实验室等离子体中强非线性阿尔芬波相互作用

中文摘要 AI 辅助

阿尔芬波及其非线性相互作用在空间和天体物理等离子体中普遍存在，被认为在这些系统的动力学中扮演重要角色，但其本质仍有待充分理解。我们描述了在大等离子体装置上进行的实验，以研究与强阿尔芬湍流相关的反向传播和同向传播波相互作用的性质。两种相互作用均被发现由于主导的二次非线性而产生广泛的非线性模式谱。反向传播相互作用可通过标准的约化MHD非线性来解释，而同向传播相互作用可通过最近提出的模型来解释，该模型包含了来自Hall MHD的二阶非线性项，这些项在大不平衡条件下占主导，并随离子惯性长度标度变化。后一模型的预测在实验和三维混合模拟中均得到了检验，发现非线性模式增长率与离子惯性尺度依赖性一致。最后，在所获得的相互作用强度下，观察到能量被转移到逐渐更小的垂直尺度，这与局域级联一致，尽管尚未达到完全发展的湍流状态。这些结果揭示并验证了平衡和不平衡湍流（以及其他阿尔芬非线性过程）中发生的机制，并代表了向在实验室中产生受控阿尔芬湍流迈出的重要一步。

英文摘要

Alfvén waves and their nonlinear interactions are ubiquitous in space and astrophysical plasmas, and are thought to play important roles in the dynamics of these systems, yet their nature remains to be fully understood. We describe experiments performed on the Large Plasma Device to study the nature of counter- and co-propagating wave interactions relevant to strong Alfvénic turbulence. Both interactions were found to produce a broad spectrum of nonlinear modes as a result of a dominant quadratic nonlinearity. The counter-propagating interaction can be explained through the standard reduced MHD nonlinearity, and the co-propagating interaction can be explained through a recently-proposed model that includes second-order nonlinear terms from Hall MHD that dominate at large imbalance and scale with the ion inertial length. The predictions of the latter model were tested in both the experiment and in 3D hybrid simulations, where the nonlinear mode growth rate and ion inertial scale dependence were found to be consistent. Finally, at the obtained interaction strengths, energy was seen to be transferred to progressively smaller perpendicular scales, consistent with a local cascade, although not a state of fully-developed turbulence. These results reveal and verify the mechanisms occurring in balanced and imbalanced turbulence (as well as other Alfvénic nonlinear processes), and represent an important step towards the generation of controlled Alfvénic turbulence in the laboratory.

发表机构

• Queen Mary University of London（伦敦大学玛丽女王学院）
• Space Science Institute（空间科学研究所）
• University of California, Los Angeles（加利福尼亚大学洛杉矶分校）
• University of Wisconsin–Madison（威斯康星大学麦迪逊分校）
• Northumbria University（诺森比亚大学）
• University of California, Berkeley（加利福尼亚大学伯克利分校）
• Oak Ridge National Laboratory（橡树岭国家实验室）

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