, 3V particle in cell (PIC) code to understand the details of the plasma potential profile as electrodes are biased to arbitrary voltages to affect flows. Additionally, measurements and simulations from the linear HelCat device are being compared with those from the toroidal TORPEX basic plasma device at the Centre de Recherche en Physique des Plasmas, École Polytechnique Fédérale de Lausanne (CRPP-EPFL) in Lausanne, Switzerland.The international research team includes researchers from UNM, the University of Alaska, Fairbanks, and CRPP-EPFL, Lausanne, Switzerland (who are participating at no cost to NSF). The team is composed of full-time faculty, graduate and undergraduate student researchers, including female and minority students, and thus has a strong STEM educational component. This work will increase the fundamental knowledge of plasma turbulence/flow shear dynamics, and is expected to have an impact on astrophysical, space, laboratory, and fusion plasmas, as well as on neutral fluid systems.