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，存在空间变化（剪切）流的热、动量和粒子的湍流和湍流传输是普遍存在的。已经观察到，当流中的剪切--“平衡”ExB流或湍流雷诺应力自驱动（带状）流--足够大时，可以减少或抑制湍流和/或湍流传输。在环形聚变装置中广泛观察到的高约束模式（H模式）是这种流动剪切抑制的一个例子。 已经有大量的工作，包括实验，计算和理论，了解充分发展的湍流（及其相关的传输）和流动剪切之间的相互作用。然而，由于所观察到的抑制或减少湍流状态的过渡通常发生在非常快的时间尺度上，很少做实验来阐明这种过渡的详细的非线性动力学。由于观测动力学的困难，也缺乏对跃迁的理论理解，因为很少有详细的模型测试是可能的。本研究的目标是提高对受控实验室环境中不稳定漂移波动（包括宽带湍流）状态和波动抑制状态之间跃迁的详细动力学的理解。 在新墨西哥州大学（UNM）的双源HelCat（Helicon-Cathode）设备中进行的实验正在研究受控磁剪切和磁X点对这些动力学的影响，以及电极偏置的详细影响。 这些实验是通过与完全非线性全局Braginskii代码以及其他计算工具进行直接比较来补充的。 主要的数字代码，一个完全三维的全球（完整的等离子体平衡和波动），正在被用来模拟和理解大规模的平衡流动和波动。该代码由线性稳定性代码补充，该代码将用于解释波动的性质，以及1D，3V粒子单元（PIC）代码，以了解电极偏置到任意电压以影响流量时等离子体电势分布的细节。 此外，正在将线性HelCat装置的测量和模拟与洛桑联邦理工学院等离子体物理研究中心的环形TORPEX基本等离子体装置的测量和模拟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CRPP-EPFL，洛桑，瑞士（参与者无需支付NSF费用）。 该团队由全职教师，研究生和本科生研究人员组成，包括女性和少数民族学生，因此具有强大的STEM教育成分。 这项工作将增加等离子体湍流/流动剪切动力学的基础知识，预计将对天体物理，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.